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

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Date:   Wed, 10 May 2023 08:09:05 +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 9, 2023, at 10:34, Eiichi Tsukata <eiichi.tsukata@...anix.com> wrote:
> 
> 
> 
>> 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.
> 

I found another case.

kauditd_thread issues wake_up(&audit_backlog_wait) once after wake up.
As waiter side is using add_wait_queue_exclusive() it only wakes up one process at once.

If kauditd wakes up a process which is sleeping in audit_log_start(), then the process will
queue skb and issue wake_up_interruptible(&kauditd_wait). No problem.
But if kauditd wakes up a process which is sleeping in audit_receive(), then the process
won’t try to wake up kauditd. In this case other processes sleeping in audit_log_start()
keep sleeping even if kauditd have flushed the queue.

At this point I’m planning to use non-exclusive wait in audit_receive() in v2.
Let me know if we should use wake_up_all() in kauditd or you have better solution.

Eiichi