From: Dmitry Vyukov <dvyukov@google.com>
Date: Sat, 4 Mar 2017 12:46:12 +0000 (+0100)
Subject: tty: fix data race in tty_ldisc_ref_wait()
X-Git-Tag: v4.11-rc4~3^2
X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=a4a3e061149f09c075f108b6f1cf04d9739a6bc2;p=karo-tx-linux.git

tty: fix data race in tty_ldisc_ref_wait()

tty_ldisc_ref_wait() checks tty->ldisc under tty->ldisc_sem.
But if ldisc==NULL it releases them sem and reloads
tty->ldisc without holding the sem. This is wrong and
can lead to returning non-NULL ldisc without protection.

Don't reload tty->ldisc second time.

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Cc: syzkaller@googlegroups.com
Cc: linux-kernel@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index c3956ca022e4..b0500a0a87b8 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -271,10 +271,13 @@ const struct file_operations tty_ldiscs_proc_fops = {
 
 struct tty_ldisc *tty_ldisc_ref_wait(struct tty_struct *tty)
 {
+	struct tty_ldisc *ld;
+
 	ldsem_down_read(&tty->ldisc_sem, MAX_SCHEDULE_TIMEOUT);
-	if (!tty->ldisc)
+	ld = tty->ldisc;
+	if (!ld)
 		ldsem_up_read(&tty->ldisc_sem);
-	return tty->ldisc;
+	return ld;
 }
 EXPORT_SYMBOL_GPL(tty_ldisc_ref_wait);