From a4a3e061149f09c075f108b6f1cf04d9739a6bc2 Mon Sep 17 00:00:00 2001
From: Dmitry Vyukov <dvyukov@google.com>
Date: Sat, 4 Mar 2017 13:46:12 +0100
Subject: [PATCH] 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>
---
drivers/tty/tty_ldisc.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
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);
--
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