| Message ID | 1400640133-19836-1-git-send-email-famz@redhat.com |
|---|---|
| State | New |
| Headers | show |
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 <uobergfe@redhat.com> > Suggested-by: Paolo Bonzini <pbonzini@redhat.com> > Signed-off-by: Fam Zheng <famz@redhat.com> > > --- > 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
diff --git a/tests/test-thread-pool.c b/tests/test-thread-pool.c index c1f8e13..aa156bc 100644 --- a/tests/test-thread-pool.c +++ b/tests/test-thread-pool.c @@ -180,7 +180,7 @@ static void test_cancel(void) /* Canceling the others will be a blocking operation. */ for (i = 0; i < 100; i++) { - if (data[i].n != 3) { + if (data[i].aiocb && data[i].n != 3) { bdrv_aio_cancel(data[i].aiocb); } } diff --git a/thread-pool.c b/thread-pool.c index fbdd3ff..dfb699d 100644 --- a/thread-pool.c +++ b/thread-pool.c @@ -224,6 +224,7 @@ static void thread_pool_cancel(BlockDriverAIOCB *acb) pool->pending_cancellations--; } qemu_mutex_unlock(&pool->lock); + event_notifier_ready(&pool->notifier); } static const AIOCBInfo thread_pool_aiocb_info = {
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 <uobergfe@redhat.com> Suggested-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Fam Zheng <famz@redhat.com> --- v3: Call event_notifier_ready after unlock. --- tests/test-thread-pool.c | 2 +- thread-pool.c | 1 + 2 files changed, 2 insertions(+), 1 deletion(-)