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[PATCH] tty: fix data race in tty_buffer_flush

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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>
---
 drivers/tty/tty_buffer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 5a3fa89..7dab1b3 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -242,7 +242,7 @@ 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) {
+	while ((next = smp_load_acquire(&buf->head->next)) != NULL) {
 		tty_buffer_free(port, buf->head);
 		buf->head = next;
 	}
-- 
2.5.0.457.gab17608

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[PATCH] tty: fix data race in tty_buffer_flush Dmitry Vyukov <dvyukov@google.com> - 2015-09-07 14:30 +0200
  [PATCH] tty: fix data race in tty_buffer_flush Dmitry Vyukov <dvyukov@google.com> - 2015-09-08 14:50 +0200
    Re: [PATCH] tty: fix data race in tty_buffer_flush Greg KH <gregkh@linuxfoundation.org> - 2015-09-09 00:20 +0200
    Re: [PATCH] tty: fix data race in tty_buffer_flush Peter Hurley <peter@hurleysoftware.com> - 2015-09-16 03:40 +0200
      Re: [PATCH] tty: fix data race in tty_buffer_flush Dmitry Vyukov <dvyukov@google.com> - 2015-09-16 20:30 +0200
        Re: [PATCH] tty: fix data race in tty_buffer_flush Dmitry Vyukov <dvyukov@google.com> - 2015-09-17 12:30 +0200
          Re: [PATCH] tty: fix data race in tty_buffer_flush Dmitry Vyukov <dvyukov@google.com> - 2015-09-17 13:10 +0200
          Re: [PATCH] tty: fix data race in tty_buffer_flush Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2015-09-17 15:00 +0200

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