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Date:   Wed, 16 Sep 2020 15:54:02 +0200
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
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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 code that detaches ring buffer is probably fine to be executed more
> than once, but the problem is in calling free_uid, which will later on
> demonstrate in related crashes and refcount warnings, like:
> 
>   refcount_t: addition on 0; use-after-free.
>   ...
>   RIP: 0010:refcount_warn_saturate+0x6d/0xf
>   ...
>   Call Trace:
>   prepare_creds+0x190/0x1e0
>   copy_creds+0x35/0x172
>   copy_process+0x471/0x1a80
>   _do_fork+0x83/0x3a0
>   __do_sys_wait4+0x83/0x90
>   __do_sys_clone+0x85/0xa0
>   do_syscall_64+0x5b/0x1e0
>   entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> Using atomic decrease and check instead of separated calls.
> This fixes CVE-2020-14351.

I'm tempted to remove that line; I can never seem to find anything
useful in a CVE.

Fixes: ?

> Acked-by: Namhyung Kim <namhyung@kernel.org>
> Tested-by: Michael Petlan <mpetlan@redhat.com>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>