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Message-ID: <CAHC9VhSh3_hiwzKN_YPKyO-s2tE5V0URTMrg_tbR5=cKkjzLrw@mail.gmail.com>
Date: Mon, 8 May 2023 10:07:31 -0400
From: Paul Moore <paul@...l-moore.com>
To: Eiichi Tsukata <eiichi.tsukata@...anix.com>
Cc: eparis@...hat.com, linux-kernel@...r.kernel.org,
audit@...r.kernel.org
Subject: Re: [PATCH 0/4] audit: refactor and fix for potential deadlock
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? The first thing that
audit_log_start() does when detecting that @audit_queue is full is to
wake kauditd_thread, which should start immediately draining
@audit_queue (even if a daemon is not listening, the @audit_queue will
be drained into one of the other queues). While it is hard to predict
scheduling behavior, it seems unlikely that kauditd_thread would be
able to process the entire queue before audit_log_start() progresses
from waking kauditd_thread() to sleeping.
In the absolute worst case, the task attempting to emit an audit
record sleeps for either the configured wait time or until it is
awakened by kauditd_thread() which could be triggered by another task
attempting to emit an audit record.
--
paul-moore.com
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