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Re: [Qemu-block] [PATCH] util/async: hold AioContext ref to prevent use-
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-block] [PATCH] util/async: hold AioContext ref to prevent use-after-free |
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 |
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 <address@hidden>
> ---
> 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 <address@hidden>
Paolo