Re: [PATCH v2] selinux: log invalid contexts in AVCs

[Ondrej Mosnacek <omosnace@redhat.com>]

On Tue, Jan 22, 2019 at 8:42 PM Paul Moore <paul@paul-moore.com> wrote:
> On Mon, Jan 21, 2019 at 10:36 AM Ondrej Mosnacek <omosnace@redhat.com> wrote:
> > In case a file has an invalid context set, in an AVC record generated
> > upon access to such file, the target context is always reported as
> > unlabeled. This patch adds new optional fields to the AVC record
> > (srawcon and trawcon) that report the actual context string if it
> > differs from the one reported in scontext/tcontext. This is useful for
> > diagnosing SELinux denials involving invalid contexts.
> >
> > To trigger an AVC that illustrates this situation:
> >
> >     # setenforce 0
> >     # touch /tmp/testfile
> >     # setfattr -n security.selinux -v system_u:object_r:banana_t:s0 /tmp/testfile
> >     # runcon system_u:system_r:sshd_t:s0 cat /tmp/testfile
> >
> > AVC before:
> >
> > type=AVC msg=audit(1547801083.248:11): avc:  denied  { open } for  pid=1149 comm="cat" path="/tmp/testfile" dev="tmpfs" ino=6608 scontext=system_u:system_r:sshd_t:s0 tcontext=system_u:object_r:unlabeled_t:s15:c0.c1023 tclass=file permissive=1
> >
> > AVC after:
> >
> > type=AVC msg=audit(1547801083.248:11): avc:  denied  { open } for  pid=1149 comm="cat" path="/tmp/testfile" dev="tmpfs" ino=6608 scontext=system_u:system_r:sshd_t:s0 tcontext=system_u:object_r:unlabeled_t:s15:c0.c1023 trawcon=system_u:object_r:banana_t:s0 tclass=file permissive=1
>
> I would like us to add new fields at the end of existing records; the
> recent audit config changes are a bit of a special case as discussed
> previously.

Okay, I happened to find a way to do this a little differently (taking
a suggestion from Stephen about avoiding the need to do strcmp()) so
now it is actually easy to move them at the end. But I didn't expect
to get a more liberal reply from Steve (who is usually more strict
about this) than you :)

>
> Also, under what cases would we ever see a srawcon field?  This is
> only going to happen if we have a running process whose domain is
> removed during a policy reload, correct?  I'm find with including this
> for the sake of completeness, but I would mention this in the patch
> description for the next revision.

I tried to find a similar reproducer as for trawcon, but it doesn't
seem to be possible to do that without reloading the policy. I will
just add a note.

>
> > Cc: Daniel Walsh <dwalsh@redhat.com>
> > Link: https://bugzilla.redhat.com/show_bug.cgi?id=1135683
> > Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
> > ---
> >
> > v2: Rename fields to "(s|t)rawcon".
> >
> >  security/selinux/avc.c | 49 +++++++++++++++++++++++++-----------------
> >  1 file changed, 29 insertions(+), 20 deletions(-)
> >
> > diff --git a/security/selinux/avc.c b/security/selinux/avc.c
> > index 9b63d8ee1687..df5490db575b 100644
> > --- a/security/selinux/avc.c
> > +++ b/security/selinux/avc.c
> > @@ -165,6 +165,32 @@ static void avc_dump_av(struct audit_buffer *ab, u16 tclass, u32 av)
> >         audit_log_format(ab, " }");
> >  }
> >
> > +static void avc_dump_sid(struct audit_buffer *ab, struct selinux_state *state,
> > +                        u32 sid, char type)
> > +{
> > +       int rc;
> > +       char *context, *rcontext;
> > +       u32 context_len, rcontext_len;
> > +
> > +       rc = security_sid_to_context(state, sid, &context, &context_len);
> > +       if (rc) {
> > +               audit_log_format(ab, "%csid=%d ", type, sid);
> > +               return;
> > +       }
> > +
> > +       audit_log_format(ab, "%ccontext=%s ", type, context);
> > +
> > +       /* in case of invalid context report also the actual context string */
> > +       rc = security_sid_to_context_force(state, sid, &rcontext,
> > +                                          &rcontext_len);
> > +       if (!rc) {
> > +               if (strcmp(context, rcontext))
> > +                       audit_log_format(ab, "%crawcon=%s ", type, rcontext);
> > +               kfree(rcontext);
> > +       }
> > +       kfree(context);
> > +}
> > +
> >  /**
> >   * avc_dump_query - Display a SID pair and a class in human-readable form.
> >   * @ssid: source security identifier
> > @@ -174,28 +200,11 @@ static void avc_dump_av(struct audit_buffer *ab, u16 tclass, u32 av)
> >  static void avc_dump_query(struct audit_buffer *ab, struct selinux_state *state,
> >                            u32 ssid, u32 tsid, u16 tclass)
> >  {
> > -       int rc;
> > -       char *scontext;
> > -       u32 scontext_len;
> > -
> > -       rc = security_sid_to_context(state, ssid, &scontext, &scontext_len);
> > -       if (rc)
> > -               audit_log_format(ab, "ssid=%d", ssid);
> > -       else {
> > -               audit_log_format(ab, "scontext=%s", scontext);
> > -               kfree(scontext);
> > -       }
> > -
> > -       rc = security_sid_to_context(state, tsid, &scontext, &scontext_len);
> > -       if (rc)
> > -               audit_log_format(ab, " tsid=%d", tsid);
> > -       else {
> > -               audit_log_format(ab, " tcontext=%s", scontext);
> > -               kfree(scontext);
> > -       }
> > +       avc_dump_sid(ab, state, ssid, 's');
> > +       avc_dump_sid(ab, state, tsid, 't');
> >
> >         BUG_ON(!tclass || tclass >= ARRAY_SIZE(secclass_map));
> > -       audit_log_format(ab, " tclass=%s", secclass_map[tclass-1].name);
> > +       audit_log_format(ab, "tclass=%s", secclass_map[tclass-1].name);
> >  }
> >
> >  /**
> > --
> > 2.20.1
>
> --
> paul moore
> www.paul-moore.com

-- 
Ondrej Mosnacek <omosnace at redhat dot com>
Associate Software Engineer, Security Technologies
Red Hat, Inc.