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

[Dmitry Vyukov <dvyukov@google.com>]

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.

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);
 }
 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);
-- 
2.5.0.457.gab17608