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Commit facd885c authored by Dmitry Vyukov's avatar Dmitry Vyukov Committed by Greg Kroah-Hartman
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tty: fix data race on tty_buffer.commit 



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: default avatarDmitry Vyukov <dvyukov@google.com>
Reviewed-by: default avatarPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 9e6b7cd7

drivers/tty/tty_buffer.c

+12 −3
Original line number Diff line number Diff line
@@ -293,7 +293,10 @@ 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
@@ -396,7 +399,10 @@ 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);
@@ -494,7 +500,10 @@ 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);