block, bfq: fix uaf for 'stable_merge_bfqq'

Before commit fd571df0ac5b ("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: fd571df0ac5b ("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>

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 8a8d444..d9ed310 100644 (file)
--- a/block/bfq-iosched.c
+++ b/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);