Re: [PATCH v2 1/2] ublk: properly serialize all FETCH_REQs

[Caleb Sander Mateos]

On Tue, Apr 8, 2025 at 6:42 PM Uday Shankar <ushankar@xxxxxxxxxxxxxxx> wrote:
>
> Most uring_cmds issued against ublk character devices are serialized
> because each command affects only one queue, and there is an early check
> which only allows a single task (the queue's ubq_daemon) to issue
> uring_cmds against that queue. However, this mechanism does not work for
> FETCH_REQs, since they are expected before ubq_daemon is set. Since
> FETCH_REQs are only used at initialization and not in the fast path,
> serialize them using the per-ublk-device mutex. This fixes a number of
> data races that were previously possible if a badly behaved ublk server
> decided to issue multiple FETCH_REQs against the same qid/tag
> concurrently.
>
> Reported-by: Caleb Sander Mateos <csander@xxxxxxxxxxxxxxx>
> Signed-off-by: Uday Shankar <ushankar@xxxxxxxxxxxxxxx>
> ---
>  drivers/block/ublk_drv.c | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> index 2fd05c1bd30b03343cb6f357f8c08dd92ff47af9..5535073ccd23dfbbd25830c1722c360146b95662 100644
> --- a/drivers/block/ublk_drv.c
> +++ b/drivers/block/ublk_drv.c
> @@ -1809,8 +1809,8 @@ static void ublk_nosrv_work(struct work_struct *work)
>
>  /* device can only be started after all IOs are ready */
>  static void ublk_mark_io_ready(struct ublk_device *ub, struct ublk_queue *ubq)
> +       __must_hold(&ub->mutex)
>  {
> -       mutex_lock(&ub->mutex);
>         ubq->nr_io_ready++;
>         if (ublk_queue_ready(ubq)) {
>                 ubq->ubq_daemon = current;
> @@ -1822,7 +1822,6 @@ static void ublk_mark_io_ready(struct ublk_device *ub, struct ublk_queue *ubq)
>         }
>         if (ub->nr_queues_ready == ub->dev_info.nr_hw_queues)
>                 complete_all(&ub->completion);
> -       mutex_unlock(&ub->mutex);
>  }
>
>  static void ublk_handle_need_get_data(struct ublk_device *ub, int q_id,
> @@ -1962,17 +1961,18 @@ static int __ublk_ch_uring_cmd(struct io_uring_cmd *cmd,
>         case UBLK_IO_UNREGISTER_IO_BUF:
>                 return ublk_unregister_io_buf(cmd, ub_cmd->addr, issue_flags);
>         case UBLK_IO_FETCH_REQ:
> +               mutex_lock(&ub->mutex);

I think this may need to be even earlier. Currently the io->flags &
UBLK_IO_FLAG_ACTIVE check happens before the mutex is acquired. Which
means buggy/malicious threads may concurrently try to initialize the
same ublk_io. ublk_mark_io_ready() would then increment nr_io_ready
multiple times for the same ublk_io, causing the ublk_queue to be
declared ready before all its ublk_ios have been initialized. An
alternative to acquiring the mutex before checking io->flags &
UBLK_IO_FLAG_ACTIVE would be to check it again after acquiring the
mutex.

Best,
Caleb

>                 /* UBLK_IO_FETCH_REQ is only allowed before queue is setup */
>                 if (ublk_queue_ready(ubq)) {
>                         ret = -EBUSY;
> -                       goto out;
> +                       goto out_unlock;
>                 }
>                 /*
>                  * The io is being handled by server, so COMMIT_RQ is expected
>                  * instead of FETCH_REQ
>                  */
>                 if (io->flags & UBLK_IO_FLAG_OWNED_BY_SRV)
> -                       goto out;
> +                       goto out_unlock;
>
>                 if (ublk_need_map_io(ubq)) {
>                         /*
> @@ -1980,15 +1980,16 @@ static int __ublk_ch_uring_cmd(struct io_uring_cmd *cmd,
>                          * DATA is not enabled
>                          */
>                         if (!ub_cmd->addr && !ublk_need_get_data(ubq))
> -                               goto out;
> +                               goto out_unlock;
>                 } else if (ub_cmd->addr) {
>                         /* User copy requires addr to be unset */
>                         ret = -EINVAL;
> -                       goto out;
> +                       goto out_unlock;
>                 }
>
>                 ublk_fill_io_cmd(io, cmd, ub_cmd->addr);
>                 ublk_mark_io_ready(ub, ubq);
> +               mutex_unlock(&ub->mutex);
>                 break;
>         case UBLK_IO_COMMIT_AND_FETCH_REQ:
>                 req = blk_mq_tag_to_rq(ub->tag_set.tags[ub_cmd->q_id], tag);
> @@ -2028,7 +2029,9 @@ static int __ublk_ch_uring_cmd(struct io_uring_cmd *cmd,
>         ublk_prep_cancel(cmd, issue_flags, ubq, tag);
>         return -EIOCBQUEUED;
>
> - out:
> +out_unlock:
> +       mutex_unlock(&ub->mutex);
> +out:
>         pr_devel("%s: complete: cmd op %d, tag %d ret %x io_flags %x\n",
>                         __func__, cmd_op, tag, ret, io->flags);
>         return ret;
>
> --
> 2.34.1
>