On Mar 08, 2023 / 10:32, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> 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@xxxxxxx>
> Link: https://lore.kernel.org/linux-block/20230307071448.rzihxbm4jhbf5krj@shindev/
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> ---
> block/bfq-iosched.c | 18 +++++++++---------
> 1 file changed, 9 insertions(+), 9 deletions(-)
>
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index 8a8d4441519c..d9ed3108c17a 100644
> --- 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;
Nit: I suggest to remove the local variable above. The two references to it
below are removed, and the argument of bfq_setup_stable_merge() can be replaced
with 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);
> --
> 2.31.1
>
--
Shin'ichiro Kawasaki
