| Message ID | 20180907161520.26349-10-kwolf@redhat.com |
|---|---|
| State | New |
| Headers | show |
| Series | Fix some jobs/drain/aio_poll related hangs | expand |
On Fri, 09/07 18:15, 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> Good one! Reviewed-by: Fam Zheng <famz@redhat.com>
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); } }
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> --- block/block-backend.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-)