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

[Eiichi Tsukata]

> On May 8, 2023, at 23:07, Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
> 
> 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?  

Yes, we hit this issue multiple times, though it’s pretty rare as you are mentioning.
In our case, sshd got stuck in audit_log_user_message(), which caused SSH connection
timeout.

Eiichi