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diff --git a/winsup/doc/ntsec.sgml b/winsup/doc/ntsec.sgml deleted file mode 100644 index 8484b9f51..000000000 --- a/winsup/doc/ntsec.sgml +++ /dev/null @@ -1,890 +0,0 @@ -<sect1 id="ntsec"><title>Using Windows security in Cygwin</title> - -<para>This section discusses how the Windows security model is -utilized in Cygwin to implement POSIX-like permissions, as well as how -the Windows authentication model is used to allow cygwin applications -to switch users in a POSIX-like fashion.</para> - -<para>The setting of POSIX-like file and directory permissions is -controlled by the <link linkend="mount-table">mount</link> option -<literal>(no)acl</literal> which is set to <literal>acl</literal> by -default.</para> - -<para>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 <ulink url="http://msdn.microsoft.com/en-us/library/aa374860(VS.85).aspx">Access Control</ulink> article in MSDN documentation.</para> - -<para>POSIX concepts and specificially 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.</para> - -<sect2 id="ntsec-common"><title>Overview</title> - -<para>In the Windows security model, almost any "object" is securable. -"Objects" are files, processes, threads, semaphores, etc.</para> - -<para>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:</para> - -<itemizedlist spacing="compact"> -<listitem><para>Flags which control several aspects of this SD. This is -not discussed here.</para></listitem> -<listitem><para>The SID of the object owner.</para></listitem> -<listitem><para>The SID of the object owner group.</para></listitem> -<listitem><para>A list of "Access Control Entries" (ACE), called the -"Discretionary Access Control List" (DACL).</para></listitem> -<listitem><para>Another list of ACEs, called the "Security Access Control List" -(SACL), which doesn't matter for our purpose. We ignore it here.</para></listitem> -</itemizedlist> - -<para>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. -</para> - -<para>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:</para> - -<para>SID of a machine "foo":</para> - -<screen> - S-1-5-21-165875785-1005667432-441284377 -</screen> - -<para>SID of a user "johndoe" of the system "foo":</para> - -<screen> - S-1-5-21-165875785-1005667432-441284377-1023 -</screen> - -<para>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.</para> - -<para>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.</para> - -<para>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.</para> - -<para>Ok, consider you created a new domain "bar" on some new domain -controller and you would like to create a domain account "johndoe":</para> - -<para>SID of a domain "bar.local":</para> - -<screen> - S-1-5-21-186985262-1144665072-740312968 -</screen> - -<para>SID of a user "johndoe" in the domain "bar.local":</para> - -<screen> - S-1-5-21-186985262-1144665072-740312968-1207 -</screen> - -<para>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 <emphasis -role='bold'>not</emphasis> 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. </para> - -<para>The last part of the SID, the so called "Relative IDentifier" (RID), -is by default used as UID and/or GID under Cygwin when you create the -<filename>/etc/passwd</filename> and <filename>/etc/group</filename> -files using the <command><link linkend="mkpasswd">mkpasswd</link></command> and <command><link linkend="mkgroup">mkgroup</link></command> -tools. Domain account UIDs and GIDs are offset by 10000 by default -which might be a bit low for very big organizations. Fortunately there's -an option in both tools to change the offset...</para> - -<para>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:</para> - -<screen> -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. -</screen> - -<para>For a full list please refer to the MSDN document <ulink -url="http://msdn.microsoft.com/en-us/library/aa379649.aspx">Well-known -SIDs</ulink>. 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.</para> - -<para>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.</para> - -<para>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:</para> - -<screen> -administrators S-1-5-32-544 -users S-1-5-32-545 -guests S-1-5-32-546 -... -</screen> - -<para>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?</para> - -<para>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:</para> - -<itemizedlist spacing="compact"> -<listitem><para>The type of the ACE (allow ACE or deny ACE).</para></listitem> -<listitem><para>Permission bits, 32 of them.</para></listitem> -<listitem><para>The SID for which the permissions are allowed or denied.</para></listitem> -</itemizedlist> - -<para>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.</para> - -<para>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 -<xref linkend="ntsec-mapping"></xref>.</para> - -<para>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 Solaris -API to access Windows ACLs in a Unixy way. At the time of writing this -document, the Cygwin implementation of the Solaris API isn't quite up -to speed. For instance, it doesn't handle access denied ACEs gracefully. -So, use with care. Online man pages for the Solaris ACL API can be -found on <ulink url="http://docs.sun.com">http://docs.sun.com</ulink>.</para> - -</sect2> - -<sect2 id="ntsec-files"><title id="ntsec-files.title">File permissions</title> - -<para>On NTFS and if the <literal>noacl</literal> mount option is not -specified for a mount point, Cygwin sets file permissions as in POSIX. -Basically this is done by defining a SD 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".</para> - -<para>To use Windows security correctly, Cygwin depends on the files -<filename>/etc/passwd</filename> and <filename>/etc/group</filename>. -These files define the translation between the Cygwin uid/gid and the -Windows SID. The SID is stored in the pw_gecos field in -<filename>/etc/passwd</filename>, and in the gr_passwd field in -<filename>/etc/group</filename>. Since the pw_gecos field can contain -more information than just a SID, there are some rules for the layout. -It's required that the SID is the last entry of the pw_gecos field, -assuming that the entries in pw_gecos are comma-separated. The -commands <command>mkpasswd</command> and <command>mkgroup</command> -usually do this for you.</para> - -<para>Another interesting entry in the pw_gecos field (which is also -usually created by running <command>mkpasswd</command>) is the Windows user -name entry. It takes the form "U-domain\username" and is sometimes used -by services to authenticate a user. Logging in through -<command>telnet</command> is a common scenario.</para> - -<para>A typical snippet from <filename>/etc/passwd</filename>:</para> - -<example id="ntsec-passwd"> -<title>/etc/passwd:</title> -<screen> -SYSTEM:*:18:544:,S-1-5-18:: -Administrators:*:544:544:,S-1-5-32-544:: -Administrator:unused:500:513:U-FOO\Administrator,S-1-5-21-790525478-115176313-839522115-500:/home/Administrator:/bin/bash -corinna:unused:11001:11125:U-BAR\corinna,S-1-5-21-2913048732-1697188782-3448811101-1001:/home/corinna:/bin/tcsh -</screen> -</example> - -<para>The SYSTEM entry is usually needed by services. The Administrators -entry (Huh? A group in /etc/passwd?) is only here to allow -<command>ls</command> and similar commands to print some file ownerships -correctly. Windows doesn't care if the owner of a file is a user or a -group. In older versions of Windows NT the default ownership for files -created by an administrator account was set to the group Administrators -instead of to the creating user account. This has changed, but you can -still switch to this setting on newer systems. So it's convenient to -have the Administrators group in -<filename>/etc/passwd</filename>.</para> - -<para>The really interesting entries are the next two. The Administrator -entry is for the local administrator, the corinna entry matches the corinna -account in the domain BAR. The information given in the pw_gecos field -are all we need to exactly identify an account, and to have a two way -translation, from Windows account name/SID to Cygwin account name uid and -vice versa. Having this complete information allows us to choose a Cygwin -user name and uid which doesn't have to match the Windows account at all. As -long as the pw_gecos information is available, we're on the safe side:</para> - -<example id="ntsec-passwd-tweaked"> -<title>/etc/passwd, tweaked:</title> -<screen> -root:unused:0:513:U-FOO\Administrator,S-1-5-21-790525478-115176313-839522115-500:/home/Administrator:/bin/bash -thursday_next:unused:11001:11125:U-BAR\corinna,S-1-5-21-2913048732-1697188782-3448811101-1001:/home/corinna:/bin/tcsh -</screen> -</example> - -<para> The above <filename>/etc/passwd</filename> will still work fine. -You can now login via <command>ssh</command> as the user "root", and -Cygwin dutifully translates "root" into the Windows user -"FOO\Administrator" and files owned by FOO\Administrator are shown to -have the uid 0 when calling <command>ls -ln</command>. All you do you're -actually doing as Administrator. Files created as root will be owned by -FOO\Administrator. And the domain user BAR\corinna can now happily -pretend to be Thursday Next, but will wake up sooner or later finding -out she's still actually the domain user BAR\corinna...</para> - -<para>Do I have to mention that you can also rename groups in -<filename>/etc/group</filename>? As long as the SID is present and correct, -all is well. This allows you to, for instance, rename the "Administrators" -group to "root" as well:</para> - -<example id="ntsec-group-tweaked"> -<title>/etc/group, tweaked:</title> -<screen> -root:S-1-5-32-544:544: -</screen> -</example> - -<para>Last but not least, you can also change the primary group of a user -in <filename>/etc/passwd</filename>. The only requirement is that the user -is actually a member of the new primary group in Windows. For instance, -normal users in a domain environment are members in the group "Domain Users", -which in turn belongs to the well-known group "Users". So, if it's -more convenient in your environment for the user's primary group to be -"Users", just set the user's primary group in <filename>/etc/passwd</filename> -to the Cygwin uid of "Users" (see in <filename>/etc/group</filename>, -default 545) and let the user create files with a default group ownership -of "Users".</para> - -<note><para> -If you wish to make these kind of changes to /etc/passwd and /etc/group, -do so only if you feel comfortable with the concepts. Otherwise, do not -be surprised if things break in either subtle or surprising ways! If you -do screw things up, revert to copies of <filename>/etc/passwd</filename> -and <filename>/etc/group</filename> files created by -<command>mkpasswd</command> and <command>mkgroup</command>. (Make -backup copies of these files before modifying them.) Especially, don't -change the UID or the name of the user SYSTEM. It may mostly work, but -some Cygwin applications running as a local service under that account -could suddenly start behaving strangely. -</para></note> - -</sect2> - -<sect2 id="ntsec-ids"><title id="ntsec-ids.title">Special values of user and group ids</title> - -<para>If the current user is not present in -<filename>/etc/passwd</filename>, that user's uid is set to a -special value of 400. The user name for the current user will always be -shown correctly. If another user (or a Windows group, treated as a -user) is not present in <filename>/etc/passwd</filename>, the uid of -that user will have a special value of -1 (which would be shown by -<command>ls</command> as 65535). The user name shown in this case will -be '????????'.</para> - -<para>If the current user is not present in -<filename>/etc/passwd</filename>, that user's login gid is set to a -special value of 401. The gid 401 is shown as 'mkpasswd', -indicating the command that should be run to alleviate the -situation.</para> - -<para>If another user is not present in -<filename>/etc/passwd</filename>, that user's login gid is set to a -special value of -1. If the user is present in -<filename>/etc/passwd</filename>, but that user's group is not in -<filename>/etc/group</filename> and is not the login group of that user, -the gid is set to a special value of -1. The name of this group -(id -1) will be shown as '????????'.</para> - -<para>If the current user is present in -<filename>/etc/passwd</filename>, but that user's login group is not -present in <filename>/etc/group</filename>, the group name will be shown -as 'mkgroup', again indicating the appropriate command.</para> - -<para>A special case is if the current user's primary group SID is noted -in the user's <filename>/etc/passwd</filename> entry using another group -id than the group entry of the same group SID in -<filename>/etc/group</filename>. This should be noted and corrected. -The group name printed in this case is -'passwd/group_GID_clash(PPP/GGG)', with PPP being the gid as noted -in <filename>/etc/passwd</filename> and GGG the gid as noted in -<filename>/etc/group</filename>.</para> - -<para>To summarize:</para> -<itemizedlist spacing="compact"> - -<listitem><para>If the current user doesn't show up in -<filename>/etc/passwd</filename>, it's <emphasis>group</emphasis> will -be named 'mkpasswd'.</para></listitem> - -<listitem><para>Otherwise, if the login group of the current user isn't -in <filename>/etc/group</filename>, it will be named 'mkgroup'.</para> -</listitem> - -<listitem><para>Otherwise a group not in <filename>/etc/group</filename> -will be shown as '????????' and a user not in -<filename>/etc/passwd</filename> will be shown as "????????".</para> -</listitem> - -<listitem><para>If different group ids are used for a group with the same -SID, the group name is shown as 'passwd/group_GID_clash(PPP/GGG)' with -PPP and GGG being the different group ids.</para></listitem> - -</itemizedlist> - -<para> -Note that, since the special user and group names are just indicators, -nothing prevents you from actually having a user named `mkpasswd' in -<filename>/etc/passwd</filename> (or a group named `mkgroup' in -<filename>/etc/group</filename>). If you do that, however, be aware of -the possible confusion. -</para> - -</sect2> - - -<sect2 id="ntsec-mapping"><title id="ntsec-mapping.title">The POSIX permission mapping leak</title> - -<para>As promised earlier, here's the problem when trying to map the -POSIX permission model onto the Windows permission model.</para> - -<para>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:</para> - -<itemizedlist spacing="compact"> -<listitem><para>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.</para></listitem> - -<listitem><para>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.</para></listitem> - -<listitem><para>All access denied ACEs <emphasis -role='bold'>should</emphasis> precede any access allowed ACE. ACLs -following this rule are called "canonical"</para></listitem> -</itemizedlist> - -<para>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.</para> - -<para>Unfortunately the security tab in the file properties dialog of -the Windows NT4 explorer is completely unable to deal with access denied ACEs -while the Windows 2000 and later properties dialog rearranges 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 <emphasis role='bold'>think</emphasis> of pressing OK...</para> - -<para>Canonical ACLs are unable to reflect each possible combination -of POSIX permissions. Example:</para> - -<screen> -rw-r-xrw- -</screen> - -<para>Ok, so here's the first try to create a matching ACL, assuming -the Windows permissions only have three bits, as their POSIX counterpart: -</para> - -<screen> -UserAllow: 110 -GroupAllow: 101 -OthersAllow: 110 -</screen> - -<para>Hmm, because of the accumulation of allow rights the user may -execute because the group may execute.</para> - -<para>Second try:</para> - -<screen> -UserDeny: 001 -GroupAllow: 101 -OthersAllow: 110 -</screen> - -<para>Now the user may read and write but not execute. Better? No! -Unfortunately the group may write now because others may write.</para> - -<para>Third try:</para> - -<screen> -UserDeny: 001 -GroupDeny: 010 -GroupAllow: 001 -OthersAllow: 110 -</screen> - -<para>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.</para> - -<para>The only chance:</para> - -<screen> -UserDeny: 001 -UserAllow: 010 -GroupDeny: 010 -GroupAllow: 001 -OthersAllow: 110 -</screen> - -<para>Again: This works on all existing versions of Windows NT, at the -time of writing from at least NT4 up to Server 2008 R2. Only the GUIs -aren't able (or willing) to deal with that order.</para> - -</sect2> - -<sect2 id="ntsec-setuid-overview"><title id="ntsec-setuid-overview.title">Switching the user context</title> - -<para>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 (since Windows 2000) 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.</para> - -<para>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 -<command>setuid</command> and <command>seteuid</command> system -calls.</para> - -<para>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.</para> - -<para>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.</para> - -</sect2> - -<sect2 id="ntsec-logonuser"><title id="ntsec-logonuser.title">Switching the user context with password authentication</title> - -<para>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 <command>LogonUser</command> 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 -<command>ImpersonateLoggedOnUser</command> to change the user context of -the current thread, or in <command>CreateProcessAsUser</command> to -change the user context of a spawned child process.</para> - -<para>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.</para> - -<para>Back to this logon with password, how can this be used to -implement <command>set(e)uid</command>? Well, it requires modification -of the calling application. Two Cygwin functions have been introduced -to support porting <command>setuid</command> applications which only -require login with passwords. You only give Cygwin the right access -token and then you can call <command>seteuid</command> or -<command>setuid</command> as usual in POSIX applications. Porting such -a <command>setuid</command> application is illustrated by a short -example:</para> - -<screen> -<![CDATA[ -/* 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_password)) - error_exit; -#endif /* CYGWIN */ - -[...] - - /* Everything else remains the same! */ - - setegid (user_pwd_entry->pw_gid); - seteuid (user_pwd_entry->pw_uid); - execl ("/bin/sh", ...); -]]> - -</screen> - -</sect2> - -<sect2 id="ntsec-nopasswd1"><title id="ntsec-nopasswd1.title">Switching the user context without password, Method 1: Create a token from scratch</title> - -<para>An unfortunate aspect of the implementation of -<command>set(e)uid</command> is the fact that the calling process -requires the password of the user to which to switch. Applications such as -<command>sshd</command> wishing to switch the user context after a -successful public key authentication, or the <command>cron</command> -application which, again, wants to switch the user without any authentication -are stuck here. But there are other ways to get new user tokens.</para> - -<para>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, <command>NtCreateToken</command>, 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.</para> - -<para>That sounds good. We just start the servers which have to switch -the user context (<command>sshd</command>, <command>inetd</command>, -<command>cron</command>, ...) as Windows services under the SYSTEM -(or LocalSystem in the GUI) account and everything just works. -Unfortunately that's too simple. Using <command>NtCreateToken</command> -has a few drawbacks.</para> - -<para>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.</para> - -<para>A more important problem is that using <command>NtCreateToken</command> -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.</para> - -<para>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 <command>NtCreateToken</command> 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 <command>LookupAccountSid</command> function will not return -"DOMAIN\my_user", but "NT AUTHORITY\SYSTEM" as the user name.</para> - -<para>You might ask "So what?" After all, this only <emphasis -role='bold'>looks</emphasis> 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++.</para> - -<para>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 -<command>NtCreateToken</command>, you only can access network shares -from the new user's process tree by using explicit authentication, on -the command line for instance:</para> - -<screen> -bash$ net use '\\server\share' /user:DOMAIN\my_user my_users_password -</screen> - -<para>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</para> - -<screen> -bash$ grep foo //server/share/foofile -</screen> - -</sect2> - -<sect2 id="ntsec-nopasswd2"><title id="ntsec-nopasswd2.title">Switching the user context without password, Method 2: LSA authentication package</title> - -<para>Caveat: The method described in this chapter only works starting -with Windows 2000. Windows NT4 users have to use one of the other -methods described in this document.</para> - -<para>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.</para> - -<para>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 <filename>/usr/bin/cyglsa-config</filename> which is part of -the Cygwin package.</para> - -<para>After running <filename>/usr/bin/cyglsa-config</filename> and -rebooting the system, the LSA authentication package is used by Cygwin -when <command>set(e)uid</command> is called by an application. The -created access token using this method has its own logon session.</para> - -<para>This method has two advantages over the <command>NtCreateToken</command> -method.</para> - -<para>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.</para> - -<para>The user is correctly identified, even by delicate native applications -which choke on that using the <command>NtCreateToken</command> method.</para> - -<para>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...</para> - -<para>Nevertheless, this is already a lot better than what we get by -using <command>NtCreateToken</command>, isn't it?</para> - -</sect2> - -<sect2 id="ntsec-nopasswd3"><title id="ntsec-nopasswd3.title">Switching the user context without password, Method 3: With password</title> - -<para>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.</para> - -<para>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.</para> - -<para>So what we do is to utilize this registry area for the purpose of -<command>set(e)uid</command>. The Cygwin command <command><link -linkend="passwd">passwd</link> -R</command> 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 -<command>set(e)uid</command> examines the LSA private registry area and -searches for a Cygwin specific key which contains the password. If it -finds it, it calls <command>LogonUser</command> under the hood, using -this password. If that works, <command>LogonUser</command> returns an -access token with all credentials necessary for network access.</para> - -<para>For good measure, and since this way to implement -<command>set(e)uid</command> 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 <command>regpwd</command>) and use that if it's -available.</para> - -<para>We got it. A full access token with its own logon session, with -all network credentials. Hmm, that's heaven...</para> - -<para>Back on earth, what about the drawbacks?</para> - -<para>First, adding a password to the LSA private registry area -requires administrative access. So calling <command>passwd -R</command> -as a normal user will fail! Cygwin provides a workaround for -this. If <command>cygserver</command> is started as a service running -under the SYSTEM account (which is the default way to run -<command>cygserver</command>) you can use <command>passwd -R</command> -as normal, non-privileged user as well.</para> - -<para>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:</para> - -<screen> -bash$ date -Tue Dec 2 16:28:03 CET 2008 -bash$ at 16:29 /interactive regedit.exe -</screen> - -<para>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.</para> - -<para>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.</para> - -<para>In all other cases, don't use this method. You have been warned.</para> - -</sect2> - -<sect2 id="ntsec-setuid-impl"><title id="ntsec-setuid-impl.title">Switching the user context, how does it all fit together?</title> - -<para>Now we learned about four different ways to switch the user -context using the <command>set(e)uid</command> system call, but -how does <command>set(e)uid</command> really work? Which method does it -use now?</para> - -<para>The answer is, all four of them. So here's a brief overview -what <command>set(e)uid</command> does under the hood:</para> - -<itemizedlist> -<listitem> -<para>When <command>set(e)uid</command> 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.</para> -</listitem> - -<listitem> -<para> -Next, it tests if an access token has been stored by an earlier call -to <command>cygwin_set_impersonation_token</command>. If so, it tests -if that token matches the requested user account. If so, the stored -token is used for the user context switch.</para> - -<para> -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.</para> -</listitem> - -<listitem> -<para>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 <command>LogonUser</command>. If this -succeeds, we use the resulting token for the user context switch.</para> -</listitem> - -<listitem> -<para>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.</para> -</listitem> - -<listitem> -<para>Last chance, try to use the <command>NtCreateToken</command> call -to create a token. If that works, use this token.</para> -</listitem> - -<listitem> -<para>If all of the above fails, our process has insufficient privileges -to switch the user context at all, so <command>set(e)uid</command> -fails and returns -1, setting errno to EPERM.</para> -</listitem> -</itemizedlist> - -</sect2> - -</sect1> |