Commit 178bd438 authored by Fam Zheng's avatar Fam Zheng
Browse files

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: default avatarFam Zheng <famz@redhat.com>
Message-Id: <20170418143044.12187-2-famz@redhat.com>
Reviewed-by: default avatarJeff Cody <jcody@redhat.com>
Tested-by: default avatarJeff Cody <jcody@redhat.com>
Signed-off-by: default avatarFam Zheng <famz@redhat.com>
parent 9c6b899f

block/io.c

+20 −3
Original line number Diff line number Diff line
@@ -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;