Re: audit regressions in 4.11

[Seth Forshee <seth.forshee@canonical.com>]

On Sun, Apr 09, 2017 at 09:14:03AM -0400, Paul Moore wrote:
> On Sat, Apr 8, 2017 at 11:02 PM, Seth Forshee
> <seth.forshee@canonical.com> wrote:
> > I've observed audit regressions in 4.11-rc when not using a userspace
> > audit daemon. The most obvious issue is that audit messages are not
> > appearing in dmesg anymore. If a sufficient number of audit messages are
> > generated the kernel will also start invoking the OOM killer.
> >
> > It looks like previously, when there's no auditd in userspace kauditd
> > would call kauditd_hold_skb(), which prints the message using printk and
> > either frees the skb or queues it (with a limit on the number of queued
> > skb's by default).
> >
> > Since 5b52330bbfe6 "audit: fix auditd/kernel connection state tracking"
> > when there's no auditd kauditd will instead use the retry queue, which
> > has no limit. But it will not process the retry queue when there's no
> > auditd, so messages pile up there indefinitely.
> 
> Hi Seth,
> 
> Thanks for the report.  Let me play with this and think on it for a
> bit, but looking at the code again I think the issue is that we check
> to see if auditd is connected at the top of the kauditd_thread() loop
> and if it isn't we skip right to the main_queue label and bypass the
> hold/retry queue processing which has the logic to ensure the retry
> queue is managed correctly.  My initial thinking is that the fix is to
> check and see if auditd is connected in kauditd_retry_skb(), if it
> isn't we skip the retry queue and call kauditd_hold_skb(), if auditd
> is connected we add the record to the retry queue (what we currently
> do).

Yeah, my first thought was to make this change:

                kauditd_send_queue(sk, portid, &audit_queue, 1,
                                   kauditd_send_multicast_skb,
-                                  kauditd_retry_skb);
+                                  sk ? kauditd_retry_skb : kauditd_hold_skb);

However some scenarios could result in unbounded queueing on the hold
queue as well, so I'm not sure if that's quite enough.

Thanks,
Seth