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From: peterz@infradead.org
To: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>, Jiri Olsa <jolsa@kernel.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	lkml <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@kernel.org>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Wade Mealing <wmealing@redhat.com>
Subject: Re: [PATCHv2] perf: Fix race in perf_mmap_close function
Date: Wed, 16 Sep 2020 16:05:32 +0200
Message-ID: <20200916140532.GA1362448@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200916115311.GE2301783@krava>

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.

  parent reply index

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-10 10:41 [PATCH] " Jiri Olsa
2020-09-10 13:48 ` Namhyung Kim
2020-09-10 14:47   ` Jiri Olsa
2020-09-11  3:05     ` Namhyung Kim
2020-09-11  7:49       ` Jiri Olsa
2020-09-14 12:48         ` Namhyung Kim
2020-09-14 20:59           ` Jiri Olsa
2020-09-15 15:35             ` Michael Petlan
2020-09-16 11:53               ` [PATCHv2] " Jiri Olsa
2020-09-16 13:54                 ` peterz
2020-09-16 14:38                   ` Jiri Olsa
2020-09-16 14:05                 ` peterz [this message]
2020-10-12 11:45                 ` [tip: perf/core] perf/core: Fix race in the perf_mmap_close() function tip-bot2 for Jiri Olsa

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