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diff --git a/winsup/doc/ntsec.xml b/winsup/doc/ntsec.xml new file mode 100644 index 000000000..72cf7bb89 --- /dev/null +++ b/winsup/doc/ntsec.xml @@ -0,0 +1,889 @@ +<?xml version="1.0" encoding='UTF-8'?> +<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook V4.5//EN" + "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"> + +<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 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.</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 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 +<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 Windows XP up to Server 2012. 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 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>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> |