Re: [PATCH 5/5] scsi: libsas: fix a race condition when smp task timeout

[John Garry]

On 12/09/2018 09:29, Jason Yan wrote:
> When the lldd is processing the complete sas task in interrupt and set
> the task stat as SAS_TASK_STATE_DONE, the smp timeout timer is able to
> be triggered at the same time. And smp_task_timedout() will complete the
> task wheter the SAS_TASK_STATE_DONE is set or not. Then the sas task may
> freed before lldd end the interrupt process. Thus a use-after-free will
> happen.
>
> Fix this by calling the complete() only when SAS_TASK_STATE_DONE is not
> set. And remove the check of the return value of the del_timer().

Hi Jason,

Please mention that once the LLDD sets DONE, it must call task->done(), 
which will call smp_task_done()->complete()

> Reported-by: chenxiang <chenxiang66@xxxxxxxxxxxxx>
> Signed-off-by: Jason Yan <yanaijie@xxxxxxxxxx>
> CC: John Garry <john.garry@xxxxxxxxxx>
> CC: Johannes Thumshirn <jthumshirn@xxxxxxx>
> CC: Ewan Milne <emilne@xxxxxxxxxx>
> CC: Christoph Hellwig <hch@xxxxxx>
> CC: Tomas Henzl <thenzl@xxxxxxxxxx>
> CC: Dan Williams <dan.j.williams@xxxxxxxxx>
> CC: Hannes Reinecke <hare@xxxxxxxx>
> ---
>  drivers/scsi/libsas/sas_expander.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/scsi/libsas/sas_expander.c b/drivers/scsi/libsas/sas_expander.c
> index 52222940d398..0d1f72752ca2 100644
> --- a/drivers/scsi/libsas/sas_expander.c
> +++ b/drivers/scsi/libsas/sas_expander.c
> @@ -48,17 +48,16 @@ static void smp_task_timedout(struct timer_list *t)
>  	unsigned long flags;
>
>  	spin_lock_irqsave(&task->task_state_lock, flags);
> -	if (!(task->task_state_flags & SAS_TASK_STATE_DONE))
> +	if (!(task->task_state_flags & SAS_TASK_STATE_DONE)) {
>  		task->task_state_flags |= SAS_TASK_STATE_ABORTED;
> +		complete(&task->slow_task->completion);

Nit: for consistency with any other time we use this lock, can we call 
complete() outside the lock? Maybe just use a flag variable for this.

> +	}
>  	spin_unlock_irqrestore(&task->task_state_lock, flags);
> -
> -	complete(&task->slow_task->completion);
>  }
>
>  static void smp_task_done(struct sas_task *task)
>  {
> -	if (!del_timer(&task->slow_task->timer))
> -		return;
> +	del_timer(&task->slow_task->timer);
>  	complete(&task->slow_task->completion);
>  }

Do we also need this change or similar:
static int smp_execute_task_sg(struct domain_device *dev,
	if ((task->task_state_flags & SAS_TASK_STATE_ABORTED)) {
		SAS_DPRINTK("smp task timed out or aborted\n");
		i->dft->lldd_abort_task(task);
-		if (!(task->task_state_flags & SAS_TASK_STATE_DONE)) {
-			SAS_DPRINTK("SMP task aborted and not done\n");
-			break;
-		}
+		break;

To me, the ABORTED and DONE states are mutually exclusive.

>

Thanks,
John