Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 16 Sep 2020 16:05:32 +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
Message-ID: <20200916140532.GA1362448@hirez.programming.kicks-ass.net>
References: <20200910104153.1672460-1-jolsa@kernel.org>
 <CAM9d7cjqq8+wcZWJ77oONKXu-FsaT_YvRxzaGbRT8PjLOw-AkQ@mail.gmail.com>
 <20200910144744.GA1663813@krava>
 <CAM9d7ciEAA_3Quo1-q7hU=Te+hBgJ2wYAjbDazXd7yS70HrhPA@mail.gmail.com>
 <20200911074931.GA1714160@krava>
 <CAM9d7cicyzZvkrXTvSrGrOE626==myvF2hnuUNiUoLXiOKHB+A@mail.gmail.com>
 <20200914205936.GD1714160@krava>
 <alpine.LRH.2.20.2009151734230.12057@Diego>
 <20200916115311.GE2301783@krava>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200916115311.GE2301783@krava>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.