From f0f81002873c06fdef9bb2a272ddfd26af65b851 Mon Sep 17 00:00:00 2001 From: Stefan Hajnoczi Date: Tue, 23 Jul 2019 20:06:23 +0100 Subject: util/async: hold AioContext ref to prevent use-after-free 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 Reviewed-by: Paolo Bonzini Message-id: 20190723190623.21537-1-stefanha@redhat.com Message-Id: <20190723190623.21537-1-stefanha@redhat.com> Signed-off-by: Stefan Hajnoczi --- util/async.c | 8 ++++++++ 1 file changed, 8 insertions(+) (limited to 'util') 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) -- cgit v1.2.3