Re: [PATCH] block, bfq: fix uaf for bfqq in bic_set_bfqq()

[Holger Hoffstätte <holger@xxxxxxxxxxxxxxxxxxxxxx>]

On 2023-01-13 10:44, Yu Kuai wrote:
> After commit 64dc8c732f5c ("block, bfq: fix possible uaf for 'bfqq->bic'"),
> bic->bfqq will be accessed in bic_set_bfqq(), however, in some context
> bic->bfqq will be freed first, and bic_set_bfqq() is called with the freed
> bic->bfqq.
>
> Fix the problem by always freeing bfqq after bic_set_bfqq().
>
> Fixes: 64dc8c732f5c ("block, bfq: fix possible uaf for 'bfqq->bic'")
> Reported-and-tested-by: Shinichiro Kawasaki <shinichiro.kawasaki-Sjgp3cTcYWE@xxxxxxxxxxxxxxxx>
> Signed-off-by: Yu Kuai <yukuai3-hv44wF8Li93QT0dZR+AlfA@xxxxxxxxxxxxxxxx>
> ---
>   block/bfq-cgroup.c  | 2 +-
>   block/bfq-iosched.c | 4 +++-
>   2 files changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
> index a6e8da5f5cfd..feb13ac25557 100644
> --- a/block/bfq-cgroup.c
> +++ b/block/bfq-cgroup.c
> @@ -749,8 +749,8 @@ static void bfq_sync_bfqq_move(struct bfq_data *bfqd,
>   		 * old cgroup.
>   		 */
>   		bfq_put_cooperator(sync_bfqq);
> -		bfq_release_process_ref(bfqd, sync_bfqq);
>   		bic_set_bfqq(bic, NULL, true, act_idx);
> +		bfq_release_process_ref(bfqd, sync_bfqq);
>   	}
>   }
>   
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index 815b884d6c5a..2ddf831221c4 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -5581,9 +5581,11 @@ static void bfq_check_ioprio_change(struct bfq_io_cq *bic, struct bio *bio)
>   
>   	bfqq = bic_to_bfqq(bic, false, bfq_actuator_index(bfqd, bio));
>   	if (bfqq) {
> -		bfq_release_process_ref(bfqd, bfqq);
> +		struct bfq_queue *old_bfqq = bfqq;
> +
>   		bfqq = bfq_get_queue(bfqd, bio, false, bic, true);
>   		bic_set_bfqq(bic, bfqq, false, bfq_actuator_index(bfqd, bio));
> +		bfq_release_process_ref(bfqd, old_bfqq);
>   	}
>   
>   	bfqq = bic_to_bfqq(bic, true, bfq_actuator_index(bfqd, bio));

Hello,

does this condition also affect the current code? I ask since the patch does not apply
as bfq_sync_bfqq_move() is only part of the multi-actuator work, which is only in
Jens' for-next. Comparing the code sections it seems it should also be necessary for
current 6.1/6.2, but I wanted to check.

thanks
Holger