newlib-cygwin/winsup/doc/ntsec.sgml

<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>