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 [thread overview]
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.
next prev parent reply other threads:[~2020-09-16 20:46 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-10 10:41 [PATCH] perf: Fix race in perf_mmap_close function 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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