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

[Ondrej Mosnacek <omosnace@redhat.com>]

On Mon, Jan 21, 2019 at 11:26 AM Steve Grubb <sgrubb@redhat.com> wrote:
> On Mon, 21 Jan 2019 09:36:43 +0100
> Ondrej Mosnacek <omosnace@redhat.com> wrote:
>
> > On Sat, Jan 19, 2019 at 2:23 PM Richard Guy Briggs <rgb@redhat.com>
> > wrote:
> > > On 2019-01-18 11:04, Ondrej Mosnacek 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 (slcon and tlcon) that report the actual context
> > > > string if it differs from the one reported in scontext/tcontext.
> > > > This is useful for diagnosing SELinux denials.
> > > >
> > > > 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
> > > > tlcon=system_u:object_r:banana_t:s0 tclass=file permissive=1
> > > >
> > > > Cc: Daniel Walsh <dwalsh@redhat.com>
> > > > Link: https://bugzilla.redhat.com/show_bug.cgi?id=1135683
> > > > Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
> > > > ---
> > > >  security/selinux/avc.c | 49
> > > > +++++++++++++++++++++++++----------------- 1 file changed, 29
> > > > insertions(+), 20 deletions(-)
> > > >
> > > > I'm not entirely sure about the record format here, so I'm Cc'ing
> > > > linux-audit and Steve for feedback. I went for optional fields to
> > > > minimize the size of the record, but maybe a different format is
> > > > preferred. If so, let me know and I'll do a respin.
> > >
> > > My understanding is that optional fields can be a problem (but not
> > > if it is a non-searchable field?).
>
> Right. Searchable fields are where we can hit problems. And selinux
> contexts are a searchable field. We have 3 ways of searching: subject
> label, object label, and any label.
>
> > Yes, I hope Steve will eventually chime in and clarify what are the
> > exact boundaries I can operate within for this particular record and
> > fields. I could for example move the new fields at the end of the
> > record or make them always there, but set to "?" or "" if the value
> > would be the same as in the other fields, but I'm not sure if that is
> > necessary or sufficient for audit-userspace tools.
>
> You can leave it right where you have it now. My question is...are
> these always going to object labels? I'd need to know how to classify
> this as subject or object and if it can be either, then how to tell the
> difference. You can also keep this as optional. There is already wide
> spread usage of optional fields in AVC's.

Right, thanks for the reply!

There are two distinct new fields: "slcon" and "tlcon" (I plan to
change them to "srawcon" and "trawcon", though). You can tell whether
they refer to object or subject based on the first character of the
field name, just like with the existing scontext/tcontext fields.

>
> -Steve
>
> > > There is an existing "invalid_context"
> > > field whose position may help indicate which one it refers to, but
> > > this sounds pretty weak.
> > >
> > > Alternatively the best I could suggest would be a new auxiliary
> > > record.
> >
> > Hm, "invalid_context" seems to be part of the SELINUX_ERR record,
> > which is already use for a similar purpose (to report that an invalid
> > context has been encountered) in compute_sid_handle_invalid_context().
> > Perhaps I could reuse it here, but "error" sounds a bit strong...
> > Maybe we need a SELINUX_WARN or SELINUX_INFO...
> >
> > >
> > > > Also, I accept suggestions for better field names than
> > > > "slcon"/"tlcon" ("lcon" is meant as an acronym for "literal
> > > > context", but I'm not sure if that's a good name...).
> > > >
> > > > diff --git a/security/selinux/avc.c b/security/selinux/avc.c
> > > > index 9b63d8ee1687..4a181ed56e37 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, *lcontext;
> > > > +     u32 context_len, lcontext_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, &lcontext,
> > > > +                                        &lcontext_len);
> > > > +     if (!rc) {
> > > > +             if (strcmp(context, lcontext))
> > > > +                     audit_log_format(ab, "%clcon=%s ", type,
> > > > lcontext);
> > > > +             kfree(lcontext);
> > > > +     }
> > > > +     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
> > > >
> > > > --
> > > > Linux-audit mailing list
> > > > Linux-audit@redhat.com
> > > > https://www.redhat.com/mailman/listinfo/linux-audit
> > >
> > > - RGB
> > >
> > > --
> > > Richard Guy Briggs <rgb@redhat.com>
> > > Sr. S/W Engineer, Kernel Security, Base Operating Systems
> > > Remote, Ottawa, Red Hat Canada
> > > IRC: rgb, SunRaycer
> > > Voice: +1.647.777.2635, Internal: (81) 32635
> >
>


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