Re: [PATCH] tty: fix data races on tty_buffer.commit

[Peter Hurley <peter@hurleysoftware.com>]

Hi Dmitry,

On 09/04/2015 03:09 PM, Dmitry Vyukov wrote:
> Race on buffer data happens in the following scenario:
> __tty_buffer_request_room does a plain write of tail->commit,
> no barriers were executed before that.
> At this point flush_to_ldisc reads this new value of commit,
> and reads buffer data, no barriers in between.
> The committed buffer data is not necessary visible to flush_to_ldisc.

Please submit one patch for each "fix", because it is not possible
to review what you believe you're fixing.

See below for an example.

> Similar bug happens when tty_schedule_flip commits data.
> 
> Another race happens in tty_buffer_flush. It uses plain reads
> to read tty_buffer.next, as the result it can free a buffer
> which has pending writes in __tty_buffer_request_room thread.
> For example, tty_buffer_flush calls tty_buffer_free which
> reads b->size, the size may not be visible to this thread.
> As the result a large buffer can hang in the freelist.
> 
> Update commit with smp_store_release and read commit with
> smp_load_acquire, as it is commit that signals data readiness.
> This is orthogonal to the existing synchronization on tty_buffer.next,
> which is required to not dismiss a buffer with unconsumed data.
> 
> The data race was found with KernelThreadSanitizer (KTSAN).
> 
> Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
> ---
>  drivers/tty/tty_buffer.c | 38 ++++++++++++++++++++++++--------------
>  1 file changed, 24 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
> index 4cf263d..4fae5d1 100644
> --- a/drivers/tty/tty_buffer.c
> +++ b/drivers/tty/tty_buffer.c
> @@ -89,7 +89,7 @@ void tty_buffer_unlock_exclusive(struct tty_port *port)
>  	struct tty_bufhead *buf = &port->buf;
>  	int restart;
>  
> -	restart = buf->head->commit != buf->head->read;
> +	restart = READ_ONCE(buf->head->commit) != buf->head->read;
>  
>  	atomic_dec(&buf->priority);
>  	mutex_unlock(&buf->lock);
> @@ -242,11 +242,14 @@ void tty_buffer_flush(struct tty_struct *tty, struct tty_ldisc *ld)
>  	atomic_inc(&buf->priority);
>  
>  	mutex_lock(&buf->lock);
> -	while ((next = buf->head->next) != NULL) {
> +	/* paired with smp_store_release in __tty_buffer_request_room();
> +	 * ensures there are no outstanding writes to buf->head when we free it
> +	 */
> +	while ((next = smp_load_acquire(&buf->head->next)) != NULL) {
>  		tty_buffer_free(port, buf->head);
>  		buf->head = next;
>  	}
> -	buf->head->read = buf->head->commit;
> +	buf->head->read = READ_ONCE(buf->head->commit);
>  
>  	if (ld && ld->ops->flush_buffer)
>  		ld->ops->flush_buffer(tty);
> @@ -290,13 +293,15 @@ static int __tty_buffer_request_room(struct tty_port *port, size_t size,
>  		if (n != NULL) {
>  			n->flags = flags;
>  			buf->tail = n;
> -			b->commit = b->used;
> -			/* paired w/ barrier in flush_to_ldisc(); ensures the
> -			 * latest commit value can be read before the head is
> -			 * advanced to the next buffer
> +			/* paired with smp_load_acquire in flush_to_ldisc();
> +			 * ensures flush_to_ldisc() sees buffer data.
>  			 */
> -			smp_wmb();
> -			b->next = n;
> +			smp_store_release(&b->commit, b->used);
> +			/* paired with smp_load_acquire in flush_to_ldisc();
> +			 * ensures the latest commit value can be read before
> +			 * the head is advanced to the next buffer
> +			 */
> +			smp_store_release(&b->next, n);
>  		} else if (change)
>  			size = 0;
>  		else
> @@ -394,7 +399,10 @@ void tty_schedule_flip(struct tty_port *port)
>  {
>  	struct tty_bufhead *buf = &port->buf;
>  
> -	buf->tail->commit = buf->tail->used;
> +	/* paired with smp_load_acquire in flush_to_ldisc(); ensures the
> +	 * committed data is visible to flush_to_ldisc()
> +	 */
> +	smp_store_release(&buf->tail->commit, buf->tail->used);
>  	schedule_work(&buf->work);

schedule_work() is an implied barrier for obvious reasons.

Regards.
Peter Hurley

>  }
>  EXPORT_SYMBOL(tty_schedule_flip);
> @@ -488,13 +496,15 @@ static void flush_to_ldisc(struct work_struct *work)
>  		if (atomic_read(&buf->priority))
>  			break;
>  
> -		next = head->next;
> -		/* paired w/ barrier in __tty_buffer_request_room();
> +		/* paired with smp_store_release in __tty_buffer_request_room();
>  		 * ensures commit value read is not stale if the head
>  		 * is advancing to the next buffer
>  		 */
> -		smp_rmb();
> -		count = head->commit - head->read;
> +		next = smp_load_acquire(&head->next);
> +		/* paired with smp_store_release in __tty_buffer_request_room();
> +		 * ensures we see the committed buffer data
> +		 */
> +		count = smp_load_acquire(&head->commit) - head->read;
>  		if (!count) {
>  			if (next == NULL) {
>  				check_other_closed(tty);
>