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Date:   Thu, 21 Jan 2021 14:09:51 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Denis Efremov <efremov@linux.com>
Cc:     Gaurav Kohli <gkohli@codeaurora.org>, linux-kernel@vger.kernel.org,
        linux-arm-msm@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH v1] trace: Fix race in trace_open and buffer resize call
Message-ID: <20210121140951.2a554a5e@gandalf.local.home>
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        <f06efd7b-c7b5-85c9-1a0e-6bb865111ede@linux.com>
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On Thu, 21 Jan 2021 17:30:40 +0300
Denis Efremov <efremov@linux.com> wrote:

> Hi,
> 
> This patch (CVE-2020-27825) was tagged with
> Fixes: b23d7a5f4a07a ("ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU")
> 
> I'm not an expert here but it seems like b23d7a5f4a07a only refactored
> ring_buffer_reset_cpu() by introducing reset_disabled_cpu_buffer() without
> significant changes. Hence, mutex_lock(&buffer->mutex)/mutex_unlock(&buffer->mutex)
> can be backported further than b23d7a5f4a07a~ and to all LTS kernels. Is
> b23d7a5f4a07a the actual cause of the bug?
> 

Ug, that looks to be a mistake. Looking back at the thread about this:

  https://lore.kernel.org/linux-arm-msm/20200915141304.41fa7c30@gandalf.local.home/

That should have been:

Depends-on: b23d7a5f4a07 ("ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU")

-- Steve