POSIX accounts, permission, and security

This section discusses how the Windows security model is utilized in Cygwin to implement POSIX account information, POSIX-like permissions, and how the Windows authentication model is used to allow cygwin applications to switch users in a POSIX-like fashion.

The setting of POSIX-like file and directory permissions is controlled by the mount option (no)acl which is set to acl by default.

We start with a short overview. Note that this overview must be necessarily short. If you want to learn more about the Windows security model, see the Access Control article in MSDN documentation.

POSIX concepts and in particular the POSIX security model are not discussed here, but assumed to be understood by the reader. If you don't know the POSIX security model, search the web for beginner documentation.

Brief overview of Windows security

In the Windows security model, almost any "object" is securable. "Objects" are files, processes, threads, semaphores, etc.

Every object has a data structure attached, called a "security descriptor" (SD). The SD contains all information necessary to control who can access an object, and to determine what they are allowed to do to or with it. The SD of an object consists of five parts:

  • Flags which control several aspects of this SD. This is not discussed here.

  • The SID of the object owner.

  • The SID of the object owner group.

  • A list of "Access Control Entries" (ACE), called the "Discretionary Access Control List" (DACL).

  • Another list of ACEs, called the "Security Access Control List" (SACL), which doesn't matter for our purpose. We ignore it here.

Every ACE contains a so-called "Security IDentifier" (SID) and other stuff which is explained a bit later. Let's talk about the SID first.

A SID is a unique identifier for users, groups, computers and Active Directory (AD) domains. SIDs are basically comparable to POSIX user ids (UIDs) and group ids (GIDs), but are more complicated because they are unique across multiple machines or domains. A SID is a structure of multiple numerical values. There's a convenient convention to type SIDs, as a string of numerical fields separated by hyphen characters. Here's an example:

SID of a machine "foo":

  S-1-5-21-165875785-1005667432-441284377

SID of a user "johndoe" of the system "foo":

  S-1-5-21-165875785-1005667432-441284377-1023

The first field is always "S", which is just a notational convention to show that this is a SID. The second field is the version number of the SID structure, So far there exists only one version of SIDs, so this field is always 1. The third and fourth fields represent the "authority" which can be thought of as a type or category of SIDs. There are a couple of builtin accounts and accounts with very special meaning which have certain well known values in these third and fourth fields. However, computer and domain SIDs always start with "S-1-5-21". The next three fields, all 32 bit values, represent the unique 96 bit identifier of the computer system. This is a hopefully unique value all over the world, but in practice it's sufficient if the computer SIDs are unique within a single Windows network.

As you can see in the above example, SIDs of users (and groups) are identical to the computer SID, except for an additional part, the so-called "relative identifier" (RID). So the SID of a user is always uniquely attached to the system on which the account has been generated.

It's a bit different in domains. The domain has its own SID, and that SID is identical to the SID of the first domain controller, on which the domain is created. Domain user SIDs look exactly like the computer user SIDs, the leading part is just the domain SID and the RID is created when the user is created.

Ok, consider you created a new domain "bar" on some new domain controller and you would like to create a domain account "johndoe":

SID of a domain "bar.local":

  S-1-5-21-186985262-1144665072-740312968

SID of a user "johndoe" in the domain "bar.local":

  S-1-5-21-186985262-1144665072-740312968-1207

So you now have two accounts called johndoe, one account created on the machine "foo", one created in the domain "bar.local". Both have different SIDs and not even the RID is the same. How do the systems know it's the same account? After all, the name is the same, right? The answer is, these accounts are not identical. All machines on the network will treat these SIDs as identifying two separate accounts. One is "FOO\johndoe", the other one is "BAR\johndoe" or "johndoe@bar.local". Different SID, different account. Full stop.

Starting with Cygwin 1.7.34, Cygwin uses an automatic, internal translation from Windows SID to POSIX UID/GID. This mechanism, which is the preferred method for the SID<=>UID/GID mapping, is described in detail in the section called “Mapping Windows accounts to POSIX accounts”.

Prior to Cygwin 1.7.34, the last part of the SID, the so called "Relative IDentifier" (RID), was by default used as UID and/or GID when you created the /etc/passwd and /etc/group files using the mkpasswd and mkgroup tools. These tools as well as reading accounts from /etc/passwd and /etc/group files is still present in recent versions of Cygwin, but you should switch to the aforementioned automatic translation, unless you have very specific needs. Again, see the section called “Mapping Windows accounts to POSIX accounts” for the details.

Do you still remember the SIDs with special meaning? In offical notation they are called "well-known SIDs". For example, POSIX has no GID for the group of "all users" or "world" or "others". The last three rwx bits in a unix-style permission value just represent the permissions for "everyone who is not the owner or is member of the owning group". Windows has a SID for these poor souls, the "Everyone" SID. Other well-known SIDs represent circumstances under which a process is running, rather than actual users or groups. Here are a few examples for well-known SIDs:

Everyone                        S-1-1-0    Simply everyone...
Batch                           S-1-5-3    Processes started via the task
					   scheduler are member of this group.
Interactive			S-1-5-4    Only processes of users which are
					   logged in via an interactive
					   session are members here.
Authenticated Users             S-1-5-11   Users which have gone through
                                           the authentication process and
					   survived.  Anonymously accessing
					   users are not incuded here.
SYSTEM                          S-1-5-18   A special account which has all
					   kinds of dangerous rights, sort of
					   an uber-root account.

For a full list please refer to the MSDN document Well-known SIDs. The Cygwin package called "csih" provides a tool, /usr/lib/csih/getAccountName.exe, which can be used to print the (possibly localized) name for the various well-known SIDS.

Naturally, well-known SIDs are the same on each machine, so they are not unique to a machine or domain. They have the same meaning across the Windows network.

Additionally, there are a couple of well-known builtin groups, which have the same SID on every machine and which have certain user rights by default:

administrators                  S-1-5-32-544
users                           S-1-5-32-545
guests                          S-1-5-32-546
...

For instance, every account is usually member in the "Users" group. All administrator accounts are member of the "Administrators" group. That's all about it as far as single machines are involved. In a domain environment it's a bit more tricky. Since these SIDs are not unique to a machine, every domain user and every domain group can be a member of these well known groups. Consider the domain group "Domain Admins". This group is by default in the "Administrators" group. Let's assume the above computer called "foo" is a member machine of the domain "bar.local". If you stick the user "BAR\johndoe" into the group "Domain Admins", this guy will automatically be a member of the administrators group on "foo" when logging on to "foo". Neat, isn't it?

Back to ACE and ACL. POSIX is able to create three different permissions, the permissions for the owner, for the group and for the world. In contrast the Windows ACL has a potentially infinite number of members... as long as they fit into 64K. Every member is an ACE. ACE consist of three parts:

  • The type of the ACE (allow ACE or deny ACE).

  • Permission bits, 32 of them.

  • The SID for which the permissions are allowed or denied.

The two (for us) important types of ACEs are the "access allowed ACE" and the "access denied ACE". As the names imply, the allow ACE tells the system to allow the given permissions to the SID, the deny ACE results in denying the specific permission bits.

The possible permissions on objects are more detailed than in POSIX. For example, the permission to delete an object is different from the permission to change object data, and even changing object data can be separated into different permission bits for different kind of data. But there's a problem with the definition of a "correct" ACL which disallows mapping of certain POSIX permissions cleanly. See the section called “File permissions”.

POSIX is able to create only three different permissions? Not quite. Newer operating systems and file systems on POSIX systems also provide access control lists. Two different APIs exist for accessing these ACLs, the Solaris API and the POSIX API. Cygwin implements the original Solaris API to access Windows ACLs in a Unixy way. Online man pages for the Solaris ACL API can be found on http://docs.oracle.com. For an overview see acl(5).

Mapping Windows accounts to POSIX accounts

For as long as Cygwin has existed, it has stored user and group information in /etc/passwd and /etc/group files. Under the assumption that these files would never be too large, the first process in a process tree, as well as every execing process within the tree would parse them into structures in memory. Thus every Cygwin process would contain an expanded copy of the full information from /etc/passwd and /etc/group.

This approach has a few downsides. One of them is that the idea that these files will always be small, is flawed. Another one is that reading the entire file is most of the time entirely useless, since most processes only need information on their own user and the primary group. Last but not least, the passwd and group files have to be maintained separately from the already existing Windows user databases, the local SAM and Active Directory.

On the other hand, we have to have this mapping between Windows SIDs and POSIX uid/gid values, so we need a mechanism to convert SIDs to uid/gid values and vice versa.

Microsoft "Services for UNIX" (SFU) (deprecated since Windows 8/Server 2012) never used passwd/group files. Rather, SFU used a fixed, computational mapping between SIDs and POSIX uid/gid which even has Active Directory support. It allows us to generate uid/gid values from SIDs and vice versa. The mechanism is documented, albeit in a confusing way and spread over multiple MSDN articles.

Starting with Cygwin 1.7.34, Cygwin utilizes an approach inspired by the mapping method as implemented by SFU, with a few differences for backward compatibility and to handle some border cases differently.

Mapping Windows SIDs to POSIX uid/gid values

The following description assumes you're comfortable with the concept of Windows SIDs and RIDs. For a brief introduction, see the section called “Brief overview of Windows security”.

Cygwin's mapping between SIDs and uid/gid values works in two ways.

  • Read /etc/passwd and /etc/group files if they exist, just as in the olden days, mainly for backward compatibility.

  • If no files are present, or if an entry is missing in the files, ask Windows.

At least, that's the default behaviour now. It will be configurable using a file /etc/nsswitch.conf, which is discussed in the section called “The /etc/nsswitch.conf file”. Let's explore the default for now.

If the passwd or group files are present, they will be scanned on demand as soon as a mapping from SIDs to uid/gid or account names is required. The new mechanism will never read the entire file into memory, but only scan for the requested entry and cache this one in memory.

If no entry is found, or no passwd or group file was present, Cygwin will ask the OS.

Note

If the first process in a Cygwin process tree determines that no /etc/passwd or /etc/group file is present, no other process in the entire process tree will try to read the files later on. This is done for self-preservation. It's rather bad if the uid or gid of a user changes during the lifetime of a process tree.

For the same reason, if you delete the /etc/passwd or /etc/group file, this will be ignored. The passwd and group records read from the files will persist in memory until either a new /etc/passwd or /etc/group is created, or you exit all processes in the current process tree.

See the note in the section called “The /etc/nsswitch.conf file” for some comprehensive examples.

So if we've drawn a blank reading the files, we're going to ask the OS. First thing, we ask the local machine for the SID or the username. The OS functions LookupAccountSid and LookupAccountName are pretty intelligent. They have all the stuff built in to ask for any account of the local machine, the Active Directory domain of the machine, the Global Catalog of the forest of the domain, as well as any trusted domain of our forest for the information. One OS call and we're practically done...

Except, the calls only return the mapping between SID, account name and the account's domain. We don't have a mapping to POSIX uid/gid and we're missing information on the user's home dir and login shell.

Let's discuss the SID<=>uid/gid mapping first. Here's how it works.

  • Well-known SIDs in the NT_AUTHORITY domain of the S-1-5-RID type, or aliases of the S-1-5-32-RID type are mapped to the uid/gid value RID. Examples:

      "SYSTEM" S-1-5-18                    <=> uid/gid: 18
      "Users"  S-1-5-32-545                <=> uid/gid: 545
    
  • Other well-known SIDs in the NT_AUTHORITY domain (S-1-5-X-RID):

      S-1-5-X-RID                          <=> uid/gid: 0x1000 * X + RID
    

    Example:

      "NTLM Authentication" S-1-5-64-10    <=> uid/gid: 0x4000A == 262154
    
  • Other well-known SIDs:

      S-1-X-Y                              <=> uid/gid: 0x10000 + 0x100 * X + Y
    

    Example:

      "LOCAL" S-1-2-0                      <=> uid/gid: 0x10200 == 66048
      "Creator Group" S-1-3-1              <=> uid/gid: 0x10301 == 66305
    
  • Logon SIDs: The LogonSid of the current user's session is converted to the fixed uid 0xfff == 4095 and named "CurrentSession". Any other LogonSid is converted to the fixed uid 0xffe == 4094 and named "OtherSession".

  • Mandatory Labels:

      S-1-16-RID                           <=> uid/gid: 0x60000 + RID
    

    Example:

      "Medium Mandatory Level" S-1-16-8192 <=> uid/gid: 0x62000 == 401408
    
  • Accounts from the local machine's user DB (SAM):

      S-1-5-21-X-Y-Z-RID                   <=> uid/gid: 0x30000 + RID
    

    Example:

      "Administrator" S-1-5-21-X-Y-Z-500   <=> uid/gid: 0x301f4 == 197108
    
  • Accounts from the machine's primary domain:

      S-1-5-21-X-Y-Z-RID                   <=> uid/gid: 0x100000 + RID
    

    Example:

      "Domain Users" S-1-5-21-X-Y-Z-513    <=> uid/gid: 0x100201 == 1049089
    
  • Accounts from a trusted domain of the machine's primary domain:

      S-1-5-21-X-Y-Z-RID                   <=> uid/gid: trustPosixOffset(domain) + RID
    

    trustPosixOffset? This needs a bit of explanation. This value exists in Windows domains already since before Active Directory days. What happens is this. If you create a domain trust between two domains, a trustedDomain entry will be added to both databases. It describes how this domain trusts the other domain. One attribute of a trust is a 32 bit value called trustPosixOffset For each new trust, trustPosixOffset will get some automatic value. In recent AD domain implementations, the first trusted domain will get trustPosixOffset set to 0x80000000. Following domains will get lower values. Unfortunately the domain admins are allowed to set the trustPosixOffset value for each trusted domain to some arbitrary 32 bit value, no matter what the other trustPosixOffset are set to, thus allowing any kind of collisions between the trustPosixOffset values of domains. That's not exactly helpful, but as the user of this value, we have to trust the domain admins to set trustPosixOffset to sensible values, or to keep it at the system chosen defaults.

    So, for the first (or only) trusted domain of your domain, the automatic offset is 0x80000000. An example for a user of that trusted domain is

      S-1-5-21-X-Y-Z-1234                  <=> uid/gid 0x800004d2 == 2147484882
    

    There's one problem with this approach. Assuming you're running in the context of a local SAM user on a domain member machine. Local users don't have the right to fetch this kind of domain information from the DC, they'll get permission denied. In this case Cygwin will fake a sensible trustPosixOffset value.

    Another problem is if the AD administrators chose an unreasonably small trustPosixOffset value. Anything below the hexadecimal value 0x100000 (the POSIX offset of the primary domain) is bound to produce collisions with system accounts as well as local accounts. The right thing to do in this case is to notify your administrator of the problem and to ask for moving the offset to a more reasonable value. However, to reduce the probability for collisions, Cygwin overrides this offset with a sensible fixed replacement offset.

  • Local accounts from another machine in the network:

    There's no SID<=>uid/gid mapping implemented for this case. The problem is, there's no way to generate a bijective mapping. There's no central place which keeps an analogue of the trustPosixOffset, and there's the additional problem that the LookupAccountSid and LookupAccountName functions cannnot resolve the SIDs, unless they know the name of the machine this SID comes from. And even then it will probably suffer a Permission denied error when trying to ask the machine for its local account.

Now we have a semi-bijective mapping between SIDs and POSIX uid/gid values, but given that we have potentially users and groups in different domains having the same name, how do we uniquely distinguish between them by name? Well, we can do that by making their names unique in a per-machine way. Dependent on the domain membership of the account, and dependent of the machine being a domain member or not, the user and group names will be generated using a domain prefix and a separator character between domain and account name. The separator character is the plus sign, +.

  • Well-known and builtin accounts will be named as in Windows:

      "SYSTEM", "LOCAL", "Medium Mandatory Level", ...
    
  • If the machine is not a domain member machine, only local accounts can be resolved into names, so for ease of use, just the account names are used as Cygwin user/group names:

      "corinna", "bigfoot", "None", ...
    
  • If the machine is a domain member machine, all accounts from the primary domain of the machine are mapped to Cygwin names without domain prefix:

      "corinna", "bigfoot", "Domain Users", ...
    

    while accounts from other domains are prepended by their domain:

      "DOMAIN1+corinna", "DOMAIN2+bigfoot", "DOMAIN3+Domain Users", ...
    
  • Local machine accounts of a domain member machine get a Cygwin user name the same way as accounts from another domain: The local machine name gets prepended:

      "MYMACHINE+corinna", "MYMACHINE+bigfoot", "MYMACHINE+None", ...
    
  • If LookupAccountSid fails, Cygwin checks the accounts against the known trusted domains. If the account is from one of the trusted domains, an artificial account name is created. It consists of the domain name, and a special name created from the account RID:

      "MY_DOM+User(1234)", "MY_DOM+Group(5678)"
    

    Otherwise we know nothing about this SID, so it will be mapped to the fake accounts Unknown+User/Unknown+Group with uid/gid -1.

Caching account information

The information fetched from the Windows account database or the /etc/passwd and /etc/group files is cached by the process. The cached information is inherited by Cygwin child processes. A Cygwin process invoked from a Windows command, such as CMD.exe, will start a new Cygwin process tree and the caching starts from scratch (unless cygserver is running, but read on).

While usually working fine, this has some drawbacks. Consider a shell calling id. id fetches all group information from the current token and caches them. Unfortunately id doesn't start any child processes, so the information is lost as soon as id exits.

But there's another caching mechanism available. If cygserver is running it will provide passwd and group entry caching for all processes in every Cygwin process tree started after cygserver. So, if you start a Cygwin Terminal and cygserver is running at the time, mintty, the shell, and all child processes will use cygserver caching. If you start a Cygwin Terminal and cygserver is not running at the time, none of the processes started inside this terminal window will use cygserver caching.

The advantage of cygserver caching is that it's system-wide and, as long as cygserver is running, unforgetful. Every Cygwin process on the system will have the cygserver cache at its service. Additionally, all information requested from cygserver once, will be cached inside the process itself and, again, propagated to child processes.

If you automatically start Cygwin processes as Windows services at system startup, you may wish to consider starting cygserver first in order to take advantage of this system-wide caching. To assure that cygserver has started prior to starting sshd or other Cygwin processes, you may wish to create service startup dependencies. Cygserver should probably wait for Windows TCPIP and AFD services before it starts, and then other Cygwin process should start after cygserver. Example Windows commands to accomplish this (after the services already exist) are shown below. You will need an administrative prompt to run the sc config commands.

  # Delay Cygserver until TCPIP and AFD have started
  # Note the (odd) required space character after "depend="

  sc config cygserver depend= tcpip/afd

  # Delay sshd until after Cygserver has started
  # Again note the (odd) required space character after "depend="

  sc config sshd depend= cygserver

  # View the Cygserver service details

  sc qc cygserver

Note that this sc config command replaces any existing dependencies. The above changes will not impact the running instance, only future instances.

  # To remove all dependencies from the cygserver service

  sc config cygserver depend= /

Cygwin user names, home dirs, login shells

Obviously, if you don't maintain passwd and group files, you need to have a way to maintain the other fields of a passwd entry as well. A couple of things come to mind:

  • You want to use a Cygwin username different from your Windows username.

    Note

    This is only supported via /etc/passwd. A Cygwin username maintained in the Windows user databases would require very costly (read: slow) search operations.

  • You want to change the primary group of a user. For AD accounts this is not supported. The primary group of a user is always the Windows primary group set in Active Directory and can't be changed. For SAM accounts, you can add the primary group to the SAM description field of the user. See the section called “The desc schema” for more info.

  • You want a home dir different from the default /home/$USERNAME.

  • You want to specify a different login shell than /bin/bash.

  • You want to add specific content to the pw_gecos field.

For simple needs you can create /etc/passwd and/or /etc/group files with entries for your account and tweak that.

For bigger installations, maintaining per-client files is rather troublesome. Also, no two environments are the same, so the needs are pretty different. Therefore Cygwin supports configuring how to fetch home directory, login shell, and gecos information in /etc/nsswitch.conf. See the next section for detailed information how to configure Cygwin's account handling.

The /etc/nsswitch.conf file

On Linux and some other UNIXy OSes, we have a file called /etc/nsswitch.conf. Among other things, it determines how passwd and group entries are generated. That's what Cygwin now provides as well.

The /etc/nsswitch.conf file is optional. If you don't have one, Cygwin uses sensible defaults.

Note

The /etc/nsswitch.conf file is read exactly once by the first process of a Cygwin process tree. If there was no /etc/nsswitch.conf file when this first process started, then no other process in the running Cygwin process tree will try to read the file.

If you create or change /etc/nsswitch.conf, you have to restart all Cygwin processes that need to see the change. If the process you want to see the change is a child of another process, you need to restart all of that process's parents, too.

For example, if you run vim inside the default Cygwin Terminal, vim is a child of your shell, which is a child of mintty. If you edit /etc/nsswitch.conf in that vim instance, your shell won't immediately see the change, nor will vim if you restart it from that same shell instance. This is because both are getting their nsswitch information from their ancestor, mintty. You have to start a fresh terminal window for the change to take effect.

By contrast, if you leave that Cygwin Terminal window open after making the change to /etc/nsswitch.conf, then restart a Cygwin service like cron, cron will see the change, because it is not a child of mintty or any other Cygwin process. (Technically, it is a child of cygrunsrv, but that instance also restarts when you restart the service.)

The reason we point all this out is that the requirements for restarting things are not quite as stringent as when you replace cygwin1.dll. If you have three process trees, you have three independent copies of the nsswitch information. If you start a fresh process tree, it will see the changes. As long as any process in an existing process tree remains running, all processes in that tree will continue to use the old information.

So, what settings can we perform with /etc/nsswitch.conf? Let's start with an example /etc/nsswitch.conf file set up to all default values:

  # /etc/nsswitch.conf
  passwd: files db
  group:  files db

  db_enum:  cache builtin
  db_home:  /home/%U
  db_shell: /bin/bash
  db_gecos: <empty>
The /etc/nsswitch.conf syntax

The first line, starting with a hash # is a comment. The hash character starts a comment, just as in shell scripts. Everything up to the end of the line is ignored. So this:

  foo:  bar # baz

means, set "foo" to value "bar", ignore everything after the hash.

The other lines define the available settings. The first word up to a colon is a keyword. Note that the colon must follow immediately after the keyword. This is a valid line:

  foo: bar

This is not valid:

  foo  :  bar

Apart from this restriction, the reminder of the line can have as many spaces and TABs as you like.

The passwd: and group: settings

The two lines starting with the keywords passwd: and group: define where Cygwin gets its passwd and group information from. files means, fetch the information from the corresponding file in the /etc directory. db means, fetch the information from the Windows account databases, the SAM for local accounts, Active Directory for domain account. Examples:

  passwd: files

Read passwd entries only from /etc/passwd.

  group: db

Read group entries only from SAM/AD.

  group: files # db

Read group entries only from /etc/group (db is only a comment).

  passwd: files db

Read passwd entries from /etc/passwd. If a user account isn't found, try to find it in SAM or AD. This is the default for both, passwd and group information.

  group: db files

This is a valid entry, but the order will be ignored by Cygwin. If both settings, files and db are specified, Cygwin will always try the files first, then the db.

passwd: and group: are the two basic settings defining where to get the account information from. The following settings starting with db_ define certain aspects of the Windows account database search and how to generate passwd and group information from the database.

The db_enum: setting

db_enum: defines the depth of a database search, if an application calls one of the enumeration functions getpwent or getgrent. The problem with these functions is, they neither allow to define how many entries will be enumerated when calling them in a loop, nor do they allow to add some filter criteria. They were designed back in the days, when only /etc/passwd and /etc/group files existed and the number of user accounts on a typical UNIX system was seldomly a three-digit number.

These days, with user and group databases sometimes going in the six-digit range, they are a potential burden. For that reason, Cygwin does not enumerate all user or group accounts by default, but rather just a very small list, consisting only of the accounts cached in memory by the current process, as well as a handful of predefined builtin accounts.

db_enum: allows to specify the accounts to enumerate in a fine-grained manner. It takes a list of sources as argument:

  db_enum:  source1 source2 ...

The recognized sources are the following:

none
No output from getpwent/getgrent at all.
all
The opposite. Enumerates accounts from all known sources, including all trusted domains.
cache
Enumerate all accounts currently cached in memory.
builtin
Enumerate the predefined builtin accounts for backward compatibility. These are five passwd accounts (SYSTEM, LocalService, NetworkService, Administrators, TrustedInstaller) and two group accounts (SYSTEM and TrustedInstaller).
files
Enumerate the accounts from /etc/passwd or /etc/group.
local
Enumerate all accounts from the local SAM.
primary
Enumerate all accounts from the primary domain.
alltrusted
Enumerate all accounts from all trusted domains.
some.domain
Enumerate all accounts from the trusted domain some.domain. The trusted domain can be given as Netbios flat name (MY_DOMAIN) or as dns domain name (my_domain.corp). In contrast to the aforementioned fixed source keywords, distinct domain names are caseinsensitive. Only domains which are actually trusted domains are enumerated. Unknown domains are simply ignored.

Please note that getpwent/getgrent do not test if an account was already listed from another source, so an account can easily show up twice or three times. Such a test would be rather tricky, nor does the Linux implementation perform such test. Here are a few examples for /etc/nsswitch.conf:

  db_enum: none

No output from getpwent/getgrent at all. The first call to the function immediately returns a NULL pointer.

  db_enum: cache files

Enumerate all accounts cached by the current process, plus all entries from either the /etc/passwd or /etc/group file.

  db_enum: cache local primary

Enumerate all accounts cached by the current process, all accounts from the SAM of the local machine, and all accounts from the primary domain of the machine.

  db_enum: local primary alltrusted

Enumerate the accounts from the machine's SAM, from the primary domain of the machine, and from all trusted domains.

  db_enum: primary domain1.corp sub.domain.corp domain2.net

Enumerate the accounts from the primary domain and from the domains domain1.corp, sub.domain.corp and domain2.net.

  db_enum: all

Enumerate everything and the kitchen sink.

Settings defining how to create the passwd entry

/etc/nsswitch.conf supports three settings to configure where to get the pw_dir, pw_shell, and pw_gecos content of a passwd entry from:

  db_home: schema...     # Define how to fetch the pw_dir entry.
  db_shell: schema...    # Define how to fetch the pw_shell entry.
  db_gecos: schema...    # Define how to fetch the pw_gecos entry.
"schema..." is a list of up to four space-separated schemata:
  db_FOO: schema1 schema2 ...

When generating a passwd entry, Cygwin tries the schemata in order. If the first schema returns an empty string, it skips to the second, and so on. Schemata only supported on AD are silently skipped for SAM accounts and on non-AD machines.

Four schemata are predefined, two schemata are variable. The predefined schemata are the following:

windows
Utilizes typical Windows settings. Supported for AD and SAM accounts.
cygwin
Utilizes the cygwinUser AD schema extension. This schema extension is available via a schema extension file /usr/share/cygwin/cygwin.ldif. See the section called “The cygwin schema” for more information.
unix
Utilizes the posixAccount schema attributes per RFC 2307. The posixAccount schema is available by default since Windows Server 2003 R2, but typically only utilized when installing the Active Directory "Server for NIS" feature (which is deprecated since Server 2012 R2). See also the section called “The unix schema”.
desc
Utilizes XML-style attributes in the description attribute. Supported for AD and SAM accounts. See the section called “The desc schema” for a more detailed description.

The variable schemata are as follows. Note that the leading characters (@ and /) are an integral part of the definition.

@ad_attribute
ad_attribute is any arbitrary AD attribute name which should (ideally) be available in the User class or in any attached auxiliary class. It's always treated as a single string argument. Only the first string of a multi-string attributes will be read.
/path
An arbitrary string, typically a path. The leading slash is required. Given that a single, fixed path for all users only makes marginal sense, the /path schema supports wildcards. A wildcard is a per-cent (%) character, followed by another character giving the meaning. The supported wildcard characters are:
%u
The Cygwin username (that's lowercase u).
%U
The Windows username (that's uppercase U).
%D
Windows domain in NetBIOS style.
%H
Windows home directory in POSIX style. Note that, for the db_home: setting, this only makes sense right after the preceeding slash, as in
db_home:  /%H/cygwin
%_
Since space and TAB characters are used to separate the schemata, a space in the filename has to be given as %_ (that's an underscore).
%%
A per-cent character.

Any other %X expression is treated as if the character X has been given alone.

The exact meaning of a schema depends on the setting it's used for. The following sections explain the settings in detail.

The db_home: setting

The db_home: setting defines how Cygwin fetches the user's home directory, or, more precise, the content of the pw_dir member of the user's passwd entry. The following list describes the meaning of each schema when used with db_home:

windows
The user's home directory is set to the same directory which is used as Windows home directory. This is the homeDirectory AD attribute. For SAM accounts, this is equivalent to the "Home folder" setting in SAM. If both attributes are unset, Cygwin falls back to the user's local profile directory, typically something along the lines of C:\Users\$USERNAME. Of course, the Windows directory is converted to POSIX-style by Cygwin.
cygwin
AD only: The user's home directory is set to the POSIX path given in the cygwinHome attribute from the cygwinUser auxiliary class. See also the section called “The cygwin schema”.
unix
AD only: The user's home directory is set to the POSIX path given in the unixHomeDirectory attribute from the posixAccount auxiliary class. See also the section called “The unix schema”.
desc
The user's home directory is set to the POSIX path given in the home="..." XML-alike setting in the user's description attribute in SAM or AD. See the section called “The desc schema” for a detailed description.
@ad_attribute
AD only: The user's home directory is set to the path given in the ad_attribute attribute. The path can be given as Windows or POSIX path.
/path
The user's home directory is set to the given POSIX path. Remember the wildcards described in the section called “Settings defining how to create the passwd entry”.
Fallback
If none of the schemes given for db_home: define a non-empty directory, the user's home directory is set to /home/$USERNAME.

As has been briefly mentioned before, the default setting for db_home: is

  db_home: /home/%U

So by default, Cygwin just sets the home dir to /home/$USERNAME.

The db_shell: setting

The db_shell: setting defines how Cygwin fetches the user's login shell, the content of the pw_shell member of the user's passwd entry. The following list describes the meaning of each schema when used with db_shell:

windows
The windows schema is ignored for now. The logical choice would be CMD, but that introduces some problems, for instance the fact that CMD has no concept of running as login shell. This may change in future.
cygwin
AD only: The user's home directory is set to the POSIX path given in the cygwinShell attribute from the cygwinUser auxiliary class. See also the section called “The cygwin schema”.
unix
AD only: The user's login shell is set to the POSIX path given in the loginShell attribute from the posixAccount auxiliary class. See also the section called “The unix schema”.
desc
The user's login shell is set to the POSIX path given in the shell="..." XML-alike setting in the user's description attribute in SAM or AD. See the section called “The desc schema” for a detailed description.
@ad_attribute
AD only: The user's login shell is set to the path given in the ad_attribute attribute. The path can be given as Windows or POSIX path.
/path
The user's login shell is set to the given POSIX path. Albeit not being as important here, the wildcards described in the section called “Settings defining how to create the passwd entry” are also available for specifying a login shell path.
Fallback
If none of the schemes given for db_shell: define a non-empty pathname, the user's login shell is set to /bin/bash.

As for db_home:, the default setting for db_shell: is pretty much a constant

  db_shell: /bin/bash
The db_gecos: setting

The db_gecos: setting defines how to fetch additional content for the pw_gecos member of the user's passwd entry. There's always a fixed, Cygwin-specific part in the pw_gecos field for identifying the account. However, an administrator might want to add informative content like, for instance, the user's full name. That's what the db_gecos: setting is for. The following list describes the meaning of each schema when used with db_gecos:

windows
Add the AD displayName attribute or, for SAM accounts, the "Full name" entry to the pw_gecos field.
cygwin
AD only: The content of the cygwinGecos attribute from the cygwinUser auxiliary class is added to pw_gecos. See also the section called “The cygwin schema”.
unix
AD only: The content of the gecos attribute from the posixAccount auxiliary class is added to pw_gecos. See also the section called “The unix schema”.
desc
The content of the gecos="..." XML-alike setting in the user's description attribute in SAM or AD is added to pw_gecos. See the section called “The desc schema” for a detailed description.
@ad_attribute
AD only: The content of the ad_attribute attribute is added to pw_gecos.
/path
The string following the slash is added to pw_gecos. Here, the wildcards described in the section called “Settings defining how to create the passwd entry” may come in handy.
Fallback
If none of the schemes given for db_gecos: define a non-empty pathname, nothing is added to pw_gecos.

The default setting for db_gecos: is the empty string.

The cygwin schema

The cygwin schema is based on a Cygwin-specific Active Directory schema extension. Using this schema extension allows to maintain Cygwin-specific settings entirely within AD, without colliding with any other schema.

The cygwin schema extension is available in a default Cygwin installation in the file /usr/share/cygwin/cygwin.ldif. To install the schema extension, you have to be schema admin, and you have to run the ldifde command on the schema master. The installation itself is rather simple. Assuming you're schema admin and running a shell with administrative privileges:

  $ cd /usr/share/cygwin
  $ ldifde -i -f cygwin.ldif -k -c "CN=schema,CN=Configuration,DC=X" #schemaNamingContext

Afterwards, the auxiliary class cygwinUser is attached to the class User, and the auxiliary class cygwinGroup is attached to the class Group. The new attributes can be immediately edited using ADSI Edit.

At the time of writing the following attributes are utilized by Cygwin:

cygwinHomeUsed as Cygwin home directory with db_home: cygwin. See the section called “The db_home: setting”.
cygwinShellUsed as Cygwin login shell with db_shell: cygwin. See the section called “The db_shell: setting”.
cygwinGecosContent will be added to the pw_gecos field with db_gecos: cygwin. See the section called “The db_gecos: setting”.
The unix schema

The unix schema utilizes the posixAccount attribute extension. This is one of two schema extensions which are connected to AD accounts, available by default starting with Windows Server 2003 R2. They are usually not set, unless used by the Active Directory Server for NIS feature (deprecated since Server 2012 R2). Two schemata are interesting for Cygwin, posixAccount, connected to user accounts, and posixGroup, connected to group accounts. Both follow the description of RFC 2307, an Approach for Using LDAP as a Network Information Service. The user attributes utilized by Cygwin are:

unixHomeDirectoryUsed as Cygwin home directory with db_home: unix. See the section called “The db_home: setting”.
loginShellUsed as Cygwin login shell with db_shell: unix. See the section called “The db_shell: setting”.
gecosContent will be added to the pw_gecos field with db_gecos: unix. See the section called “The db_gecos: setting”.
uidNumberSee the section called “NFS account mapping” and the section called “Samba account mapping”.

The group attributes utilized by Cygwin are:

Apart from power shell scripting or inventing new CLI tools, these attributes can be changed using the Attribute Editor tab in the user properties dialog of the Active Directory Users and Computers MMC snap-in. Alternatively, if the Server for NIS administration feature has been installed, there will be a UNIX Attributes tab which contains the required fields, except for the gecos field. Last resort is ADSI Edit.

The desc schema

When using user accounts from the local account database, the SAM, there are only a very limited number of settings available. In contrast to Active Directory there's no way to add fields to a user's entry. You have to make do with the fields available. The method to utilize the description field has been mainly introduced for those accounts, usually the only ones a home user has. However, for symmetry, and because there may be a reason to use this in an AD environment, this schema is also supported for AD users.

Note

The presentation of local user account settings on Windows is confusing, to say the least. The description field is not visible at all in the user settings available via the User Accounts control settings. And while it's called Description in the Local Users and Groups MMC snap-in (available, for instance, via the Computer Management GUI), in the command line tool net user the same field is called comment. The latter is especially confusing for AD admins, because the comment attribute in AD is called usercomment on the command line. Confused? Never mind, you're not the only one...

Fortunately you can utilize the description field even if you're running a "home edition" of Windows, by using the command line. The net user command allows to set all values in the SAM, even if the GUI is crippled.

A Cygwin SAM comment entry looks like this:

<cygwin key="value" key="value" [...] />

The supported keys are:

home="value"Sets the Cygwin home dir to value.
shell="value"Sets the Cygwin login shell to value.
gecos="value"Adds the string value to the user's gecos field.

The next two settings are only supported for SAM accounts.

group="value"Sets the Cygwin primary group of the account to value, provided that the user is already a member of that group. This allows to override the default None primary group for local accounts. One nice idea here is, for instance, group="Users".
unix="value"Sets the NFS/Samba uid of the user to the decimal value. See the section called “NFS account mapping” and the section called “Samba account mapping”.

The <cygwin .../> string can start at any point in the comment, but you have to follow the rules:

  • It starts with "<cygwin " and ends with "/>".
  • The "cygwin" string and the key names have to be lowercase.
  • No spaces between key and "value", just the equal sign.
  • The value must be placed within double quotes and it must not contain a double quote itself. The double quotes are required for the decimal values as well!

Note

There's also a length restriction imposed by Windows. The description entry has a maximum length of 1023 characters.

CMD example:

net user corinna /comment:"<cygwin home=\"/home/foo\"/>"

Bash example (use single quotes):

net user corinna /comment:'<cygwin home="/home/foo"/>'

For changing group comments, use the `net localgroup' command. The supported key/value pair for SAM groups are:

unix="value"Sets the NFS/Samba gid of the group to the decimal value. See the section called “NFS account mapping” and the section called “Samba account mapping”.

NFS account mapping

Microsoft's NFS client does not map the uid/gid values on the NFS shares to SIDs. There's no such thing as a (fake) security descriptor returned to the application. Rather, via an undocumented API an application can fetch RFC 1813 compatible NFSv3 stat information from the share. This is what Cygwin is using to show stat information for files on NFS shares.

The problem is, while all other information in this stat record, like timestamps, file size etc., can be used by Cygwin, Cygwin had no way to map the values of the st_uid and st_gid members to a Windows SID for a long time. So it just faked the file owner info and claimed that it's you.

However, SFU has, over time, developed multiple methods to map UNIX uid/gid values on NFS shares to Windows SIDs. You'll find the full documentation of the mapping methods in NFS Identity Mapping in Windows Server 2012

Cygwin now utilizes the RFC 2307 mapping for this purpose. This is most of the time provided by an AD domain, but it could also be a standalone LDAP mapping server. Per RFC 2307, the uid is in the attribute uidNumber. For groups, the gid is in the gidNumber attribute. See the section called “The unix schema”.

When Cygwin stat()s files on an NFS share, it asks the mapping server via LDAP in two different ways, depending on the role of the mapping server.

  • If the server is an AD domain controller, it asks for an account with uidNumber attribute == st_uid field of the stat record returned by NFS. If an account matches, AD returns the Windows SID, so we have an immediate mapping from UNIX uid to a Windows SID, if the user account has a valid uidNumber attribute. For groups, the method is the same, just that Cygwin asks for a group with gidNumber attribute == st_gid field of the stat record.
  • If the server is a standalone LDAP mapping server Cygwin asks for the same uidNumber/gidNumber attributes, but it can't expect that the LDAP server knows anything about Windows SIDs. Rather, the mapping server returns the account name. Cygwin then asks the DC for an account with this name, and if that succeeds, we have a mapping between UNIX uid/gid and Windows SIDs.

The mapping will be cached for the lifetime of the process, and inherited by child processes.

Samba account mapping

A fully set up Samba file server with domain integration is running winbindd to map Windows SIDs to artificially created UNIX uids and gids, and this mapping is transparent within the domain, so Cygwin doesn't have to do anything special.

However, setting up winbindd isn't for everybody, and it fails to map Windows accounts to already existing UNIX users or groups. In contrast to NFS, Samba returns security descriptors, but unmapped UNIX accounts get special SIDs:

  • A UNIX user account with uid X is mapped to the Windows SID S-1-22-1-X.
  • A UNIX group account with gid X is mapped to SID S-1-22-2-X.

As you can see, even though we have SIDs, they just reflect the actual uid/gid values on the UNIX box in the RID value. It's only marginally different from the NFS method, so why not just use the same method as for NFS?

That's what Cygwin will do. If it encounters a S-1-22-x-y SID, it will perform the same RFC 2307 mapping as for NFS shares.

For home users without any Windows domain or LDAP server per RFC 2307, but with a Linux machine running Samba, just add this information to your SAM account. Assuming the uid of your Linux user account is 505 and the gid of your primary group is, say, 100, just add the values to your SAM user and group accounts. The following example assumes you didn't already add something else to the comment field.

To your user's SAM comment (remember: called Description in the GUI), add:

  <cygwin group="Users" unix="505"/>

To the Users group SAM comment add:

  <cygwin unix="100"/>

This should be sufficient to work on your Samba share and to see all files owned by your Linux user account as your files.

File permissions

On NTFS and if the noacl mount option is not specified for a mount point, Cygwin sets file permissions as on POSIX systems. Basically this is done by defining a Security Descriptor with the matching owner and group SIDs, and a DACL which contains ACEs for the owner, the group and for "Everyone", which represents what POSIX calls "others".

There's just one problem when trying to map the POSIX permission model onto the Windows permission model.

There's a leak in the definition of a "correct" ACL which disallows a certain POSIX permission setting. The official documentation explains in short the following:

  • The requested permissions are checked against all ACEs of the user as well as all groups the user is member of. The permissions given in these user and groups access allowed ACEs are accumulated and the resulting set is the set of permissions of that user given for that object.

  • The order of ACEs is important. The system reads them in sequence until either any single requested permission is denied or all requested permissions are granted. Reading stops when this condition is met. Later ACEs are not taken into account.

  • All access denied ACEs should precede any access allowed ACE. ACLs following this rule are called "canonical".

Note that the last rule is a preference or a definition of correctness. It's not an absolute requirement. All Windows kernels will correctly deal with the ACL regardless of the order of allow and deny ACEs. The second rule is not modified to get the ACEs in the preferred order.

Unfortunately the security tab in the file properties dialog of the Windows Explorer insists to rearrange the order of the ACEs to canonical order before you can read them. Thank God, the sort order remains unchanged if one presses the Cancel button. But don't even think of pressing OK...

Canonical ACLs are unable to reflect each possible combination of POSIX permissions. Example:

rw-r-xrw-

Ok, so here's the first try to create a matching ACL, assuming the Windows permissions only have three bits, as their POSIX counterpart:

UserAllow:   110
GroupAllow:  101
OthersAllow: 110

Hmm, because of the accumulation of allow rights the user may execute because the group may execute.

Second try:

UserDeny:    001
GroupAllow:  101
OthersAllow: 110

Now the user may read and write but not execute. Better? No! Unfortunately the group may write now because others may write.

Third try:

UserDeny:    001
GroupDeny:   010
GroupAllow:  001
OthersAllow: 110

Now the group may not write as intended but unfortunately the user may not write anymore, either. How should this problem be solved? According to the canonical order a UserAllow has to follow the GroupDeny but it's easy to see that this can never be solved that way.

The only chance:

UserDeny:    001
UserAllow:   010
GroupDeny:   010
GroupAllow:  001
OthersAllow: 110

Again: This works on all existing versions of Windows NT, at the time of writing from at least Windows XP up to Server 2012 R2. Only the GUIs aren't able (or willing) to deal with that order.

Switching the user context

Since Windows XP, Windows users have been accustomed to the "Switch User" feature, which switches the entire desktop to another user while leaving the original user's desktop "suspended". Another Windows feature is the "Run as..." context menu entry, which allows you to start an application using another user account when right-clicking on applications and shortcuts.

On POSIX systems, this operation can be performed by processes running under the privileged user accounts (usually the "root" user account) on a per-process basis. This is called "switching the user context" for that process, and is performed using the POSIX setuid and seteuid system calls.

While this sort of feature is available on Windows as well, Windows does not support the concept of these calls in a simple fashion. Switching the user context in Windows is generally a tricky process with lots of "behind the scenes" magic involved.

Windows uses so-called `access tokens' to identify a user and its permissions. Usually the access token is created at logon time and then it's attached to the starting process. Every new process within a session inherits the access token from its parent process. Every thread can get its own access token, which allows, for instance, to define threads with restricted permissions.

Switching the user context with password authentication

To switch the user context, the process has to request such an access token for the new user. This is typically done by calling the Win32 API function LogonUser with the user name and the user's cleartext password as arguments. If the user exists and the password was specified correctly, the access token is returned and either used in ImpersonateLoggedOnUser to change the user context of the current thread, or in CreateProcessAsUser to change the user context of a spawned child process.

Later versions of Windows define new functions in this context and there are also functions to manipulate existing access tokens (usually only to restrict them). Windows Vista also adds subtokens which are attached to other access tokens which plays an important role in the UAC (User Access Control) facility of Vista and later. However, none of these extensions to the original concept are important for this documentation.

Back to this logon with password, how can this be used to implement set(e)uid? Well, it requires modification of the calling application. Two Cygwin functions have been introduced to support porting setuid applications which only require login with passwords. You only give Cygwin the right access token and then you can call seteuid or setuid as usual in POSIX applications. Porting such a setuid application is illustrated by a short example:


/* First include all needed cygwin stuff. */
#ifdef __CYGWIN__
#include <windows.h>
#include <sys/cygwin.h>
#endif

[...]

  struct passwd *user_pwd_entry = getpwnam (username);
  char *cleartext_password = getpass ("Password:");

[...]

#ifdef __CYGWIN__
  /* Patch the typical password test. */
  {
    HANDLE token;

    /* Try to get the access token from Windows. */
    token = cygwin_logon_user (user_pwd_entry, cleartext_password);
    if (token == INVALID_HANDLE_VALUE)
       error_exit;
    /* Inform Cygwin about the new impersonation token. */
    cygwin_set_impersonation_token (token);
    /* Cygwin is now able, to switch to that user context by setuid or seteuid calls. */
  }
#else
    /* Use standard method on non-Cygwin systems. */
    hashed_password = crypt (cleartext_password, salt);
    if (!user_pwd_entry ||
        strcmp (hashed_password, user_pwd_entry->pw_passwd))
      error_exit;
#endif /* CYGWIN */

[...]

  /* Everything else remains the same! */

  setegid (user_pwd_entry->pw_gid);
  seteuid (user_pwd_entry->pw_uid);
  execl ("/bin/sh", ...);


Switching the user context without password, Method 1: Create a token from scratch

An unfortunate aspect of the implementation of set(e)uid is the fact that the calling process requires the password of the user to which to switch. Applications such as sshd wishing to switch the user context after a successful public key authentication, or the cron application which, again, wants to switch the user without any authentication are stuck here. But there are other ways to get new user tokens.

One way is just to create a user token from scratch. This is accomplished by using an (officially undocumented) function on the NT function level. The NT function level is used to implement the Win32 level, and, as such is closer to the kernel than the Win32 level. The function of interest, NtCreateToken, allows you to specify user, groups, permissions and almost everything you need to create a user token, without the need to specify the user password. The only restriction for using this function is that the calling process needs the "Create a token object" user right, which only the SYSTEM user account has by default, and which is considered the most dangerous right a user can have on Windows systems.

That sounds good. We just start the servers which have to switch the user context (sshd, inetd, cron, ...) as Windows services under the SYSTEM (or LocalSystem in the GUI) account and everything just works. Unfortunately that's too simple. Using NtCreateToken has a few drawbacks.

First of all, beginning with Windows Server 2003, the permission "Create a token object" gets explicitly removed from the SYSTEM user's access token, when starting services under that account. That requires us to create a new account with this specific permission just to run this kind of services. But that's a minor problem.

A more important problem is that using NtCreateToken is not sufficient to create a new logon session for the new user. What does that mean? Every logon usually creates a new logon session. A logon session has a couple of attributes which are unique to the session. One of these attributes is the fact, that Windows functions identify the user domain and user name not by the SID of the access token owner, but only by the logon session the process is running under.

This has the following unfortunate consequence. Consider a service started under the SYSTEM account (up to Windows XP) switches the user context to DOMAIN\my_user using a token created directly by calling the NtCreateToken function. A process running under this new access token might want to know under which user account it's running. The corresponding SID is returned correctly, for instance S-1-5-21-1234-5678-9012-77777. However, if the same process asks the OS for the user name of this SID something wierd happens. For instance, the LookupAccountSid function will not return "DOMAIN\my_user", but "NT AUTHORITY\SYSTEM" as the user name.

You might ask "So what?" After all, this only looks bad, but functionality and permission-wise everything should be ok. And Cygwin knows about this shortcoming so it will return the correct Cygwin username when asked. Unfortunately this is more complicated. Some native, non-Cygwin Windows applications will misbehave badly in this situation. A well-known example are certain versions of Visual-C++.

Last but not least, you don't have the usual comfortable access to network shares. The reason is that the token has been created without knowing the password. The password are your credentials necessary for network access. Thus, if you logon with a password, the password is stored hidden as "token credentials" within the access token and used as default logon to access network resources. Since these credentials are missing from the token created with NtCreateToken, you only can access network shares from the new user's process tree by using explicit authentication, on the command line for instance:

bash$ net use '\\server\share' /user:DOMAIN\my_user my_users_password

Note that, on some systems, you can't even define a drive letter to access the share, and under some circumstances the drive letter you choose collides with a drive letter already used in another session. Therefore it's better to get used to accessing these shares using the UNC path as in

bash$ grep foo //server/share/foofile

Switching the user context without password, Method 2: LSA authentication package

We're looking for another way to switch the user context without having to provide the password. Another technique is to create an LSA authentication package. LSA is an acronym for "Local Security Authority" which is a protected part of the operating system which only allows changes to become active when rebooting the system after the change. Also, as soon as the LSA encounters serious problems (for instance, one of the protected LSA processes died), it triggers a system reboot. LSA is the part of the OS which cares for the user logons and which also creates logon sessions.

An LSA authentication package is a DLL which has to be installed as part of the LSA. This is done by tweaking a special registry key. Cygwin provides such an authentication package. It has to be installed and the machine has to be rebooted to activate it. This is the job of the shell script /usr/bin/cyglsa-config which is part of the Cygwin package.

After running /usr/bin/cyglsa-config and rebooting the system, the LSA authentication package is used by Cygwin when set(e)uid is called by an application. The created access token using this method has its own logon session.

This method has two advantages over the NtCreateToken method.

The very special and very dangerous "Create a token object" user right is not required by a user using this method. Other privileged user rights are still necessary, especially the "Act as part of the operating system" right, but that's just business as usual.

The user is correctly identified, even by delicate native applications which choke on that using the NtCreateToken method.

Disadvantages? Yes, sure, this is Windows. The access token created using LSA authentication still lacks the credentials for network access. After all, there still hasn't been any password authentication involved. The requirement to reboot after every installation or deinstallation of the cygwin LSA authentication DLL is just a minor inconvenience compared to that...

Nevertheless, this is already a lot better than what we get by using NtCreateToken, isn't it?

Switching the user context without password, Method 3: With password

Ok, so we have solved almost any problem, except for the network access problem. Not being able to access network shares without having to specify a cleartext password on the command line or in a script is a harsh problem for automated logons for testing purposes and similar stuff.

Fortunately there is a solution, but it has its own drawbacks. But, first things first, how does it work? The title of this section says it all. Instead of trying to logon without password, we just logon with password. The password gets stored two-way encrypted in a hidden, obfuscated area of the registry, the LSA private registry area. This part of the registry contains, for instance, the passwords of the Windows services which run under some non-default user account.

So what we do is to utilize this registry area for the purpose of set(e)uid. The Cygwin command passwd -R allows a user to specify his/her password for storage in this registry area. When this user tries to login using ssh with public key authentication, Cygwin's set(e)uid examines the LSA private registry area and searches for a Cygwin specific key which contains the password. If it finds it, it calls LogonUser under the hood, using this password. If that works, LogonUser returns an access token with all credentials necessary for network access.

For good measure, and since this way to implement set(e)uid is not only used by Cygwin but also by Microsoft's SFU (Services for Unix), we also look for a key stored by SFU (using the SFU command regpwd) and use that if it's available.

We got it. A full access token with its own logon session, with all network credentials. Hmm, that's heaven...

Back on earth, what about the drawbacks?

First, adding a password to the LSA private registry area requires administrative access. So calling passwd -R as a normal user will fail! Cygwin provides a workaround for this. If cygserver is started as a service running under the SYSTEM account (which is the default way to run cygserver) you can use passwd -R as normal, non-privileged user as well.

Second, as aforementioned, the password is two-way encrypted in a hidden, obfuscated registry area. Only SYSTEM has access to this area for listing purposes, so, even as an administrator, you can't examine this area with regedit. Right? No. Every administrator can start regedit as SYSTEM user:

bash$ date
Tue Dec  2 16:28:03 CET 2008
bash$ at 16:29 /interactive regedit.exe

Additionally, if an administrator knows under which name the private key is stored (which is well-known since the algorithms used to create the Cygwin and SFU keys are no secret), every administrator can access the password of all keys stored this way in the registry.

Conclusion: If your system is used exclusively by you, and if you're also the only administrator of your system, and if your system is adequately locked down to prevent malicious access, you can safely use this method. If your machine is part of a network which has dedicated administrators, and you're not one of these administrators, but you (think you) can trust your administrators, you can probably safely use this method.

In all other cases, don't use this method. You have been warned.

Switching the user context, how does it all fit together?

Now we learned about four different ways to switch the user context using the set(e)uid system call, but how does set(e)uid really work? Which method does it use now?

The answer is, all four of them. So here's a brief overview what set(e)uid does under the hood:

  • When set(e)uid is called, it tests if the user context had been switched by an earlier call already, and if the new user account is the privileged user account under which the process had been started originally. If so, it just switches to the original access token of the process it had been started with.

  • Next, it tests if an access token has been stored by an earlier call to cygwin_set_impersonation_token. If so, it tests if that token matches the requested user account. If so, the stored token is used for the user context switch.

    If not, there's no predefined token which can just be used for the user context switch, so we have to create a new token. The order is as follows.

  • Check if the user has stored the logon password in the LSA private registry area, either under a Cygwin key, or under a SFU key. If so, use this to call LogonUser. If this succeeds, we use the resulting token for the user context switch.

  • Otherwise, check if the Cygwin-specifc LSA authentication package has been installed and is functional. If so, use the appropriate LSA calls to communicate with the Cygwin LSA authentication package and use the returned token.

  • Last chance, try to use the NtCreateToken call to create a token. If that works, use this token.

  • If all of the above fails, our process has insufficient privileges to switch the user context at all, so set(e)uid fails and returns -1, setting errno to EPERM.