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Re: [Qemu-devel] [PATCH v3] aio: Fix use-after-free in cancellation path
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH v3] aio: Fix use-after-free in cancellation path |
Date: |
Wed, 21 May 2014 16:18:23 +0200 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
On Wed, May 21, 2014 at 10:42:13AM +0800, Fam Zheng wrote:
> The current flow of canceling a thread from THREAD_ACTIVE state is:
>
> 1) Caller wants to cancel a request, so it calls thread_pool_cancel.
>
> 2) thread_pool_cancel waits on the conditional variable
> elem->check_cancel.
>
> 3) The worker thread changes state to THREAD_DONE once the task is
> done, and notifies elem->check_cancel to allow thread_pool_cancel
> to continue execution, and signals the notifier (pool->notifier) to
> allow callback function to be called later. But because of the
> global mutex, the notifier won't get processed until step 4) and 5)
> are done.
>
> 4) thread_pool_cancel continues, leaving the notifier signaled, it
> just returns to caller.
>
> 5) Caller thinks the request is already canceled successfully, so it
> releases any related data, such as freeing elem->common.opaque.
>
> 6) In the next main loop iteration, the notifier handler,
> event_notifier_ready, is called. It finds the canceled thread in
> THREAD_DONE state, so calls elem->common.cb, with an (likely)
> dangling opaque pointer. This is a use-after-free.
>
> Fix it by calling event_notifier_ready before leaving
> thread_pool_cancel.
>
> Test case update: This change will let cancel complete earlier than
> test-thread-pool.c expects, so update the code to check this case: if
> it's already done, done_cb sets .aiocb to NULL, skip calling
> bdrv_aio_cancel on them.
>
> Reported-by: Ulrich Obergfell <address@hidden>
> Suggested-by: Paolo Bonzini <address@hidden>
> Signed-off-by: Fam Zheng <address@hidden>
>
> ---
> v3: Call event_notifier_ready after unlock.
> ---
> tests/test-thread-pool.c | 2 +-
> thread-pool.c | 1 +
> 2 files changed, 2 insertions(+), 1 deletion(-)
Thanks, applied to my block tree:
https://github.com/stefanha/qemu/commits/block
Stefan