Commit 5ca9d21b authored Sep 07, 2018 by Kevin Wolf

block-backend: Fix potential double blk_delete()

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 <kwolf@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>

parent fe5258a5

block/block-backend.c

+8 −1

Original line number	Diff line number	Diff line
		@@ -435,6 +435,7 @@ int blk_get_refcnt(BlockBackend *blk)
		*/
		void blk_ref(BlockBackend *blk)
		{
		assert(blk->refcnt > 0);
		blk->refcnt++;
		}

		@@ -447,7 +448,13 @@ 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_drain() cannot resurrect blk, nobody held a reference */
		assert(blk->refcnt == 1);
		blk->refcnt = 0;
		blk_delete(blk);
		}
		}