block: Fix use after free in blockdev_mark_auto_del()

job_cancel_locked() drops the job list lock temporarily and it may call
aio_poll(). We must assume that the list has changed after this call.
Also, with unlucky timing, it can end up freeing the job during
job_completed_txn_abort_locked(), making the job pointer invalid, too.

For both reasons, we can't just continue at block_job_next_locked(job).
Instead, start at the head of the list again after job_cancel_locked()
and skip those jobs that we already cancelled (or that are completing
anyway).

Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230503140142.474404-1-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

blockdev.c

@@ -153,12 +153,22 @@ void blockdev_mark_auto_del(BlockBackend *blk)
 
     JOB_LOCK_GUARD();
 
-    for (job = block_job_next_locked(NULL); job;
-         job = block_job_next_locked(job)) {
-        if (block_job_has_bdrv(job, blk_bs(blk))) {
+    do {
+        job = block_job_next_locked(NULL);
+        while (job && (job->job.cancelled ||
+                       job->job.deferred_to_main_loop ||
+                       !block_job_has_bdrv(job, blk_bs(blk))))
+        {
+            job = block_job_next_locked(job);
+        }
+        if (job) {
+            /*
+             * This drops the job lock temporarily and polls, so we need to
+             * restart processing the list from the start after this.
+             */
             job_cancel_locked(&job->job, false);
         }
-    }
+    } while (job);
 
     dinfo->auto_del = 1;
 }