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diff --git a/winsup/doc/ntsec.sgml b/winsup/doc/ntsec.sgml deleted file mode 100644 index 4859feb48..000000000 --- a/winsup/doc/ntsec.sgml +++ /dev/null @@ -1,834 +0,0 @@ -<sect1 id="ntsec"><title>NT security and usage of <literal>ntsec</literal></title> - -<para>The setting of UNIX like object permissions is controlled by the -<link linkend="using-cygwinenv"><envar>CYGWIN</envar> environment -variable</link> setting <literal>(no)ntsec</literal> which is set to -<literal>ntsec</literal> by default.</para> - -<para>The design goal of <literal>ntsec</literal> is to get a more UNIX-like -permission structure based upon the security features of Windows NT. -To describe the changes, I will first give a short overview in -<xref linkend="ntsec-common"></xref>. -</para> -<para><link linkend="ntsec-processes" endterm="ntsec-processes.title"></link> -discusses the changes in ntsec related to privileges on processes.</para> - -<para><link linkend="ntsec-files" endterm="ntsec-files.title"></link> shows -the basics of UNIX-like setting of file permissions.</para> - -<para><link linkend="ntsec-sids" endterm="ntsec-sids.title"></link> -talks about using SIDs in <filename>/etc/passwd</filename> and -<filename>/etc/group</filename>.</para> - -<para><link linkend="ntsec-mapping" endterm="ntsec-mapping.title"></link> -illustrates the permission mapping leak of Windows NT.</para> - -<para><link linkend="ntsec-aclfuncs" endterm="ntsec-aclfuncs.title"></link> -describes in short the ACL API since release 1.1.</para> - -<para><link linkend="ntsec-setuid" endterm="ntsec-setuid.title"></link> -describes the new support of a setuid concept introduced with release -1.1.3.</para> - -<para><link linkend="ntsec-switch" endterm="ntsec-switch.title"></link> -gives the basics of using the SYSTEM user to switch user context. -</para> - -<para><link linkend="ntsec-ids" endterm="ntsec-ids.title"></link> -explains the way Cygwin shows users and groups that are not in -<filename>/etc/passwd</filename> or <filename>/etc/group</filename>. -</para> - -<sect2 id="ntsec-common"><title>NT security</title> - -<para>The NT security allows a process to allow or deny access of -different kind to `objects'. `Objects' are files, processes, -threads, semaphores, etc.</para> - -<para>The main data structure of NT security is the `security descriptor' -(SD) structure. It explains the permissions, that are granted (or denied) -to an object and contains information, that is related to so called -`security identifiers' (SID).</para> - -<para>A SID is a unique identifier for users, groups and domains. -SIDs are comparable to UNIX UIDs and GIDs, but are more complicated -because they are unique across networks. Example:</para> - -<para>SID of a system `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 above example shows the convention for printing SIDs. The leading -`S' should show that it is a SID. The next number is a version number which -is always 1. The next number is the so called `top-level authority' that -identifies the source that issued the SID.</para> - -<para>While each system in a NT network has it's own SID, the situation -is modified in NT domains: The SID of the domain controller is the -base SID for each domain user. If an NT user has one account as domain -user and another account on his local machine, these accounts are under -any circumstances DIFFERENT, regardless of the usage of the same user -name and password!</para> - -<para>SID of a domain `bar':</para> - -<screen> - S-1-5-21-186985262-1144665072-740312968 -</screen> - -<para>SID of a user `johndoe' in the domain `bar':</para> - -<screen> - S-1-5-21-186985262-1144665072-740312968-1207 -</screen> - -<para>The last part of the SID, the so called `relative identifier' (RID), -is by default used as UID and/or GID under Cygwin. As the name and the -above example implies, this id is unique only relative to one system or -domain.</para> - -<para>Note, that it's possible that a user has the same RID on two -different systems. The resulting SIDs are nevertheless different, so -the SIDs are representing different users in an NT network.</para> - -<para>There is a big difference between UNIX IDs and NT SIDs: the existence of -the so called `well known groups'. For example UNIX has no GID for the -group of `all users'. NT has an SID for them, called `Everyone' in the -English versions. The SIDs of well-known groups are not unique across -an NT network but their meanings are unmistakable. -Examples of well-known groups:</para> - -<screen> -everyone S-1-1-0 -creator/owner S-1-3-0 -batch process (via `at') S-1-5-3 -authenticated users S-1-5-11 -system S-1-5-18 -</screen> - -<para>The last important group of SIDs are the `predefined groups'. This -groups are used mainly on systems outside of domains to simplify the -administration of user permissions. The corresponding SIDs are not unique -across the network so they are interpreted only locally:</para> - -<screen> -administrators S-1-5-32-544 -users S-1-5-32-545 -guests S-1-5-32-546 -... -</screen> - -<para>Now, how are permissions given to objects? A process may assign an SD -to the object. The SD of an object consists of three parts:</para> - -<itemizedlist spacing="compact"> -<listitem><para>the SID of the owner </para></listitem> -<listitem><para>the SID of the group </para></listitem> -<listitem><para>a list of SIDs with their permissions, called -`access control list' (ACL) </para></listitem> -</itemizedlist> - -<para>UNIX is able to create three different permissions, the permissions -for the owner, for the group and for the world. In contrast the ACL -has a potentially infinite number of members. Every member is a so called -`access control element' (ACE). An ACE contains three parts:</para> - -<itemizedlist spacing="compact"> -<listitem><para>the type of the ACE </para></listitem> -<listitem><para>permissions, described with a DWORD </para></listitem> -<listitem><para>the SID, for which the above mentioned permissions are -set </para></listitem> -</itemizedlist> - -<!-- Is the historical note really important here? we're at version 1.5.9, after all.. --> -<para>The two important types of ACEs are the `access allowed ACE' and the -`access denied ACE'. The ntsec functionality only used `access allowed ACEs' up -to Cygwin version 1.1.0. Later versions also use `access denied ACEs' -to reflect the UNIX permissions as well as possible.</para> - -<para>The possible permissions on objects are more detailed than in -UNIX. For example, the permission to delete an object is different -from the write permission.</para> - -<para>With the aforementioned method NT is able to grant or revoke permissions -to objects in a far more specific way. But what about cygwin? In a POSIX -environment it would be fine to have the security behavior of a POSIX -system. The NT security model is MOSTLY able to reproduce the POSIX model. -The ntsec method tries to do this in cygwin.</para> - -<para>You ask "Mostly? Why mostly???" Because there's a leak in the NT model. -I will describe that in detail in chapter 5.</para> - -<para>Creating explicit object security is not that easy so you will often -see only two simple variations in use:</para> - -<itemizedlist spacing="compact"> -<listitem><para>default permissions, computed by the operating system </para></listitem> -<listitem><para>each permission to everyone </para></listitem> -</itemizedlist> - -<para>For parameters to functions that create or open securable objects another -data structure is used, the `security attributes' (SA). This structure -contains an SD and a flag that specifies whether the returned handle -to the object is inherited to child processes or not. -This property is not important for ntsec so in -this document the difference between SDs and SAs is ignored.</para> - -</sect2> - -<sect2 id="ntsec-processes"><title id="ntsec-processes.title">Process privileges</title> - -<para>Any process started under control of Cygwin has a semaphore attached -to it, that is used for signaling purposes. The creation of this semaphore -can be found in sigproc.cc, function `getsem'. The first parameter to the -function call `CreateSemaphore' is an SA. Without ntsec this SA -assigns default security to the semaphore. There is a simple disadvantage: -Only the owner of the process may send signals to it. Or, in other words, -if the owner of the process is not a member of the administrators' group, -no administrator may kill the process! This is especially annoying, if -processes are started via service manager.</para> - -<para>Ntsec now assigns an SA to the process control semaphore, that -has each permission set for the user of the process, for the -administrators' group and for `system', which is a synonym for the -operating system itself. The creation of this SA is done by the function -`sec_user', that can be found in `shared.cc'. Each member of the -administrators' group is now allowed to send signals to any process -created in Cygwin, regardless of the process owner.</para> - -<para>Moreover, each process now has the appropriate security settings, when -it is started via `CreateProcess'. You will find this in function -`spawn_guts' in module `spawn.cc'. The security settings for starting a -process in another user context have to add the SID of the new user, too. -In the case of the `CreateProcessAsUser' call, sec_user creates an SA with -an additional entry for the sid of the new user.</para> - -</sect2> - -<sect2 id="ntsec-files"><title id="ntsec-files.title">File permissions</title> - -<para>If ntsec is turned on, file permissions are set as in UNIX. An SD is -assigned to the file containing the owner and group and ACEs for the -owner, the group and `Everyone'.</para> - -<para>The complete settings of UNIX like permissions can be found in the file -`security.cc'. The two functions `get_nt_attribute' and `set_nt_attribute' -are the main code. The reading and writing of the SDs is done by the -functions `read_sd' and `write_sd'. `write_sd' uses the function `BackupRead' -instead of the simpler function `SetFileSecurity' because the latter is -unable to set owners different from the caller.</para> - -<para>If you are creating a file `foo' outside of Cygwin, you will see something -like the following on <command>ls -ln</command>:</para> - -<para>If your login is member of the administrators' group:</para> -<screen> - rwxrwxrwx 1 544 513 ... foo -</screen> -<para>if not:</para> -<screen> - rwxrwxrwx 1 1000 513 ... foo -</screen> - -<para>Note the user and group IDs. 544 is the UID of the administrators' group. -This is a `feature' <literal>:-P</literal> of WinNT. If you are a member of -the administrators' group, every file that you create is owned by the -administrators' group, instead of by you.</para> - -<para>The second example shows the UID of the first user, that has been -created with NT's the user administration tool. The users and groups are -sequentially numbered, starting with 1000. Users and groups are using the -same numbering scheme, so a user and a group don't share the same ID.</para> - -<para>In both examples the GID 513 is of special interest. This GID is a -well known group with different naming in local systems and domains. -Outside of domains the group is named 'None' (`Kein' in German, `Aucun' -in French, etc.), in domains it is named 'Domain Users'. Unfortunately, -the group `None' is never shown in the user admin tool outside of domains! -This is very confusing but this seems to have no negative consequences.</para> - -<para>To work correctly, ntsec depends on the files -<filename>/etc/passwd</filename> and <filename>/etc/group</filename>. -In Cygwin release 1.0 the names and the IDs must correspond to the -appropriate NT IDs! The IDs used in Cygwin are the RID of the NT SID, as -mentioned earlier. -A SID of e.g. the user `corinna' on my NT workstation:</para> - -<screen> - S-1-5-21-165875785-1005667432-441284377-1000 -</screen> - -<para>Note the last number: It's the RID 1000, Cygwin's UID.</para> - -<para>Unfortunately, workstations and servers outside of domains are not -able to set primary groups! In these cases, where there is no correlation -of users to primary groups, NT returns 513 (None) as primary group, -regardless of the membership to existing local groups.</para> - -<para>When using <command>mkpasswd -l -g</command> on such systems, you -have to change the primary group by hand if `None' as primary group is -not what you want (and I'm sure, it's not what you want!)</para> - -<para>Look at the following examples, which were parts of my files before -storing SIDs in /etc/passwd and /etc/group had been introduced (See next -chapter for details). With the exception of my personal user entry, all -entries are well known entries.</para> - -<example> -<title>/etc/passwd</title> -<screen> -everyone:*:0:0::: -system:*:18:18::: -administrator::500:544::/home/root:/bin/bash -guest:*:501:546::: -administrators:*:544:544::/home/root: -corinna::1000:547:Corinna Vinschen:/home/corinna:/bin/tcsh -</screen> -</example> - -<example> -<title>/etc/group</title> -<screen> -everyone::0: -system::18: -none::513: -administrators::544: -users::545: -guests::546: -powerusers::547: -</screen> -</example> - -<para>As you can see, I changed my primary group membership from 513 (None) -to 547 (powerusers). So all files I created inside of Cygwin were now owned -by the powerusers group instead of None. This is the way I liked it.</para> - -<para>Groups may be mentioned in the passwd file, too. This has two -advantages:</para> - -<itemizedlist spacing="compact"> -<listitem><para>Because NT assigns them to files as owners, a -<command>ls -l</command> is often more readable.</para></listitem> -<listitem><para>Moreover it's possible to assigned them to files as -owners with Cygwin's <command>chown</command>.</para></listitem> -</itemizedlist> - -<para>The group `system' is the aforementioned synonym for the operating system -itself and is normally the owner of processes that are started through -service manager. The same is true for files that are created by -processes, which are started through service manager.</para> - -</sect2> - -<sect2 id="ntsec-sids"><title id="ntsec-sids.title">NT SIDs in Cygwin</title> - -<para>In Cygwin release 1.1 a new technique of using the -<filename>/etc/passwd</filename> and <filename>/etc/group</filename> - was introduced.</para> - -<para>Both files may now contain SIDs of users and groups. They -are saved in the last field of pw_gecos in <filename>/etc/passwd</filename> -and in the gr_passwd field in <filename>/etc/group</filename>.</para> - -<para>This has the following advantages:</para> -<itemizedlist spacing="compact"> -<listitem><para>ntsec works better in domain environments.</para></listitem> -<listitem><para>Accounts (users and groups) may get another name in -Cygwin than their NT account name. The name in <filename>/etc/passwd</filename> -or <filename>/etc/group</filename> is transparently used by Cygwin -applications (e.g. <command>chown</command>, <command>chmod</command>, -<command>ls</command>):</para> - -<screen> -root::500:513::/home/root:/bin/sh -</screen> - -<para>instead of</para> - -<screen> -adminstrator::500:513::/home/root:/bin/sh -</screen> - -<para>Caution: If you like to use the account as login account via -<command>telnet</command> etc. you have to remain the name unchanged or -you have to use the special version of <command>login</command> which is -part of the standard Cygwin distribution since 1.1.</para></listitem> -<listitem><para>Cygwin UIDs and GIDs are now not necessarily the RID -part of the NT SID:</para> - -<screen> -root::0:513:S-1-5-21-54355234-56236534-345635656-500:/home/root:/bin/sh -</screen> - -<para>instead of</para> - -<screen> -root::500:513::/home/root:/bin/sh -</screen> - -</listitem> -<listitem><para>As in U*X systems UIDs and GIDs numbering scheme now -don't influence each other. So it's possible to have same Id's for a -user and a group:</para> -<example> -<title>/etc/passwd:</title> -<screen> -root::0:0:S-1-5-21-54355234-56236534-345635656-500:/home/root:/bin/sh -</screen> -</example> - -<example> -<title>/etc/group:</title> -<screen> -root:S-1-5-32-544:0: -</screen> -</example> -</listitem> -</itemizedlist> - -<para>The tools <command>mkpasswd</command> and <command>mkgroup</command> -create the needed entries by default. If you don't want that you can use -the options <literal>-s</literal> or <literal>--no-sids</literal>. I suggest -not to do this since ntsec works better when having the SIDs available.</para> - -<para>Please note that the pw_gecos field in <filename>/etc/passwd</filename> -is defined as a comma separated list. The SID has to be the last field!</para> - -<para>As aforementioned you are able to use Cygwin account names different -from the NT account names. If you want to login through `telnet' or something -else you have to use the special <command>login</command>. You may then -add another field to pw_gecos which contains the NT user name including -it's domain. So you are able to login as each domain user. The syntax -is easy: Just add an entry of the form U-ntdomain\ntusername to the pw_gecos -field. Note that the SID must still remain the last field in pw_gecos!</para> - -<screen> -the_king::1:1:Elvis Presley,U-STILLHERE\elvis,S-1-5-21-1234-5678-9012-1000:/bin/sh -</screen> - -<para>For a local user just drop the domain:</para> - -<screen> -the_king::1:1:Elvis Presley,U-elvis,S-1-5-21-1234-5678-9012-1000:/bin/sh -</screen> - -<para>In either case the password of the user is taken from the NT user -database, NOT from the passwd file!</para> - -<para>As in the previous chapter I give my personal -<filename>/etc/passwd</filename> and <filename>/etc/group</filename> as -examples. Please note that I've changed these files heavily! There's no -need to change them that way, it's just for testing purposes and... -for fun.</para> - -<example> -<title>/etc/passwd</title> -<screen> -root:*:0:0:Administrators group,S-1-5-32-544:: -SYSTEM:*:18:18:,S-1-5-18:/home/system:/bin/bash -admin:*:500:513:,S-1-5-21-1844237615-436374069-1060284298-500:/home/Administrator:/bin/bash -corinna:*:100:0:Corinna Vinschen,S-1-5-21-1844237615-436374069-1060284298-1003:/home/corinna:/bin/tcsh -Guest:*:501:546:,S-1-5-21-1844237615-436374069-1060284298-501:/home/Guest:/bin/bash -</screen> -</example> - -<example> -<title>/etc/group</title> -<screen> -root:S-1-5-32-544:0: -local:S-1-2-0:2: -network:S-1-5-2:3: -interactive:S-1-5-4:4: -authenticatedusers:S-1-5-11:5: -SYSTEM:S-1-5-18:18: -local_svc:S-1-5-19:19: -netwrk_svc:S-1-5-20:20: -none:S-1-5-21-1844237615-436374069-1060284298-513:513: -bckup_op:S-1-5-32-551:551: -guests:S-1-5-32-546:546: -pwrusers:S-1-5-32-547:547: -replicator:S-1-5-32-552:552: -users:S-1-5-32-545:545: -</screen> -</example> - -<para>If you want to do similar changes to your files, please do that only -if you're feeling comfortably with the concepts. Otherwise don't be surprised -if some stuff doesn't work anymore. If you screwed up things, revert to files -created by mkpasswd and mkgroup. Especially don't change the UID or the name -of user SYSTEM. Even if that works mostly, some Cygwin applications running -as local service under that account could suddenly start behaving strangely. -</para> - -</sect2> - -<sect2 id="ntsec-mapping"><title id="ntsec-mapping.title">The mapping leak</title> - -<para>Now its time to point out the leak in the NT permissions. -The official documentation explains in short the following:</para> -<itemizedlist spacing="compact"> -<listitem><para>access allow ACEs are accumulated regarding to the -group membership of the caller.</para></listitem> -<listitem><para>The order of ACEs is important. The system reads them -in sequence until either any needed right is denied or all needed rights -are granted. Later ACEs are then not taken into account.</para></listitem> -<listitem><para>All access denied ACEs _should_ precede any -access allowed ACE.</para></listitem> -</itemizedlist> - -<para>Note that the last rule is a preference, not a law. NT will correctly -deal with the ACL regardless of the sequence order. The second rule is -not modified to get the ACEs in the preferred order.</para> - -<para>Unfortunately the security tab of the NT4 explorer is completely -unable to deal with access denied ACEs while the explorer of W2K rearranges -the order of the ACEs before you can read them. Thank God, the sort order -remains unchanged if one presses the Cancel button.</para> - -<para>You still ask "Where is the leak?" NT ACLs are unable to reflect each -possible combination of POSIX permissions. Example:</para> - -<screen> -rw-r-xrw- -</screen> - -<para>1st try:</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>2st 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>3rd 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 official rules 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 for both, NT4 and W2K. Only the GUIs aren't -able to deal with that order.</para> - -</sect2> - -<sect2 id="ntsec-aclfuncs"><title id="ntsec-aclfuncs.title">The ACL API</title> - -<para>For dealing with ACLs Cygwin now has the ACL API as it's -implemented in newer versions of Solaris. The new data structure -for a single ACL entry (ACE in NT terminology) is defined in -<filename>sys/acl.h</filename> as:</para> - -<screen> -typedef struct acl { - int a_type; /* entry type */ - uid_t a_id; /* UID | GID */ - mode_t a_perm; /* permissions */ -} aclent_t; -</screen> - -<para>The a_perm member of the aclent_t type contains only the bits -for read, write and execute as in the file mode. If e.g. read permission -is granted, all read bits (S_IRUSR, S_IRGRP, S_IROTH) are set. -CLASS_OBJ or MASK ACL entries are not fully implemented yet.</para> - -<para>The new API calls are</para> - -<screen> -acl(2), facl(2) -aclcheck(3), -aclsort(3), -acltomode(3), aclfrommode(3), -acltopbits(3), aclfrompbits(3), -acltotext(3), aclfromtext(3) -</screen> - -<para>Like in Solaris, Cygwin has two new commands for working with -ACLs on the command line: <command>getfacl</command> and -<command>setfacl</command>.</para> - -<para>Online man pages for the aforementioned commands and API calls can be -found on <ulink url="http://docs.sun.com">http://docs.sun.com</ulink> </para> - -</sect2> - -<sect2 id="ntsec-setuid"><title id="ntsec-setuid.title">New setuid concept</title> - -<para>UNIX applications which have to switch the user context are using -the <command>setuid</command> and <command>seteuid</command> calls which -are not part of the Windows API. -Nevertheless these calls are supported under Windows NT/W2K since Cygwin -release 1.1.3. Because of the nature of NT security an application which -needs the ability has to be patched, though.</para> - -<para>NT uses so-called `access tokens' to identify a user and it's -permissions. To switch the user context the application has to request -such an `access token'. This is typically done by calling the NT API -function <command>LogonUser</command>. The access token is returned and -either used in <command>ImpersonateLoggedOnUser</command> to change user -context of the current process or in <command>CreateProcessAsUser</command> -to change user context of a spawned child process. An important restriction -is that the application using <command>LogonUser</command> must have special -permissions:</para> - -<screen> -"Act as part of the operating system" -"Replace process level token" -"Increase quotas" -</screen> - -<para>Note that administrators do not have all these user rights set -by default.</para> - -<para>Two new Cygwin calls are introduced to support porting -<command>setuid</command> applications with a minimum of effort. 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. The call to <command>sexec</command> is not needed -anymore. Porting 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> -/* Use the following define to determine the Windows version */ -#define is_winnt (GetVersion() < 0x80000000) -#endif - -[...] - - struct passwd *user_pwd_entry = getpwnam (username); - char *cleartext_password = getpass ("Password:"); - -[...] - -#ifdef __CYGWIN__ - /* Patch the typical password test. */ - if (is_winnt) - { - HANDLE token; - - /* Try to get the access token from NT. */ - token = cygwin_logon_user (user_pwd_entry, cleartext_password); - if (token == INVALID_HANDLE_VALUE) - error_exit; - /* Inform Cygwin about the new impersonation token. - Cygwin is able now, to switch to that user context by - setuid or seteuid calls. */ - cygwin_set_impersonation_token (token); - } - else -#endif /* CYGWIN */ - /* Use standard method for W9X as well. */ - hashed_password = crypt (cleartext_password, salt); - if (!user_pwd_entry || - strcmp (hashed_password, user_pwd_entry->pw_password)) - error_exit; - -[...] - - /* Everything else remains the same! */ - - setegid (user_pwd_entry->pw_gid); - seteuid (user_pwd_entry->pw_uid); - execl ("/bin/sh", ...); -]]> - -</screen> - -<para>The new Cygwin call to retrieve an access token is defined as follows:</para> - -<screen> -#include <windows.h> -#include <sys/cygwin.h> - -HANDLE -cygwin_logon_user (struct passwd *pw, const char *cleartext_password) -</screen> - -<para>You can call that function as often as you want for different user -logons and remember the access tokens for further calls to the second function.</para> - -<screen> -#include <windows.h> -#include <sys/cygwin.h> - -void -cygwin_set_impersonation_token (HANDLE hToken); -</screen> - -<para> is the call to inform Cygwin about the user context to which further -calls to <command>setuid</command>/<command>seteuid</command> should switch to. -While you always need the correct access token to do a -<command>setuid</command>/<command>seteuid</command> to another user's context, -you are always able to use <command>setuid</command>/<command>seteuid</command> -to return to your own user context by giving your own uid as parameter.</para> - -<para>If you have remembered several access tokens from calls to -<command>cygwin_logon_user</command> you can switch to different user -contexts by observing the following order:</para> - -<screen> - - cygwin_set_impersonation_token (user1_token); - seteuid (user1_uid); - -[...] - - seteuid (own_uid); - cygwin_set_impersonation_token (user2_token); - seteuid (user2_uid); - -[...] - - seteuid (own_uid); - cygwin_set_impersonation_token (user1_token); - seteuid (user1_uid); - -etc. - -</screen> - -</sect2> - -<sect2 id="ntsec-switch"><title id="ntsec-switch.title">Switching User -Context</title> - -<para> -Since Cygwin release 1.3.3, applications that are members of the -Administrators group and have the <command>Create a token -object</command>, <command>Replace a process level token</command> and -<command>Increase Quota</command> user rights can switch user -context without giving a password by just calling the usual -<command>setuid</command>, <command>seteuid</command>, -<command>setgid</command> and <command>setegid</command> functions. -</para> -<para> -On NT and Windows 2000 the <systemitem -class="username">SYSTEM</systemitem> user has these privileges and can -run services such as <command>sshd</command>. However, on Windows 2003 -<systemitem class="username">SYSTEM</systemitem> lacks the -<command>Create a token object</command> right, so it is necessary to -create a special user with all the necessary rights, as -well as <command>Logon as a service</command>, to run such services. -For security reasons this user should be denied the rights to logon -interactively or over the network. All this is done by configuration -scripts such as <command>ssh-host-config</command>. -</para> -<para> -An important restriction of this method is that a process started -without a password cannot access network shares which require -authentication. This also applies to subprocesses which switched user -context without a password. Therefore, when using -<command>ssh</command> or <command>rsh</command> without a password, it -is typically not possible to access network drives. -</para> - -</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 user id 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 user id 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 group id is set to a special value of 401. If another -user is not present in <filename>/etc/passwd</filename>, that user's login -group id 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 group id is set to a special value of -1. The name of this group -(id -1) will be shown as '????????'. -In releases of Cygwin before 1.3.20, the group id 401 had a group name -'None'. Since Cygwin release 1.3.20, the group id 401 is shown as -'mkpasswd', indicating the command that should be run to alleviate the -situation. -</para> - -<para> -Also, since Cygwin release 1.3.20, 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>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> - -</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> - -</sect1> |