block, bfq: fix uaf for 'stable_merge_bfqq'

Before commit fd571df0 ("block, bfq: turn bfqq_data into an array
in bfq_io_cq"), process reference is read before bfq_put_stable_ref(),
and it's safe if bfq_put_stable_ref() put the last reference, because
process reference will be 0 and 'stable_merge_bfqq' won't be accessed
in this case. However, the commit changed the order and  will cause
uaf for 'stable_merge_bfqq'.

In order to emphasize that bfq_put_stable_ref() can drop the last
reference, fix the problem by moving bfq_put_stable_ref() to the end of
bfq_setup_stable_merge().

Fixes: fd571df0 ("block, bfq: turn bfqq_data into an array in bfq_io_cq")
Reported-and-tested-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Link: https://lore.kernel.org/linux-block/20230307071448.rzihxbm4jhbf5krj@shindev/Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-iosched.c

@@ -2854,11 +2854,11 @@ bfq_setup_stable_merge(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 {
 	int proc_ref = min(bfqq_process_refs(bfqq),
 			   bfqq_process_refs(stable_merge_bfqq));
-	struct bfq_queue *new_bfqq;
+	struct bfq_queue *new_bfqq = NULL;
 
-	if (idling_boosts_thr_without_issues(bfqd, bfqq) ||
-	    proc_ref == 0)
-		return NULL;
+	bfqq_data->stable_merge_bfqq = NULL;
+	if (idling_boosts_thr_without_issues(bfqd, bfqq) || proc_ref == 0)
+		goto out;
 
 	/* next function will take at least one ref */
 	new_bfqq = bfq_setup_merge(bfqq, stable_merge_bfqq);
@@ -2873,6 +2873,11 @@ bfq_setup_stable_merge(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 			new_bfqq_data->stably_merged = true;
 		}
 	}
+
+out:
+	/* deschedule stable merge, because done or aborted here */
+	bfq_put_stable_ref(stable_merge_bfqq);
+
 	return new_bfqq;
 }
 
@@ -2933,11 +2938,6 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 			struct bfq_queue *stable_merge_bfqq =
 				bfqq_data->stable_merge_bfqq;
 
-			/* deschedule stable merge, because done or aborted here */
-			bfq_put_stable_ref(stable_merge_bfqq);
-
-			bfqq_data->stable_merge_bfqq = NULL;
-
 			return bfq_setup_stable_merge(bfqd, bfqq,
 						      stable_merge_bfqq,
 						      bfqq_data);