From 178bd438af5c95deef5073416c60396f88e97ec9 Mon Sep 17 00:00:00 2001 From: Fam Zheng Date: Tue, 18 Apr 2017 22:30:43 +0800 Subject: block: Walk bs->children carefully in bdrv_drain_recurse The recursive bdrv_drain_recurse may run a block job completion BH that drops nodes. The coming changes will make that more likely and use-after-free would happen without this patch Stash the bs pointer and use bdrv_ref/bdrv_unref in addition to QLIST_FOREACH_SAFE to prevent such a case from happening. Since bdrv_unref accesses global state that is not protected by the AioContext lock, we cannot use bdrv_ref/bdrv_unref unconditionally. Fortunately the protection is not needed in IOThread because only main loop can modify a graph with the AioContext lock held. Signed-off-by: Fam Zheng Message-Id: <20170418143044.12187-2-famz@redhat.com> Reviewed-by: Jeff Cody Tested-by: Jeff Cody Signed-off-by: Fam Zheng --- block/io.c | 23 ++++++++++++++++++++--- 1 file changed, 20 insertions(+), 3 deletions(-) diff --git a/block/io.c b/block/io.c index 8706bfa578..a7142e00e8 100644 --- a/block/io.c +++ b/block/io.c @@ -158,7 +158,7 @@ bool bdrv_requests_pending(BlockDriverState *bs) static bool bdrv_drain_recurse(BlockDriverState *bs) { - BdrvChild *child; + BdrvChild *child, *tmp; bool waited; waited = BDRV_POLL_WHILE(bs, atomic_read(&bs->in_flight) > 0); @@ -167,8 +167,25 @@ static bool bdrv_drain_recurse(BlockDriverState *bs) bs->drv->bdrv_drain(bs); } - QLIST_FOREACH(child, &bs->children, next) { - waited |= bdrv_drain_recurse(child->bs); + QLIST_FOREACH_SAFE(child, &bs->children, next, tmp) { + BlockDriverState *bs = child->bs; + bool in_main_loop = + qemu_get_current_aio_context() == qemu_get_aio_context(); + assert(bs->refcnt > 0); + if (in_main_loop) { + /* In case the recursive bdrv_drain_recurse processes a + * block_job_defer_to_main_loop BH and modifies the graph, + * let's hold a reference to bs until we are done. + * + * IOThread doesn't have such a BH, and it is not safe to call + * bdrv_unref without BQL, so skip doing it there. + */ + bdrv_ref(bs); + } + waited |= bdrv_drain_recurse(bs); + if (in_main_loop) { + bdrv_unref(bs); + } } return waited; -- cgit v1.2.1