[Qemu-block] [PATCH v2 10/17] 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 <address@hidden>
Reviewed-by: Fam Zheng <address@hidden>
---
 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