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 <dvyu...@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
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