linux-kernel - [PATCH v2] tty: fix data race in flush_to

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Date:	Thu, 17 Sep 2015 12:39:36 +0200
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	gregkh@...uxfoundation.org, peter@...leysoftware.com,
	jslaby@...e.com, linux-kernel@...r.kernel.org
Cc:	jslaby@...e.cz, andreyknvl@...gle.com, kcc@...gle.com,
	glider@...gle.com, paulmck@...ux.vnet.ibm.com, hboehm@...gle.com,
	Dmitry Vyukov <dvyukov@...gle.com>
Subject: [PATCH v2] tty: fix data race in flush_to_ldisc

flush_to_ldisc reads port->itty and checks that it is not NULL,
concurrently release_tty sets port->itty to NULL. It is possible
that flush_to_ldisc loads port->itty once, ensures that it is
not NULL, but then reloads it again and uses. The second load
can already return NULL, which will cause a crash.

Use READ_ONCE to read port->itty.

The data race was found with KernelThreadSanitizer (KTSAN).

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

---

Changed since first version:
 - remove WRITE_ONCE when updating port->itty
---
 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..23de97d 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -467,7 +467,7 @@ static void flush_to_ldisc(struct work_struct *work)
 	struct tty_struct *tty;
 	struct tty_ldisc *disc;
 
-	tty = port->itty;
+	tty = READ_ONCE(port->itty);
 	if (tty == NULL)
 		return;
 
-- 
2.6.0.rc0.131.gf624c3d

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