linux-kernel - [ 37/82] tty: Correct tty buffer flush.

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Date:	Mon, 18 Mar 2013 04:22:21 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	akpm@...ux-foundation.org, Ilya Zykov <ilya@...x.ru>
Subject: [ 37/82] tty: Correct tty buffer flush.

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Ilya Zykov <ilya@...x.ru>

commit 64325a3be08d364a62ee8f84b2cf86934bc2544a upstream.

  The root of problem is carelessly zeroing pointer(in function __tty_buffer_flush()),
when another thread can use it. It can be cause of "NULL pointer dereference".
  Main idea of the patch, this is never free last (struct tty_buffer) in the active buffer.
Only flush the data for ldisc(buf->head->read = buf->head->commit).
At that moment driver can collect(write) data in buffer without conflict.
It is repeat behavior of flush_to_ldisc(), only without feeding data to ldisc.

Signed-off-by: Ilya Zykov <ilya@...x.ru>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -114,11 +114,14 @@ static void __tty_buffer_flush(struct tt
 {
 	struct tty_buffer *thead;

-	while ((thead = tty->buf.head) != NULL) {
-		tty->buf.head = thead->next;
-		tty_buffer_free(tty, thead);
+	if (tty->buf.head == NULL)
+		return;
+	while ((thead = tty->buf.head->next) != NULL) {
+		tty_buffer_free(tty, tty->buf.head);
+		tty->buf.head = thead;
 	}
-	tty->buf.tail = NULL;
+	WARN_ON(tty->buf.head != tty->buf.tail);
+	tty->buf.head->read = tty->buf.head->commit;
 }

 /**

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