linux-kernel - Re: [PATCHv2] perf: Fix race in perf_mmap

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Message-ID: <20200916140532.GA1362448@hirez.programming.kicks-ass.net>
Date:   Wed, 16 Sep 2020 16:05:32 +0200
From:   peterz@...radead.org
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Namhyung Kim <namhyung@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Wade Mealing <wmealing@...hat.com>
Subject: Re: [PATCHv2] perf: Fix race in perf_mmap_close function

On Wed, Sep 16, 2020 at 01:53:11PM +0200, Jiri Olsa wrote:
> There's a possible race in perf_mmap_close when checking ring buffer's
> mmap_count refcount value. The problem is that the mmap_count check is
> not atomic because we call atomic_dec and atomic_read separately.
> 
>   perf_mmap_close:
>   ...
>    atomic_dec(&rb->mmap_count);
>    ...
>    if (atomic_read(&rb->mmap_count))
>       goto out_put;
> 
>    <ring buffer detach>
>    free_uid
> 
> out_put:
>   ring_buffer_put(rb); /* could be last */
> 
> The race can happen when we have two (or more) events sharing same ring
> buffer and they go through atomic_dec and then they both see 0 as refcount
> value later in atomic_read. Then both will go on and execute code which
> is meant to be run just once.

The trival case should be protected by mmap_sem, we call vm_ops->close()
with mmap_sem held for writing IIRC. But yes, I think it's possible to
construct a failure here.