[PATCH] tty: plug a use-after-free in TIOCGETD ioctl

[Mateusz Guzik <mguzik@redhat.com>]

When the line discipline is being changed, the old one is freed.
However, the handler for TIOCGETD would dereference it without taking
any locks, in effect possibly reading freed memory.

Line discipline changes are protected with tty lock. Use it on reader
side as well.

CVE: CVE-2016-0723
Found-by: Milos Vyletel <milos@redhat.com>
Signed-off-by: Mateusz Guzik <mguzik@redhat.com>
---
 drivers/tty/tty_io.c | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 892c923..1b10469 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -2626,6 +2626,27 @@ static int tiocgsid(struct tty_struct *tty, struct tty_struct *real_tty, pid_t _
 }
 
 /**
+ *	tiocgetd	-	get line discipline
+ *	@tty: tty device
+ *	@p: pointer to returned line discipline
+ *
+ *	Get the line discipline associated with the tty.
+ *
+ *	Locking: none
+ */
+
+static int tiocgetd(struct tty_struct *tty, int __user *p)
+{
+	int ldisc;
+
+	tty_lock(tty);
+	ldisc = tty->ldisc->ops->num;
+	tty_unlock(tty);
+
+	return put_user(ldisc, p);
+}
+
+/**
  *	tiocsetd	-	set line discipline
  *	@tty: tty device
  *	@p: pointer to user data
@@ -2874,7 +2895,7 @@ long tty_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 	case TIOCGSID:
 		return tiocgsid(tty, real_tty, p);
 	case TIOCGETD:
-		return put_user(tty->ldisc->ops->num, (int __user *)p);
+		return tiocgetd(tty, p);
 	case TIOCSETD:
 		return tiocsetd(tty, p);
 	case TIOCVHANGUP:
-- 
1.8.3.1