blk_unref() first decreases the refcount of the BlockBackend and calls blk_delete() if the refcount reaches zero. Requests can still be in flight at this point, they are only drained during blk_delete():
At this point, arbitrary callbacks can run. If any callback takes a temporary BlockBackend reference, it will first increase the refcount to 1 and then decrease it to 0 again, triggering another blk_delete(). This will cause a use-after-free crash in the outer blk_delete(). Fix it by draining the BlockBackend before decreasing to refcount to 0. Assert in blk_ref() that it never takes the first refcount (which would mean that the BlockBackend is already being deleted). Signed-off-by: Kevin Wolf <kw...@redhat.com> Reviewed-by: Fam Zheng <f...@redhat.com> --- block/block-backend.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/block/block-backend.c b/block/block-backend.c index efa8d8011c..1b2d7a6ff5 100644 --- a/block/block-backend.c +++ b/block/block-backend.c @@ -435,6 +435,7 @@ int blk_get_refcnt(BlockBackend *blk) */ void blk_ref(BlockBackend *blk) { + assert(blk->refcnt > 0); blk->refcnt++; } @@ -447,7 +448,11 @@ void blk_unref(BlockBackend *blk) { if (blk) { assert(blk->refcnt > 0); - if (!--blk->refcnt) { + if (blk->refcnt > 1) { + blk->refcnt--; + } else { + blk_drain(blk); + blk->refcnt = 0; blk_delete(blk); } } -- 2.13.6