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diff --git a/winsup/doc/ntsec.sgml b/winsup/doc/ntsec.sgml new file mode 100644 index 000000000..5f95dd288 --- /dev/null +++ b/winsup/doc/ntsec.sgml @@ -0,0 +1,316 @@ + +<sect1 id="ntsec"><title>NTSEC Documentation</title> + +<para>The design goal of the ntsec patch was to get a more UNIX like +permission structure based upon the security features of Windows NT. +To describe the changes, I will give a short overview of NT security +in chapter one.</para> +<para>Chapter two discusses the changes in ntsec related to privileges on +processes.</para> +<para>Chapter three shows the UNIX like setting of file permissions.</para> + +<para>The setting of UNIX like object permissions is controlled by the new +<EnVar>CYGWIN</EnVar> variable setting <literal>(no)ntsec</literal>.</para> + +<para>On NT ntsec is now turned on by default.</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>An 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> + +<example> +<screen> +SID of a system `foo': + + S-1-5-21-165875785-1005667432-441284377 + +SID of a user `johndoe' of the system `foo': + + S-1-5-21-165875785-1005667432-441284377-1023 +</screen> +</example> + +<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, this accounts are under +any circumstances DIFFERENT, regardless of the usage of the same user +name and password!</para> + +<example> +<screen> +SID of a domain `bar': + + S-1-5-21-186985262-1144665072-740312968 + +SID of a user `johndoe' in the domain `bar': + + S-1-5-21-186985262-1144665072-740312968-1207 +</screen> +</example> + +<para>The last part of the SID, the so called `relative identifier' (RID), +is 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 an 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> +<example> +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> +</example> + +<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> +<example> +administrators S-1-5-32-544 +users S-1-5-32-545 +guests S-1-5-32-546 +... +</screen> +</example> + +<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> + +<para>The two important types of ACEs are the `access allowed ACE' and the +`access denied ACE'. The ntsec patch only uses `access allowed ACEs'.</para> + +<para>The possible permissions on objects are more complicated 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 able to reproduce the POSIX model. +The ntsec patch tries to do this in cygwin.</para> + +<para>The creation of explicit object security is a bit complicated, so +typically only two simple variations are used:</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 created or opened object is inherited to child processes or not. +This property is not important for the ntsec patch description, so in +this document SDs and SAs are more or less identical.</para> + +</sect2> + +<sect2 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 patch 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>The ntsec patch 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>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'. If the group of the file is not the +administrators' group, the administrators' group gets the permissions +to read the permissions (yes, this is an own permission flag +<literal>:-)</literal>) and to take the ownership on this file. +If the file's group is the administrators group itself, this behaviour +is modified to support the typical behaviour of NT better: +As you know, if one is member of admin group, all her files are owned +by the group instead of by her. This is not the case with ntsec but the +other admins should have easier access to the administrative files. +So in this case the admin group gets additionally the permissions to +write permissions and to write extended attributes, also in the case +where group permissions are set to 0.</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 `ReadSD' and `WriteSD'. They are using the Backup API functions +`BackupRead' and `BackupWrite', that have the advantage not to crash, +if they are used on non NTFS file systems! These crashes are the default +behavior of the security API, if it's used on, e.g., FAT or SAMBA +file systems <literal>:-(</literal></para> + +<para>Unfortunately, the settings of NT file security are only available +on NTFS. SAMBA doesn't support them.</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 one is a member of +the administrators' group, every file, that he has created is owned by the +administrators' group, instead by him.</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 it seems that this has no negativ influences.</para> + +<para>To work correctly the ntsec patch depends on reasoned files +<filename>/etc/passwd/</filename> and <filename>/etc/group</filename>. +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 aforementioned. +An SID of e.g. the user `corinna' on my NT workstation:</para> + +<example> +<screen> + S-1-5-21-165875785-1005667432-441284377-1000 +</screen> +</example> + +<para>Note the last number: It's the RID 1000, the 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 regular groups of these users.</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>To get help in creating correct passwd and group files, look at +the following examples, that are part of my files. With the exception +of my personal user entry, all entries are well known entries. For a +better understanding, the names are translated to the equivalents of the +English NT version:</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>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 better 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> + +</sect1> |