798 lines
30 KiB
Plaintext
798 lines
30 KiB
Plaintext
<sect1 id="ntsec"><title>NT security</title>
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<para>The setting of POSIX like object permissions is controlled by the
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<link linkend="mount-table">mount option</link> <literal>(no)acl</literal>
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which is set to <literal>acl</literal> by default. The design goal
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is to utilize the Windows access control API to implement real POSIX
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permissions.</para>
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<para>We start with a short overview. Note that this overview must
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be necessarily short. If you want to learn more about the Windows security
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model, see the <ulink url="http://msdn.microsoft.com/en-us/library/aa374860(VS.85).aspx">Access Control</ulink> article in MSDN documentation.</para>
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<sect2 id="ntsec-common"><title>NT security</title>
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<para>In the NT security model, almost any "object" is securable.
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"Objects" are files, processes, threads, semaphores, etc.</para>
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<para>Every object has a data structure called "security descriptor" (SD)
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attached. The SD contains all information necessary to control who can
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how access an object. The SD of an object consists of five parts:</para>
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<itemizedlist spacing="compact">
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<listitem><para>Flags which control several aspects of this SD. This is
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not discussed here.</para></listitem>
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<listitem><para>The SID of the object owner.</para></listitem>
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<listitem><para>The SID of the object owner group.</para></listitem>
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<listitem><para>A list of "Access Control Entries" (ACE), called the
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"Discretionary Access Control List" (DACL).</para></listitem>
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<listitem><para>Another list of ACEs, called the
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"Security Access Control List" (SACL), which doesn't matter for our
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purpose.</para></listitem>
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</itemizedlist>
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<para>Every ACE contains a so-called "Security IDentifier" (SID) and
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other stuff which is explained a bit later. Let's talk about the SID first.
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</para>
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<para>A SID is a unique identifier for users, groups, computers and AD
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domains. SIDs are basically comparable to POSIX UIDs and GIDs, but are
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more complicated because they are unique across multiple machines or
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domains. A SID is a structure of multiple numerical values. There's
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a convenient convention to type SIDs. Here's an example:</para>
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<para>SID of a machine "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 leading "S" has no further meaning except to show that this is
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a SID. The next number is a version number which is always 1 so far.
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The next two numbers are the authority which shows the initiated what
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kind of SID this is. There are a couple of builtin accounts and
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accounts with very special meaning. However, computer and domain SIDs
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always start with "S-1-5-21". The next three numbers, all 32 bit values,
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are the unique 96 bit identifier of the comupter system. This is
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hopefully unique all over the world, but in practice it's sufficient if
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the comuter SIDs are unique within a single Windows network.</para>
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<para>As you can see in the above example, SIDs of users (and groups)
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are identical to the computer SID, except for an additional part, the
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so-called "relative identifier" (RID). So the SID of a user is always
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uniquely attached to the system on which the account has been generated.</para>
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<para>It's a bit different in domains. The domain has its own SID, and
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that SID is identical to the SID of the first domain controller, on
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which the domain is created. Domain user SIDs look exactly like the
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computer user SIDs, the leading part is just the domain SID and the RID
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is created when the user is created.</para>
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<para>Ok, consider you created a new domain "bar" on some new domain
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controller and you would like to create a domain account "johndoe":</para>
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<para>SID of a domain "bar.local":</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.local":</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>Ok, so you now have two accounts called johndoe, one account
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created on the machine "foo", one created in the domain "bar.local".
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Both have different SIDs and not even the RID is the same. How do
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the systems know it's the same account? After all, the name is
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the same, right? The answer is, these accounts are NOT identical.
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For all the machines know there are two different accounts, one is
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"FOO\johndoe", the other one is "BAR\johndoe" or "johndoe@bar.local".
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Different SID, different account. Full stop.
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</para>
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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 when you create the
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<filename>/etc/passwd</filename> and <filename>/etc/group</filename>
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files using the <command>mkpasswd</command> and <command>mkgroup</command>
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tools. Domain account UIDs and GIDs are offset by 10000 by default
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which might be a bit low for very big organizations. Fortunately there's
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an option in both tools to change the offset...</para>
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<para>Do you still remember the SIDs with special meaning? In offical
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notation they are called "well-known SIDs". For example, POSIX has no GID
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for the group of "all users" or "world" or "others". The last three rwx
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bits in a permission value just represent the permissions for "everyone
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who is not the owner or is member of the owning group". Windows has a
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SID for these poor souls, the "Everyone" SID. Other well-known SIDs
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represent more circumstances instead of actual users or groups. Here
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are a few examples for well-known SIDs:</para>
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<screen>
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Everyone S-1-1-0 Simply everyone...
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Batch S-1-5-3 Processes started via the task
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scheduler are member of this group.
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Interactive S-1-5-4 Only processes of users which are
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logged in via an interactive
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session are members here.
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Authenticated Users S-1-5-11 Users which have gone through
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the authentication process and
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survived. Anonymously accessing
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users are not incuded here.
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SYSTEM S-1-5-18 A special account which has all
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kinds of dangerous rights, sort of
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an uber-root account.
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</screen>
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<para>For a full list please refer to
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<ulink url="http://msdn.microsoft.com/en-us/library/aa379649.aspx">Well-known SIDs</ulink>.
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Naturally well-known SIDs are the same on each machine, so they are
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not unique to a machine or domain. They have the same meaning across
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the Windows network.</para>
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<para>Additionally there are a couple of well-known builtin groups,
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which have the same SID on every machine and which have certain user
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rights by default:</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>For instance, every account is usually member in the "Users"
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group. All administrator accounts are member of the "Administrators"
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group. That's all about it as far as single machines are involved. In
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a domain environment it's a bit more tricky. Since these SIDs are not
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unique to a machine, every domain user and every domain group can be a
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member of these well known groups. Consider the domain group "Domain
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Admins". This group is by default in the "Administrators" group. Let's
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assume the above computer called "foo" is a member machine of the domain
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"bar.local". If you stick the user "BAR\johndoe" into the group "Domain
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Admins", this guy will automatically be a mamber of the administrators
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group on "foo", when logging in on "foo". Neat, isn't it?</para>
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<para>Back to ACE and ACL. POSIX is able to create three different
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permissions, the permissions for the owner, for the group and for the
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world. In contrast the Windows ACL has a potentially infinite number of
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members... as long as they fit into 64K. Every member is an ACE.
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ACE consist of three parts:</para>
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<itemizedlist spacing="compact">
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<listitem><para>The type of the ACE (allow ACE or deny ACE).</para></listitem>
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<listitem><para>Permission bits, 32 of them.</para></listitem>
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<listitem><para>The SID for which the permissions are allowed or denied.</para></listitem>
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</itemizedlist>
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<para>The two (for us) important types of ACEs are the "access allowed
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ACE" and the "access denied ACE". As the names imply, the allow ACE
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tells the system to allow the given permissions to the SID, the deny ACE
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results in denying the specific permission bits.</para>
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<para>The possible permissions on objects are more detailed than in
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POSIX. For example, the permission to delete an object is different
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from the permission to change object data, and even changing object data
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can be separated into different permission bits for different kind of
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data.</para>
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</sect2>
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<sect2 id="ntsec-files"><title id="ntsec-files.title">File permissions</title>
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<para>On NTFS and if the <literal>noacl</literal> mount option is not
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specified for a mount point, Cygwin sets file permissions as in POSIX.
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Basically this is done by defining a SD with the matching owner and group
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SIDs, and a DACL which contains ACEs for the owner, the group and for
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"Everyone", which represents what POSIX calls "others".</para>
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<para>To use NT security correctly, Cygwin depends on the files
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<filename>/etc/passwd</filename> and <filename>/etc/group</filename>.
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These files define the traslation between the Cygwin uid/gid and the
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Windows SID. The SID is stored in the pw_gecos field in
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<filename>/etc/passwd</filename>, and in the gr_passwd field in
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<filename>/etc/group</filename>. Since the pw_gecos field can contain
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more information than just a SID, there are some rules for the layout.
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It's required that the SID is the last entry of the pw_gecos field,
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assuming that the entries in pw_gecos are comma-separated. The
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commands <command>mkpasswd</command> and <command>mkgroup</command>
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usually do this for you.</para>
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<para>Another interesting entry in the pw_gecos field (which is also
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usually created by running <command>mkpasswd</command>) is the Windows user
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name entry. It takes the form "U-domain\username" and is typically used
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by services to authenticate a user. Logging in through <command>ssh</command>
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or <command>telnet</command> are two typical scenarios.
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</para>
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<para>A typical snippet from <filename>/etc/passwd</filename>:</para>
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<example id="ntsec-passwd">
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<title>/etc/passwd:</title>
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<screen>
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SYSTEM:*:18:544:,S-1-5-18::
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Administrators:*:544:544:,S-1-5-32-544::
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Administrator:unused:500:513:U-FOO\Administrator,S-1-5-21-790525478-115176313-839522115-500:/home/Administrator:/bin/bash
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corinna:unused:11001:11125:U-BAR\corinna,S-1-5-21-2913048732-1697188782-3448811101-1001:/home/corinna:/bin/tcsh
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</screen>
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</example>
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<para>The SYSTEM entry is usually needed by services. The Administrators
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entry (Huh? A group in /etc/passwd?) is only here to allow
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<command>ls</command> to print some file ownerships correctly. Windows
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doesn't care if the owner of a file is a user or a group. In older
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versions of Windows NT the default ownership for files created by an
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administrator account was set to the group Administrators instead of to
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the creating user account. This has changed, but for those older
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systems it's convenient to have the Administrators group in
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<filename>/etc/passwd</filename>.</para>
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<para>The really interesting entries are the next two. The Administrator
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entry is for the local administrator, the corinna entry matches the corinna
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account in the domain BAR. The information given in the pw_gecos field
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are all we need to exactly identify an account, and to have a two way
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translation, from Windows account name/SID to Cygwin account name uid and
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vice versa. Having this complete information allows us to choose a Cygwin
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name and uid which doesn't have to match the Windows account at all. As
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long as the pw_gecos information is available, we're on the safe side:</para>
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<example id="ntsec-passwd-tweaked">
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<title>/etc/passwd, tweaked:</title>
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<screen>
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root:unused:0:513:U-FOO\Administrator,S-1-5-21-790525478-115176313-839522115-500:/home/Administrator:/bin/bash
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thursday_next:unused:11001:11125:U-BAR\corinna,S-1-5-21-2913048732-1697188782-3448811101-1001:/home/corinna:/bin/tcsh
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</screen>
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</example>
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<para> The above <filename>/etc/passwd</filename> will still work fine.
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You can now login via <command>ssh</command> as the user "root", and
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Cygwin dutyfully translates "root" into the Windows user
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"FOO\Administrators" and files owned by FOO\Administrators are shown to
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have the uid 0 when calling <command>ls -ln</command>. All you do you're
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actually doing as Administrator. Files created as root will be owned by
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FOO\Administrator. And the domain user BAR\corinna can now happily
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pretend to be Thursday Next, but will wake up sooner or later finding
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out she's still actually the domain user BAR\corinna...</para>
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<para>Do I have to mention that you can also rename groups in
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<filename>/etc/group</filename>? As long as the SID is present and correct,
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all is well. This allows for instance to rename the "Administrators" group
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to "root" as well:</para>
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<example id="ntsec-group-tweaked">
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<title>/etc/group, tweaked:</title>
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<screen>
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root:S-1-5-32-544:544:
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</screen>
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</example>
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<para>Last but not least you can also change the primary group of a user
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in <filename>/etc/passwd</filename>. The only requirement is that the user
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is actually a member of the new primary group in Windows. For instance,
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normal users in a domain environment are members in the group "Domain Users",
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which in turn is member of the well-known group "Users". Additionally let's
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assume the user is also a member of the newly created group . The default
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primary group for users is
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<!-- TODO: The rest of the file... -->
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</para>
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<para>As you can see, I changed my primary group membership from 513 (None)
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to 547 (powerusers). So all files I created inside of Cygwin were now owned
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by the powerusers group instead of None. This is the way I liked it.</para>
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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 more 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-sids"><title id="ntsec-sids.title">NT SIDs in Cygwin</title>
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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 than 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 (e.g. <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 the special version of <command>login</command> which is
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part of the standard Cygwin distribution since 1.1.</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 id="ntsec-passwd-root">
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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 id="ntsec-group-root">
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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>. I suggest
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not to do this since ntsec works better when having the SIDs available.</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 separated list. The SID has to be the last field!</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 either 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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<para>As in the previous chapter I give my personal
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<filename>/etc/passwd</filename> and <filename>/etc/group</filename> as
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examples. Please note that I've changed these files heavily! There's no
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need to change them that way, it's just for testing purposes and...
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for fun.</para>
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<example id="ntsec-passwd-ex-2">
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<title>/etc/passwd</title>
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<screen>
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root:*:0:0:Administrators group,S-1-5-32-544::
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SYSTEM:*:18:18:,S-1-5-18:/home/system:/bin/bash
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admin:*:500:513:,S-1-5-21-1844237615-436374069-1060284298-500:/home/Administrator:/bin/bash
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corinna:*:100:0:Corinna Vinschen,S-1-5-21-1844237615-436374069-1060284298-1003:/home/corinna:/bin/tcsh
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Guest:*:501:546:,S-1-5-21-1844237615-436374069-1060284298-501:/home/Guest:/bin/bash
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</screen>
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</example>
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<example id="ntsec-group-ex-2">
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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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local:S-1-2-0:2:
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network:S-1-5-2:3:
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interactive:S-1-5-4:4:
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authenticatedusers:S-1-5-11:5:
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SYSTEM:S-1-5-18:18:
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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>POSIX 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>
|