Re: [PATCH 0/4] audit: refactor and fix for potential deadlock

[Paul Moore]

On Mon, May 8, 2023 at 3:58 AM Eiichi Tsukata
<eiichi.tsukata@xxxxxxxxxxx> wrote:
> Commit 7ffb8e317bae ("audit: we don't need to
> __set_current_state(TASK_RUNNING)") accidentally moved queue full check
> before add_wait_queue_exclusive() which introduced the following race:
>
>     CPU1                           CPU2
>   ========                       ========
>   (in audit_log_start())         (in kauditd_thread())
>
>   queue is full
>                                  wake_up(&audit_backlog_wait)
>                                  wait_event_freezable()
>   add_wait_queue_exclusive()
>   ...
>   schedule_timeout()
>
> Once this happens, both audit_log_start() and kauditd_thread() can cause
> deadlock for up to backlog_wait_time waiting for each other. To prevent
> the race, this patch adds queue full check after
> prepare_to_wait_exclusive().

Have you seen this occur in practice?  The first thing that
audit_log_start() does when detecting that @audit_queue is full is to
wake kauditd_thread, which should start immediately draining
@audit_queue (even if a daemon is not listening, the @audit_queue will
be drained into one of the other queues).  While it is hard to predict
scheduling behavior, it seems unlikely that kauditd_thread would be
able to process the entire queue before audit_log_start() progresses
from waking kauditd_thread() to sleeping.

In the absolute worst case, the task attempting to emit an audit
record sleeps for either the configured wait time or until it is
awakened by kauditd_thread() which could be triggered by another task
attempting to emit an audit record.

-- 
paul-moore.com