[PATCH v35 27/29] Audit: Add record for multiple object contexts

[Paul Moore]

On Mon, Apr 25, 2022 at 11:38 PM John Johansen
<john.johansen at canonical.com> wrote:
> On 4/18/22 07:59, Casey Schaufler wrote:
> > Create a new audit record AUDIT_MAC_OBJ_CONTEXTS.
> > An example of the MAC_OBJ_CONTEXTS (1421) record is:
> >
> >     type=MAC_OBJ_CONTEXTS[1421]
> >     msg=audit(1601152467.009:1050):
> >     obj_selinux=unconfined_u:object_r:user_home_t:s0
> >
> > When an audit event includes a AUDIT_MAC_OBJ_CONTEXTS record
> > the "obj=" field in other records in the event will be "obj=?".
> > An AUDIT_MAC_OBJ_CONTEXTS record is supplied when the system has
> > multiple security modules that may make access decisions based
> > on an object security context.
> >
> > Signed-off-by: Casey Schaufler <casey at schaufler-ca.com>
> > ---
> >  include/linux/audit.h      |  5 +++
> >  include/uapi/linux/audit.h |  1 +
> >  kernel/audit.c             | 47 +++++++++++++++++++++++
> >  kernel/auditsc.c           | 79 ++++++++++++--------------------------
> >  4 files changed, 77 insertions(+), 55 deletions(-)

...

> > diff --git a/kernel/audit.c b/kernel/audit.c
> > index 8ed2d717c217..a8c3ec6ba60b 100644
> > --- a/kernel/audit.c
> > +++ b/kernel/audit.c
> > @@ -2226,6 +2226,53 @@ static void audit_buffer_aux_end(struct audit_buffer *ab)
> >       ab->skb = skb_peek(&ab->skb_list);
> >  }
> >
> > +void audit_log_object_context(struct audit_buffer *ab, struct lsmblob *blob)
> > +{
> > +     int i;
> > +     int error;
> > +     struct lsmcontext context;
> > +
> > +     if (!lsm_multiple_contexts()) {
> > +             error = security_secid_to_secctx(blob, &context, LSMBLOB_FIRST);
> > +             if (error) {
> > +                     if (error != -EINVAL)
> > +                             goto error_path;
> > +                     return;
> > +             }
> > +             audit_log_format(ab, " obj=%s", context.context);
> > +             security_release_secctx(&context);
> > +     } else {
> > +             audit_log_format(ab, " obj=?");
> > +             error = audit_buffer_aux_new(ab, AUDIT_MAC_OBJ_CONTEXTS);
> > +             if (error)
> > +                     goto error_path;
> > +
> > +             for (i = 0; i < LSMBLOB_ENTRIES; i++) {
> > +                     if (blob->secid[i] == 0)
> > +                             continue;
> > +                     error = security_secid_to_secctx(blob, &context, i);
> > +                     if (error) {
> > +                             audit_log_format(ab, "%sobj_%s=?",
> > +                                              i ? " " : "",
> > +                                              lsm_slot_to_name(i));
> > +                             if (error != -EINVAL)
> > +                                     audit_panic("error in audit_log_object_context");
> > +                     } else {
> > +                             audit_log_format(ab, "%sobj_%s=%s",
> > +                                              i ? " " : "",
> > +                                              lsm_slot_to_name(i),
> > +                                              context.context);
> > +                             security_release_secctx(&context);
> > +                     }
> > +             }
> > +
> > +             audit_buffer_aux_end(ab);
> > +     }
> > +     return;
> > +
> > +error_path:
> > +     audit_panic("error in audit_log_object_context");
>
> This moves the audit_panic around, so certain operations are not
> done before the call. I am currently not sure of the implications.

Short version: It's okay.

Longer version: The audit_panic() call is either going to panic the
kernel (NOT the default), do a pr_err(), or essentially be a no-op.
In the case of the full blown kernel panic we don't really care, the
system is going to die before there is any chance of this record in
progress getting logged.  In the case of a pr_err() or no-op the key
part is making sure we leave the audit_buffer in a consistent state so
that we preserve whatever information is already present.  In the
!lsm_multiple_contexts case we simply return without making any
changes to the audit_buffer so we're good there; in the multiple LSM
case we always end the aux record properly (using a "?" when
necessary) if an aux record has been successfully created.

Feel free to point out a specific scenario that you think looks wrong
- I may have missed it - but I believe this code to be correct.

> > diff --git a/kernel/auditsc.c b/kernel/auditsc.c
> > index 557713954a69..04bf3c04ef3d 100644
> > --- a/kernel/auditsc.c
> > +++ b/kernel/auditsc.c
> > @@ -1420,18 +1409,10 @@ static void show_special(struct audit_context *context, int *call_panic)
>
> If pushing audit_panic into audit_log_object_context() is acceptable then this call_panic arg is
> no longer needed. The same goes for the call_panic arg in audit_log_name(). And call_panic can
> be dropped from audit_log_exit()

Good catch.

I suspect this is a vestige from when audit_log_end() used to do the
record's skb write to userspace, meaning it was possible that you
might get some of the records written to userspace before the system
killed itself.  Now with all of the queuing involved it's less likely
that this would be the case, and even if it does happen in some cases,
it's basically a toss up depending on how the system is loaded, the
scheduler, etc.

-- 
paul-moore.com