On 13/09/2018 14:52, Kevin Wolf wrote:
> 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>
> ---
> 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);
Maybe "assert(blk->refcnt == 1);" here to ensure that blk_drain() cannot
resurrect the block device, or alternatively
+ if (blk->refcnt == 1) {
+ blk_drain(blk);
+ assert(blk->refcnt == 1);
+ }
if (!--blk->refcnt) {
blk_delete(blk);
}
Paolo