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

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Message-ID: <506299C8-B1D0-457D-881B-BF639224A3AD@nutanix.com>
Date:   Tue, 9 May 2023 01:34:26 +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 0/4] audit: refactor and fix for potential deadlock



> On May 8, 2023, at 23:07, Paul Moore <paul@...l-moore.com> wrote:
> 
> On Mon, May 8, 2023 at 3:58 AM 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())
>> 
>>  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