linux-kernel - [PATCH] tty: Correct tty buffer flush.

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Date:	Mon, 03 Dec 2012 13:54:49 +0400
From:	Ilya Zykov <ilya@...x.ru>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Ben Hutchings <ben@...adent.org.uk>,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: [PATCH] tty: Correct tty buffer flush.

  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 release last (struct tty_buffer) in the active buffer.
Only flush data for ldisc(tty->buf.head->read = tty->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.
Test program and bug report you can see:
https://lkml.org/lkml/2012/11/29/368

Cc: stable@...r.kernel.org
Signed-off-by: Ilya Zykov <ilya@...x.ru>
---
diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 6c9b7cd..4f02f9c 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -114,11 +114,14 @@ static void __tty_buffer_flush(struct tty_struct *tty)
 {
 	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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