linux-kernel - Re: [PATCH v2 4/5] audit: check if audit_queue is full after prepare_to_wait

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Date:   Mon, 22 May 2023 04:28:14 +0000
From:   Eiichi Tsukata <eiichi.tsukata@...anix.com>
To:     Paul Moore <paul@...l-moore.com>
CC:     "eparis@...hat.com" <eparis@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "audit@...r.kernel.org" <audit@...r.kernel.org>
Subject: Re: [PATCH v2 4/5] audit: check if audit_queue is full after
  prepare_to_wait_exclusive()



> On May 20, 2023, at 5:54, Paul Moore <paul@...l-moore.com> wrote:
> 
> On May 11, 2023 Eiichi Tsukata <eiichi.tsukata@...anix.com> 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@...anix.com>
>> ---
>> 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