520 lines
19 KiB
Plaintext
520 lines
19 KiB
Plaintext
<sect1 id="ntsec"><title>NT security and the <literal>ntsec</literal> usage</title>
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<para>The design goal of the ntsec patch was to get a more UNIX like
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permission structure based upon the security features of Windows NT.
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To describe the changes, I will give a short overview of NT security
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in chapter one.</para>
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<para>Chapter two discusses the changes in ntsec related to privileges on
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processes.</para>
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<para>Chapter three shows the basics of UNIX like setting of
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file permissions.</para>
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<para>Chapter four talks about the advanced settings introduced in
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release 1.1</para>
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<para>Chapter five illustrates the permission mapping leak of Windows NT.</para>
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<para>Chapter six describes in short the new acl API since release 1.1</para>
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<para>The setting of UNIX like object permissions is controlled by the new
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<EnVar>CYGWIN</EnVar> variable setting <literal>(no)ntsec</literal>.
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On NT ntsec is now turned on by default.</para>
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<sect2 id="ntsec-common"><title>NT security</title>
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<para>The NT security allows a process to allow or deny access of
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different kind to `objects'. `Objects' are files, processes,
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threads, semaphores, etc.</para>
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<para>The main data structure of NT security is the `security descriptor'
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(SD) structure. It explains the permissions, that are granted (or denied)
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to an object and contains information, that is related to so called
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`security identifiers' (SID).</para>
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<para>A SID is a unique identifier for users, groups and domains.
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SIDs are comparable to UNIX UIDs and GIDs, but are more complicated
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because they are unique across networks. Example:</para>
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<para>SID of a system `foo':</para>
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<screen>
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S-1-5-21-165875785-1005667432-441284377
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</screen>
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<para>SID of a user `johndoe' of the system `foo':</para>
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<screen>
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S-1-5-21-165875785-1005667432-441284377-1023
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</screen>
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<para>The above example shows the convention for printing SIDs. The leading
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`S' should show that it is a SID. The next number is a version number which
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is always 1. The next number is the so called `top-level authority' that
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identifies the source that issued the SID.</para>
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<para>While each system in a NT network has it's own SID, the situation
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is modified in NT domains: The SID of the domain controller is the
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base SID for each domain user. If an NT user has one account as domain
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user and another account on his local machine, this accounts are under
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any circumstances DIFFERENT, regardless of the usage of the same user
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name and password!</para>
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<para>SID of a domain `bar':</para>
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<screen>
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S-1-5-21-186985262-1144665072-740312968
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</screen>
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<para>SID of a user `johndoe' in the domain `bar':</para>
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<screen>
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S-1-5-21-186985262-1144665072-740312968-1207
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</screen>
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<para>The last part of the SID, the so called `relative identifier' (RID),
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is by default used as UID and/or GID under cygwin. As the name and the
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above example implies, this id is unique only relative to one system or
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domain.</para>
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<para>Note, that it's possible, that an user has the same RID on two
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different systems. The resulting SIDs are nevertheless different, so
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the SIDs are representing different users in an NT network.</para>
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<para>There is a big difference between UNIX IDs and NT SIDs, the existence of
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the so called `well known groups'. For example UNIX has no GID for the
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group of `all users'. NT has an SID for them, called `Everyone' in the
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English versions. The SIDs of well-known groups are not unique across
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an NT network but their meanings are unmistakable.
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Examples of well-known groups:</para>
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<screen>
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everyone S-1-1-0
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creator/owner S-1-3-0
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batch process (via `at') S-1-5-3
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authenticated users S-1-5-11
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system S-1-5-18
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</screen>
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<para>The last important group of SIDs are the `predefined groups'. This
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groups are used mainly on systems outside of domains to simplify the
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administration of user permissions. The corresponding SIDs are not unique
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across the network so they are interpreted only locally:</para>
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<screen>
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administrators S-1-5-32-544
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users S-1-5-32-545
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guests S-1-5-32-546
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...
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</screen>
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<para>Now, how are permissions given to objects? A process may assign an SD
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to the object. The SD of an object consists of three parts:</para>
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<itemizedlist spacing="compact">
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<listitem><para>the SID of the owner </para></listitem>
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<listitem><para>the SID of the group </para></listitem>
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<listitem><para>a list of SIDs with their permissions, called
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`access control list' (ACL) </para></listitem>
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</itemizedlist>
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<para>UNIX is able to create three different permissions, the permissions
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for the owner, for the group and for the world. In contrast the ACL
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has a potentially infinite number of members. Every member is a so called
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`access control element' (ACE). An ACE contains three parts:</para>
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<itemizedlist spacing="compact">
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<listitem><para>the type of the ACE </para></listitem>
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<listitem><para>permissions, described with a DWORD </para></listitem>
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<listitem><para>the SID, for which the above mentioned permissions are
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set </para></listitem>
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</itemizedlist>
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<para>The two important types of ACEs are the `access allowed ACE' and the
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`access denied ACE'. The ntsec patch only uses `access allowed ACEs'.</para>
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<para>The possible permissions on objects are more complicated than in
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UNIX. For example, the permission to delete an object is different
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from the write permission.</para>
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<para>With the aforementioned method NT is able to grant or revoke permissions
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to objects in a far more specific way. But what about cygwin? In a POSIX
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environment it would be fine to have the security behavior of a POSIX
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system. The NT security model is MOSTLY able to reproduce the POSIX model.
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The ntsec patch tries to do this in cygwin.</para>
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<para>You ask "Mostly? Why mostly???" Because there's a leak in the NT model.
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I will describe that in detail in chapter 4.</para>
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<para>The creation of explicit object security is a bit complicated, so
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typically only two simple variations are used:</para>
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<itemizedlist spacing="compact">
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<listitem><para>default permissions, computed by the operating system </para></listitem>
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<listitem><para>each permission to everyone </para></listitem>
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</itemizedlist>
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<para>For parameters to functions that create or open securable objects another
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data structure is used, the `security attributes' (SA). This structure
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contains an SD and a flag, that specifies whether the returned handle
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to the created or opened object is inherited to child processes or not.
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This property is not important for the ntsec patch description, so in
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this document SDs and SAs are more or less identical.</para>
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</sect2>
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<sect2 id="ntsec-processes"><title>Process privileges</title>
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<para>Any process started under control of cygwin has a semaphore attached
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to it, that is used for signaling purposes. The creation of this semaphore
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can be found in sigproc.cc, function `getsem'. The first parameter to the
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function call `CreateSemaphore' is an SA. Without ntsec patch this SA
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assigns default security to the semaphore. There is a simple disadvantage:
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Only the owner of the process may send signals to it. Or, in other words,
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if the owner of the process is not a member of the administrators' group,
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no administrator may kill the process! This is especially annoying, if
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processes are started via service manager.</para>
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<para>The ntsec patch now assigns an SA to the process control semaphore, that
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has each permission set for the user of the process, for the
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administrators' group and for `system', which is a synonym for the
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operating system itself. The creation of this SA is done by the function
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`sec_user', that can be found in `shared.cc'. Each member of the
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administrators' group is now allowed to send signals to any process
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created in cygwin, regardless of the process owner.</para>
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<para>Moreover, each process now has the appropriate security settings, when
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it is started via `CreateProcess'. You will find this in function
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`spawn_guts' in module `spawn.cc'. The security settings for starting a
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process in another user context have to add the sid of the new user, too.
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In the case of the `CreateProcessAsUser' call, sec_user creates an SA with
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an additional entry for the sid of the new user.</para>
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</sect2>
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<sect2 id="ntsec-files"><title>File permissions</title>
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<para>If ntsec is turned on, file permissions are set as in UNIX. An SD is
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assigned to the file containing the owner and group and ACEs for the
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owner, the group and `Everyone'.</para>
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<para>The complete settings of UNIX like permissions can be found in the file
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`security.cc'. The two functions `get_nt_attribute' and `set_nt_attribute'
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are the main code. The reading and writing of the SDs is done by the
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functions `read_sd' and `write_sd'. `write_sd' uses the function `BackupRead'
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instead of the simpler function `SetFileSecurity' because the latter is
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unable to set owners different from the caller.</para>
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<para>If you are creating a file `foo' outside of cygwin, you will see something
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like the following on <command>ls -ln</command>:</para>
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<para>If your login is member of the administrators' group:</para>
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<screen>
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rwxrwxrwx 1 544 513 ... foo
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</screen>
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<para>if not:</para>
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<screen>
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rwxrwxrwx 1 1000 513 ... foo
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</screen>
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<para>Note the user and group IDs. 544 is the UID of the administrators' group.
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This is a `feature' <literal>:-P</literal> of WinNT. If one is a member of
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the administrators' group, every file, that he has created is owned by the
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administrators' group, instead by him.</para>
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<para>The second example shows the UID of the first user, that has been
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created with NT's the user administration tool. The users and groups are
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sequentially numbered, starting with 1000. Users and groups are using the
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same numbering scheme, so a user and a group don't share the same ID.</para>
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<para>In both examples the GID 513 is of special interest. This GID is a
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well known group with different naming in local systems and domains.
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Outside of domains the group is named 'None' (`Kein' in German, `Aucun'
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in French, etc.), in domains it is named 'Domain Users'. Unfortunately,
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the group `None' is never shown in the user admin tool outside of domains!
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This is very confusing but it seems that this has no negativ influences.</para>
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<para>To work correctly the ntsec patch depends on reasoned files
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<filename>/etc/passwd/</filename> and <filename>/etc/group</filename>.
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In cygwin release 1.0 the names and the IDs must correspond to the
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appropriate NT IDs! The IDs used in cygwin are the RID of the NT SID, as
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mentioned earlier.
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An SID of e.g. the user `corinna' on my NT workstation:</para>
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<screen>
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S-1-5-21-165875785-1005667432-441284377-1000
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</screen>
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<para>Note the last number: It's the RID 1000, the cygwin's UID.</para>
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<para>Unfortunately, workstations and servers outside of domains are not
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able to set primary groups! In these cases, where there is no correlation
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of users to primary groups, NT returns 513 (None) as primary group,
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regardless of the membership to existing local groups.</para>
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<para>when using <command>mkpasswd -l -g</command> on such systems, you
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have to change the primary group by hand if `None' as primary group is
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not what you want (and I'm sure, it's not what you want!)</para>
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<para>To get help in creating correct passwd and group files, look at
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the following examples, that are part of my files. With the exception
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of my personal user entry, all entries are well known entries. For a
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better understanding, the names are translated to the equivalents of the
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English NT version.</para>
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<example>
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<title>/etc/passwd</title>
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<screen>
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everyone:*:0:0:::
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system:*:18:18:::
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administrator::500:544::/home/root:/bin/bash
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guest:*:501:546:::
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administrators:*:544:544::/home/root:
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corinna::1000:547:Corinna Vinschen:/home/corinna:/bin/tcsh
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</screen>
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</example>
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<example>
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<title>/etc/group</title>
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<screen>
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everyone::0:
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system::18:
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none::513:
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administrators::544:
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users::545:
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guests::546:
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powerusers::547:
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</screen>
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</example>
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<para>Groups may be mentioned in the passwd file, too. This has two
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advantages:</para>
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<itemizedlist spacing="compact">
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<listitem><para>Because NT assigns them to files as owners, a
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<command>ls -l</command> is often better readable.</para></listitem>
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<listitem><para>Moreover it's possible to assigned them to files as
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owners with cygwin's <command>chown</command>.</para></listitem>
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</itemizedlist>
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<para>The group `system' is the aforementioned synonym for the operating system
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itself and is normally the owner of processes, that are started through
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service manager. The same is true for files, that are created by
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processes, which are started through service manager.</para>
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</sect2>
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<sect2 id="ntsec-release1.1"><title>New since Cygwin release 1.1</title>
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<para>In Cygwin release 1.1 a new technique of using the
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<filename>/etc/passwd</filename> and <filename>/etc/group</filename>
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is introduced.</para>
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<para>Both files may now contain SIDs of users and groups. They
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are saved in the last field of pw_gecos in <filename>/etc/passwd</filename>
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and in the gr_passwd field in <filename>/etc/group</filename>.</para>
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<para>This has the following advantages:</para>
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<itemizedlist spacing="compact">
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<listitem><para>ntsec works better in domain environments.</para></listitem>
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<listitem><para>Accounts (users and groups) may get another name in
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cygwin that their NT account name. The name in <filename>/etc/passwd</filename>
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or <filename>/etc/group</filename> is transparently used by cygwin
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applications (eg. <command>chown</command>, <command>chmod</command>,
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<command>ls</command>):</para>
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<screen>
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root::500:513::/home/root:/bin/sh
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</screen>
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<para>instead of</para>
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<screen>
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adminstrator::500:513::/home/root:/bin/sh
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</screen>
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<para>Caution: If you like to use the account as login account via
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<command>telnet</command> etc. you have to remain the name unchanged or
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you have to use a special version of <command>login</command> which will
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be part of the release 1.1 soon.</para></listitem>
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<listitem><para>Cygwin UIDs and GIDs are now not necessarily the RID
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part of the NT SID:</para>
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<screen>
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root::0:513:S-1-5-21-54355234-56236534-345635656-500:/home/root:/bin/sh
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</screen>
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<para>instead of</para>
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<screen>
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root::500:513::/home/root:/bin/sh
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</screen>
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</listitem>
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<listitem><para>As in U*X systems UIDs and GIDs numbering scheme now
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don't influence each other. So it's possible to have same Id's for a
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user and a group:</para>
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<example>
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<title>/etc/passwd:</title>
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<screen>
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root::0:0:S-1-5-21-54355234-56236534-345635656-500:/home/root:/bin/sh
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</screen>
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</example>
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<example>
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<title>/etc/group:</title>
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<screen>
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root:S-1-5-32-544:0:
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</screen>
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</example>
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</listitem>
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</itemizedlist>
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<para>The tools <command>mkpasswd</command> and <command>mkgroup</command>
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create the needed entries by default. If you don't want that you can use
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the options <literal>-s</literal> or <literal>--no-sids</literal>. In this
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case ntsec behaves like the previous version.</para>
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<para>Please note that the pw_gecos field in <filename>/etc/passwd</filename>
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is defined as a comma seperated list. The SID has to be the last field!</para>
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<para>As aforementioned you are able to use cygwin account names different
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from the NT account names. If you want to login thru `telnet' or something
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else you have to use the special <command>login</command>. You may then
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add another field to pw_gecos which contains the NT user name including
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it's domain. So you are able to login as each domain user. The syntax
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is easy: Just add an entry of the form U-ntdomain\ntusername to the pw_gecos
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field. Note that the SID must still remain the last field in pw_gecos!</para>
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<screen>
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the_king::1:1:Elvis Presley,U-STILLHERE\elvis,S-1-5-21-1234-5678-9012-1000:/bin/sh
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</screen>
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<para>For a local user just drop the domain:</para>
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<screen>
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the_king::1:1:Elvis Presley,U-elvis,S-1-5-21-1234-5678-9012-1000:/bin/sh
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</screen>
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<para>In each case the password of the user is taken from the NT user
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database, NOT from the passwd file!</para>
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</sect2>
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<sect2 id="ntsec-mapping"><title>The mapping leak</title>
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<para>Now its time to point out the leak in the NT permissions.
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The official documentation explains in short the following:</para>
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<itemizedlist spacing="compact">
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<listitem><para>access allow ACEs are accumulated regarding to the
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group membership of the caller.</para></listitem>
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<listitem><para>The order of ACEs is important. The system reads them
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in sequence until either any needed right is denied or all needed rights
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are granted. Later ACEs are then not taken into account.</para></listitem>
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<listitem><para>ALl access denied ACEs _should_ precede any
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access allowed ACE.</para></listitem>
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</itemizedlist>
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<para>Note that the last rule is a preference, not a law. NT will correctly
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deal with the ACL regardless of the sequence order. The second rule is
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not modified to get the ACEs in the prefered order.</para>
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<para>Unfortunately the security tab of the NT4 explorer is completely
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unable to deal with access denied ACEs while the explorer of W2K rearranges
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the order of the ACEs before you can read them. Thank God, the sort order
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remains unchanged if one presses the Cancel button.</para>
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<para>You still ask "Where is the leak?" NT ACLs are unable to reflect each
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possible combination of POSIX permissions. Example:</para>
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<screen>
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rw-r-xrw-
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</screen>
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<para>1st try:</para>
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<screen>
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UserAllow: 110
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GroupAllow: 101
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OthersAllow: 110
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</screen>
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<para>Hmm, because of the accumulation of allow rights the user may
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execute because the group may execute.</para>
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<para>2st try:</para>
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<screen>
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UserDeny: 001
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GroupAllow: 101
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OthersAllow: 110
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</screen>
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<para>Now the user may read and write but not execute. Better? No!
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Unfortunately the group may write now because others may write.</para>
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<para>3rd try:</para>
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<screen>
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UserDeny: 001
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GroupDeny: 010
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GroupAllow: 001
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OthersAllow: 110
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|
</screen>
|
|
|
|
<para>Now the group may not write as intended but unfortunately the user may
|
|
not write anymore, too. 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>New 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 eg. 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 eg. http://docs.sun.com</para>
|
|
|
|
</sect2>
|
|
|
|
</sect1>
|