[PATCH] tty: fix data race on tty_buffer.commit

[PATCH] tty: fix data race on tty_buffer.commit

[Dmitry Vyukov]

Race on buffer data happens when newly committed data is
picked up by an old flush work 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.

Similar bug happens when tty_schedule_flip commits data.

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 | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 5a3fa89..0e39d96 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -290,7 +290,10 @@ 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/ acquire in flush_to_ldisc(); ensures
+			 * flush_to_ldisc() sees buffer data.
+			 */
+			smp_store_release(&b->commit, b->used);
 			/* paired w/ acquire in flush_to_ldisc(); ensures the
 			 * latest commit value can be read before the head is
 			 * advanced to the next buffer
@@ -393,7 +396,10 @@ void tty_schedule_flip(struct tty_port *port)
 {
 	struct tty_bufhead *buf = &port->buf;
 
-	buf->tail->commit = buf->tail->used;
+	/* paired w/ acquire in flush_to_ldisc(); ensures
+	 * flush_to_ldisc() sees buffer data.
+	 */
+	smp_store_release(&b->commit, buf->tail->used);
 	schedule_work(&buf->work);
 }
 EXPORT_SYMBOL(tty_schedule_flip);
@@ -491,7 +497,10 @@ static void flush_to_ldisc(struct work_struct *work)
 		 * is advancing to the next buffer
 		 */
 		next = smp_load_acquire(&head->next);
-		count = head->commit - head->read;
+		/* paired w/ release in __tty_buffer_request_room() or in
+		 * tty_buffer_flush(); ensures we see the committed buffer data
+		 */
+		count = smp_load_acquire(&head->commit) - head->read;
 		if (!count) {
 			if (next == NULL) {
 				check_other_closed(tty);
-- 
2.5.0.457.gab17608

[PATCH] tty: fix data race on tty_buffer.commit

[Dmitry Vyukov]

Race on buffer data happens when newly committed data is
picked up by an old flush work 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.

Similar bug happens when tty_schedule_flip commits data.

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 | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 5a3fa89..7ed2006 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -290,7 +290,10 @@ 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/ acquire in flush_to_ldisc(); ensures
+			 * flush_to_ldisc() sees buffer data.
+			 */
+			smp_store_release(&b->commit, b->used);
 			/* paired w/ acquire in flush_to_ldisc(); ensures the
 			 * latest commit value can be read before the head is
 			 * advanced to the next buffer
@@ -393,7 +396,10 @@ void tty_schedule_flip(struct tty_port *port)
 {
 	struct tty_bufhead *buf = &port->buf;
 
-	buf->tail->commit = buf->tail->used;
+	/* paired w/ acquire in flush_to_ldisc(); ensures
+	 * flush_to_ldisc() sees buffer data.
+	 */
+	smp_store_release(&buf->tail->commit, buf->tail->used);
 	schedule_work(&buf->work);
 }
 EXPORT_SYMBOL(tty_schedule_flip);
@@ -491,7 +497,10 @@ static void flush_to_ldisc(struct work_struct *work)
 		 * is advancing to the next buffer
 		 */
 		next = smp_load_acquire(&head->next);
-		count = head->commit - head->read;
+		/* paired w/ release in __tty_buffer_request_room() or in
+		 * tty_buffer_flush(); ensures we see the committed buffer data
+		 */
+		count = smp_load_acquire(&head->commit) - head->read;
 		if (!count) {
 			if (next == NULL) {
 				check_other_closed(tty);
-- 
2.5.0.457.gab17608

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

[Peter Hurley]

On Tue, Sep 8, 2015 at 9:03 AM, Dmitry Vyukov <dvyukov@google.com> wrote:
> Race on buffer data happens when newly committed data is
> picked up by an old flush work 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.
>
> Similar bug happens when tty_schedule_flip commits data.
>
> 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).

Looks good.

Same comments as the other patch; needs changelog revision
information.

Thanks for your work on this; really good stuff.

Regards,
Peter Hurley

> Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
> ---
>  drivers/tty/tty_buffer.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
> index 5a3fa89..7ed2006 100644
> --- a/drivers/tty/tty_buffer.c
> +++ b/drivers/tty/tty_buffer.c
> @@ -290,7 +290,10 @@ 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/ acquire in flush_to_ldisc(); ensures
> +                        * flush_to_ldisc() sees buffer data.
> +                        */
> +                       smp_store_release(&b->commit, b->used);
>                         /* paired w/ acquire in flush_to_ldisc(); ensures the
>                          * latest commit value can be read before the head is
>                          * advanced to the next buffer
> @@ -393,7 +396,10 @@ void tty_schedule_flip(struct tty_port *port)
>  {
>         struct tty_bufhead *buf = &port->buf;
>
> -       buf->tail->commit = buf->tail->used;
> +       /* paired w/ acquire in flush_to_ldisc(); ensures
> +        * flush_to_ldisc() sees buffer data.
> +        */
> +       smp_store_release(&buf->tail->commit, buf->tail->used);
>         schedule_work(&buf->work);
>  }
>  EXPORT_SYMBOL(tty_schedule_flip);
> @@ -491,7 +497,10 @@ static void flush_to_ldisc(struct work_struct *work)
>                  * is advancing to the next buffer
>                  */
>                 next = smp_load_acquire(&head->next);
> -               count = head->commit - head->read;
> +               /* paired w/ release in __tty_buffer_request_room() or in
> +                * tty_buffer_flush(); ensures we see the committed buffer data
> +                */
> +               count = smp_load_acquire(&head->commit) - head->read;
>                 if (!count) {
>                         if (next == NULL) {
>                                 check_other_closed(tty);
> --
> 2.5.0.457.gab17608
>