Re: [Qemu-block] [PATCH 09/14] block-backend: Fix potential double blk_delete()

[Fam Zheng]

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 

Good one!

Reviewed-by: Fam Zheng 

[Qemu-block] [PATCH 09/14] block-backend: Fix potential double blk_delete()

[Kevin Wolf]

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 
---
 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