[PATCH v4] tty: fix data race in tty_buffer_flush

[Dmitry Vyukov <dvyukov@google.com>]

tty_buffer_flush frees not acquired buffers.
As the result, for example, read of b->size in tty_buffer_free
can return garbage value which will lead to a huge buffer
hanging in the freelist. This is just the benignest
manifestation of freeing of a not acquired object.
If the object is passed to kfree, heap can be corrupted.

Acquire visibility over the buffer before freeing it.

The data race was found with KernelThreadSanitizer (KTSAN).

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
---

v4: Corrected commit log and patch revision notes

v3: Added code comment re: paired smp barrier

v2: Split from 'tty: fix data races on tty_buffer.commit'

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
---
 drivers/tty/tty_buffer.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 5a3fa89..a2a8cd0 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -242,7 +242,10 @@ 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 w/ release in __tty_buffer_request_room; ensures there are
+	 * no pending memory accesses to the freed buffer
+	 */
+	while ((next = smp_load_acquire(&buf->head->next)) != NULL) {
 		tty_buffer_free(port, buf->head);
 		buf->head = next;
 	}
-- 
2.6.0.rc0.131.gf624c3d