tty: fix data race on tty_buffer.commit (facd885c) · Commits · 戴 / test

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> Reviewed-by:

Peter Hurley <peter@hurleysoftware.com> Signed-off-by:

Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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);

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