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([2001:b07:6468:f312:10f7:67c8:abb4:8512]) by smtp.gmail.com with ESMTPSA id o11sm41889770wmh.37.2019.07.24.01.47.28 (version=TLS1_3 cipher=AEAD-AES128-GCM-SHA256 bits=128/128); Wed, 24 Jul 2019 01:47:29 -0700 (PDT) To: Stefan Hajnoczi , qemu-devel@nongnu.org References: <20190723190623.21537-1-stefanha@redhat.com> From: Paolo Bonzini Openpgp: preference=signencrypt Message-ID: <7287eae6-0c19-e082-9a8c-4fbd47afcd94@redhat.com> Date: Wed, 24 Jul 2019 10:47:28 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20190723190623.21537-1-stefanha@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.85.128.68 Subject: Re: [Qemu-devel] [PATCH] util/async: hold AioContext ref to prevent use-after-free X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Fam Zheng , =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= , qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 23/07/19 21:06, Stefan Hajnoczi wrote: > The tests/test-bdrv-drain /bdrv-drain/iothread/drain test case does the > following: > > 1. The preadv coroutine calls aio_bh_schedule_oneshot() and then yields. > 2. The one-shot BH executes in another AioContext. All it does is call > aio_co_wakeup(preadv_co). > 3. The preadv coroutine is re-entered and returns. > > There is a race condition in aio_co_wake() where the preadv coroutine > returns and the test case destroys the preadv IOThread. aio_co_wake() > can still be running in the other AioContext and it performs an access > to the freed IOThread AioContext. > > Here is the race in aio_co_schedule(): > > QSLIST_INSERT_HEAD_ATOMIC(&ctx->scheduled_coroutines, > co, co_scheduled_next); > <-- race: co may execute before we invoke qemu_bh_schedule()! > qemu_bh_schedule(ctx->co_schedule_bh); > > So if co causes ctx to be freed then we're in trouble. Fix this problem > by holding a reference to ctx. > > Signed-off-by: Stefan Hajnoczi > --- > util/async.c | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/util/async.c b/util/async.c > index 8d2105729c..4e4c7af51e 100644 > --- a/util/async.c > +++ b/util/async.c > @@ -459,9 +459,17 @@ void aio_co_schedule(AioContext *ctx, Coroutine *co) > abort(); > } > > + /* The coroutine might run and release the last ctx reference before we > + * invoke qemu_bh_schedule(). Take a reference to keep ctx alive until > + * we're done. > + */ > + aio_context_ref(ctx); > + > QSLIST_INSERT_HEAD_ATOMIC(&ctx->scheduled_coroutines, > co, co_scheduled_next); > qemu_bh_schedule(ctx->co_schedule_bh); > + > + aio_context_unref(ctx); > } > > void aio_co_wake(struct Coroutine *co) > This must have been painful to debug. Reviewed-by: Paolo Bonzini Paolo