[refpolicy] refpolicy-2.20101213 (mcs) and dbus messages

[guido@trentalancia.com (Guido Trentalancia)]

Hello again Dominick !

On Thu, 13/01/2011 at 13.34 +0100, Dominick Grift wrote:
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> On 01/13/2011 01:25 PM, Guido Trentalancia wrote:
> > Hello Dominick and thanks very much for your message !
> > 
> > The problem appears to be mostly sorted out now. The point is that I
am
> > not able to reproduce it again.
> > 
> > As far as I remember, there were only a wrong PAM file for gdm and
an
> > old dbus-daemon-launch-helper in /usr/libexec. But then reverting
back
> > those changes doesn't reproduce the problem.
> > 
> > In any case at the moment nothing except init runs in any init*
context.
> > 
> > But there is still something interesting to speculate. The main
effect
> > of the problem as I had originally reported was that it was not
possible
> > to login into gdm which I use as the graphical login manager.
> > 
> > What is worrying is that when the init binary was not labelled
correctly
> > and thus init/upstart was running in a wrong context (something like
> > system_u:system_r:insmod_t as far as I can remember) it was possible
to
> > login into gdm. If you remember I mentioned in my original message
about
> > the fact that refpolicy unfortunately does not yet
label /sbin/upstart
> > correctly because it's missing from the default file_contexts...
> 
> I sincerely doubt that, but then again i may be wrong. Can you
reproduce
> this?

Yes confirmed and it can be easily reproduced.

When /sbin/upstart is mislabelled as system_u:object_r:bin_t:s0 as it
would happen with an unmodified refpolicy-2.20101213, sestatus reports:

Process contexts:
Current context:                system_u:system_r:kernel_t:s0
Init context:                   system_u:system_r:kernel_t:s0
/sbin/mingetty                  system_u:system_r:kernel_t:s0

But the system appears to run even "better" than when init correctly
transitions into its proper context !

I believe in other cases, for some reason that is not evident now, the
context was system_u:system_r:insmod_t with a completely similar effect.

So, for example, with the wrong init context, not only it is possible to
login into gdm, but also things that were broken with the right init
context works perfectly fine. A few examples: gedit, evince,
gnome-terminal and openoffice refusing to start up without leaving any
trace of denied AVCs in the audit logs and finally no more denied
send_msg with dbus !

The output of ps auxZ in this case ? Everything runs in
system_u:system_r:kernel_t:s0 context apart from udev.

What do you say ?

Is this not a sort of security-flaw ? Someone manages to
relabel /sbin/init and then nobody checks that in enforcing mode init
has effectively re-executed itself in the proper context ?

> > This is quite worrying.
> > 
> > So, despite init was running in a wrong context (and despite the
wrong
> > gdm PAM file), it was still possible to login into gdm while it
wasn't
> > possible when init was running in the proper context...
> > 
> > However, in the current situation I am still getting some USER_AVC
> > "denied" send_msg about dbus messages (in the audit log only). At a
very
> > quick first check, at least some of those messages should have been
> > allowed according to /etc/dbus-1/system.d/* policies. I now need to
look
> > at that in more detail. In general what should be done if those
messages
> > are allowed by dbus policies but then are (mysteriously) denied by
> > SELinux ?
> 
> There should not be anything mysterious about SELinux. I will not
> speculate as to your specific issue. I would need to see the AVC
denials
> to be able to make a decent suggestion.

These are some examples of the USER_AVC denials (when init is running in
the proper context and the system has a few problems):

type=USER_AVC msg=audit(1294930848.646:34): user pid=2243 uid=81
auid=4294967295 ses=4294967295
subj=system_u:system_r:system_dbusd_t:s0-s0:c0.c1023 msg='avc:
denied  { send_msg } for msgtype=method_call
interface=org.freedesktop.Hal.Manager member=FindDeviceByCapability
dest=org.freedesktop.Hal spid=2928 tpid=2314
scontext=system_u:system_r:xdm_t:s0-s0:c0.c1023
tcontext=system_u:system_r:hald_t:s0 tclass=dbus :
exe="/bin/dbus-daemon" sauid=81 hostname=? addr=? terminal=?'

type=USER_AVC msg=audit(1294930839.360:29): user pid=2243 uid=81
auid=4294967295 ses=4294967295
subj=system_u:system_r:system_dbusd_t:s0-s0:c0.c1023 msg='avc:
denied  { send_msg } for msgtype=method_call
interface=org.freedesktop.Hal.Manager member=FindDeviceByCapability
dest=org.freedesktop.Hal spid=2868 tpid=2314
scontext=system_u:system_r:xdm_t:s0-s0:c0.c1023
tcontext=system_u:system_r:hald_t:s0 tclass=dbus :
exe="/bin/dbus-daemon" sauid=81 hostname=? addr=? terminal=?'

type=USER_AVC msg=audit(1294930838.020:26): user pid=2243 uid=81
auid=4294967295 ses=4294967295
subj=system_u:system_r:system_dbusd_t:s0-s0:c0.c1023 msg='avc:
denied  { send_msg } for msgtype=method_call
interface=org.freedesktop.Hal.Manager member=FindDeviceByCapability
dest=org.freedesktop.Hal spid=2868 tpid=2314
scontext=system_u:system_r:xdm_t:s0-s0:c0.c1023
tcontext=system_u:system_r:hald_t:s0 tclass=dbus :
exe="/bin/dbus-daemon" sauid=81 hostname=? addr=? terminal=?'

type=USER_AVC msg=audit(1294926149.131:118): user pid=2242 uid=81
auid=4294967295 ses=4294967295
subj=system_u:system_r:system_dbusd_t:s0-s0:c0.c1023 msg='avc:
 denied  { send_msg } for msgtype=signal
interface=org.freedesktop.PolicyKit1.Authority member=Changed
dest=org.freedesktop.DBus spid=2613 tpid=2546
scontext=system_u:system_r:system_dbusd_t:s0-s0:c0.c1023
tcontext=system_u:system_r:consolekit_t:s0-s0:c0.c1023 tclass=dbus :
exe="/bin/dbus-daemon" sauid=81 hostname=? addr=? terminal=?'

type=USER_AVC msg=audit(1294948907.764:95): user pid=2242 uid=81
auid=4294967295 ses=4294967295
subj=system_u:system_r:system_dbusd_t:s0-s0:c0.c1023 msg='avc:
denied  { send_msg } for msgtype=method_call
interface=org.freedesktop.DBus.Properties member=GetAll
dest=org.freedesktop.UDisks spid=3567 tpid=3569
scontext=system_u:system_r:xdm_t:s0-s0:c0.c1023
tcontext=system_u:system_r:devicekit_disk_t:s0-s0:c0.c1023 tclass=dbus :
exe="/bin/dbus-daemon" sauid=81 hostname=? addr=?
terminal=?'

At this point is probably pointless that I bother looking
into /etc/dbus-1/system.d configuration files because it seems something
which depends only on the SELinux policy.

> I will however say that you also have to be aware of any constraints
> that may influence access decisions (not only type enforcement)
> 
> > Best regards,
> > 
> > Guido Trentalancia

Thanks again for offering your advice.

Regards,

Guido