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 <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);
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