[PATCH v3] tty: fix data race on tty_buffer.commit

From: Dmitry Vyukov
Date: Thu Sep 17 2015 - 06:58:48 EST


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@xxxxxxxxxx>
---

v3: Added patch revision notes

v2: Removed unrelated changed
---
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);
--
2.6.0.rc0.131.gf624c3d

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