On Thu, 2010-03-25 at 12:49 -0400, Paul Moore wrote:
> On Thursday 25 March 2010 10:02:48 am Stephen Smalley wrote:
> > On Wed, 2010-03-24 at 22:42 -0400, Eric Paris wrote:
> > > On Tue, 2010-03-23 at 11:44 +0000, Daniel P. Berrange wrote:
> > > > On Tue, Mar 23, 2010 at 07:35:13AM -0400, Daniel J Walsh wrote:
> > > > > On 03/22/2010 07:47 PM, Eric Paris wrote:
> > > > > >On Mon, 2010-03-22 at 17:44 -0400, Daniel J Walsh wrote:
> > > > > >>type=AVC msg=audit(1269293509.223:4753): avc: denied { write }
> > > > > >>for pid=28549 comm="qemu-kvm" path="socket:[4417531]" dev=sockfs
> > > > > >>ino=4417531 scontext=system_u:system_r:svirt_t:s0:c1
> > > > > >>tcontext=system_u:system_r:svirt_t:s0-s15:c0.c1023
> > > > > >>tclass=unix_stream_socket
> > > > > >>
> > > > > >>I have Static Virtualization working on an MLS box except for this
> > > > > >>strange AVC.
>
> ...
>
> > > > > >># ps -eZ | grep virt
> > > > > >>system_u:system_r:virtd_t:s0-s15:c0.c1023 27344 ? 05:34:47 libvirtd
> > > > > >>system_u:system_r:svirt_t:s0:c1 28549 ? 00:00:01 qemu-kvm
>
> ...
>
> > > > > >># ls -lZ /proc/28549/fd/ | grep 4417531
> > > > > >>lrwx------. qemu qemu system_u:system_r:svirt_t:s0:c1 17 ->
> > > > > >>socket:[4417531]
> > > > > >>
> > > > > >> lsof | grep 4417531
> > > > > >>
> > > > > >>qemu-kvm 28549 qemu 17u unix 0xffff88003e1f7900
> > > > > >>0t0 4417531 /var/lib/libvirt/qemu/xguest.monitor
> > > > > >>
> > > > > >># lsof /var/lib/libvirt/qemu/xguest.monitor
> > > > > >>COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE
> > > > > >>NAME qemu-kvm 28549 qemu 3u unix 0xffff88003a853000 0t0
> > > > > >>4417518 /var/lib/libvirt/qemu/xguest.monitor
> > > > > >>qemu-kvm 28549 qemu 17u unix 0xffff88003e1f7900 0t0 4417531
> > > > > >>/var/lib/libvirt/qemu/xguest.monitor
> > > > > >>
> > > > > >>So it looks like we have a process that is running as both labels?
> > > > > >
> > > > > >This is a check between the type of the process and that of the
> > > > > >socket itself, not between 2 processes. So, the type of the socket
> > > > > >is wrong. Just as a question, what does ls -lZ
> > > > > >/var/lib/libvirt/qemu/ show? c0-c1023 for xguest.monitor? What
> > > > > >created that socket? Did they get it correct? (I admit it looks
> > > > > >correct on my F13ish system)
> > > > >
> > > > > The socket file is labeled svirt_var_run_t and has the correct level.
> > > > >
> > > > > I believe the socket file was created by qemu. Dan can you confirm
> > > > > this.
> > > >
> > > > Yes, these sockets are created by QEMU when it starts. libvirt just
> > > > gives it the path at which to create the socket.
> > > >
> > > > > # ls -lZa /var/lib/libvirt/qemu/
> > > > >
> > > > > drwx------. qemu qemu
> > > > > system_u:object_r:svirt_var_run_t:s0-s15:c0.c1023 . drwxr-xr-x. root
> > > > > root system_u:object_r:virt_var_lib_t:s0 .. srwxr-xr-x. qemu qemu
> > > > > system_u:object_r:svirt_var_run_t:s0:c1 xguest.monitor
> > >
> > > And then libvirt attaches to the other end?
> > >
> > > In any case, doing some digging the problem (where we first end up with
> > > this crazy context with the type of svirt_t but the MLS label of
> > > libvirt) is in selinux_socket_unix_stream_connect(). We never saw this
> > > in MCS because we don't have constraints on unix domain sockets in
> > > targetted/MCS policy. At this hour of the night my brain isn't running
> > > well enough nor is my networking foo strong enough to understand exactly
> > > which object is supposed to be labeled what where, but it has to be
> > > something with the call to security_sid_mls_copy().
> > >
> > > I'll certainly be looking at this again in the morning.
> >
> > That's intentional behavior for MLS.
>
> Stephen is correct, the general idea is that when a connected child socket is
> created on a socket accepting incoming connections it is labeled using the
> type of the listening socket and the MLS attributes of the remote peer. As an
> example, imagine client client_t:s0:c1 connecting to server server_t:s0-
> s15:c0.c1023, the client's connected socket would be labeled client_t:s0:c1
> (it inherits the label from the client process) while the server's connected
> child socket would be labeled server_t:s0:c1 (labeled as described above).
>
> Now, while the code (looking at Linus' current tree, but this hasn't changed
> in a while) it does handle labeling UNIX sockets correctly but there are a few
> things which strike me as odd, if not wrong:
>
> 1. The "peer_sid" field of the client's socket is set to the label of the
> server's listening socket, NOT the derived label used for the server's child
> socket. This means that the MLS attributes of the "peer_sid" stored in the
> client's socket do not match the MLS attributes of the server's child socket.
> This isn't consistent with how we handle INET sockets, but then again with
> UNIX sockets we know the labels of both the remote socket and the remote peer;
> with INET sockets we only get one label. In some ways this gets back to the
> socket as an endpoint argument and I'm not sure we want to dig that up.
That should likely be changed.
> 2. We don't currently update the server's child socket inode label to reflect
> the derived label used in the socket. A potential difference between INET and
> UNIX socket handling if security_sock_graft() is not called at some point in
> the connect process (need to track this down, but it didn't jump out at me in
> unix_stream_connect()).
unix_accept() calls sock_graft, so I think that is already covered.
> 3. Somewhat unrelated I think, but selinux_socket_unix_may_send() doesn't use
> the socket/sock labels, it relies on the inode labels. As has been mentioned
> several times in the past, we need to unify the inode/sock labels better.
>
> There may be more issues, but these are the ones that caught my eye when
> scanning the UNIX socket code quickly. Item #1 is probably only an annoyance
> that you would see in getpeercon() but we should still probably fix. Item's
> #2 and #3 are potentially a bit more serious as the file descriptor access
> controls are going to use the inode's label so a mis-match between the socket
> and inode labels could cause some rather strange behavior. I can go through
> and cleanup this code (it is long overdue), but I want to get some consensus
> first on how we want UNIX sockets to behave.
>
--
Stephen Smalley
National Security Agency
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