Re: [PATCH v2 4/5] audit: check if audit_queue is full after prepare_to_wait_exclusive()

[Eiichi Tsukata]

> On May 20, 2023, at 5:54, Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
> 
> On May 11, 2023 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())
>> 
>>  @audit_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 @audit_queue full check after
>> prepare_to_wait_exclusive() and call schedule_timeout() only if the
>> queue is full.
>> 
>> Fixes: 7ffb8e317bae ("audit: we don't need to __set_current_state(TASK_RUNNING)")
>> Signed-off-by: Eiichi Tsukata <eiichi.tsukata@xxxxxxxxxxx>
>> ---
>> kernel/audit.c | 12 ++++++++++--
>> 1 file changed, 10 insertions(+), 2 deletions(-)
> 
> I discussed my concerns with this patch in the last patchset, and I
> believe they still apply here.
> 

Please refer to the implementation of ___wait_event().
It checks the condition *after* prepare_to_wait_event().

Another similar example in the kernel code is unix_wait_for_peer().
It checks unix_recvq_full() after prepare_to_wait_exclusive().

I’m assuming this is a logical bug needs to be fixed.

Eiichi