Patch "tty: Fix unsafe ldisc reference via ioctl(TIOCGETD)" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

tty: Fix unsafe ldisc reference via ioctl(TIOCGETD)

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
tty-fix-unsafe-ldisc-reference-via-ioctl-tiocgetd.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Peter Hurley <[email protected]>
Date: Sun, 10 Jan 2016 22:40:55 -0800
Subject: tty: Fix unsafe ldisc reference via ioctl(TIOCGETD)

From: Peter Hurley <[email protected]>

commit 5c17c861a357e9458001f021a7afa7aab9937439 upstream.

ioctl(TIOCGETD) retrieves the line discipline id directly from the ldisc because the line discipline id (c_line) in termios is untrustworthy; userspace may have set termios via ioctl(TCSETS*) without actually changing the line discipline via ioctl(TIOCSETD).

However, directly accessing the current ldisc via tty->ldisc is unsafe; the ldisc ptr dereferenced may be stale if the line discipline is changing via ioctl(TIOCSETD) or hangup.

Wait for the line discipline reference (just like read() or write()) to retrieve the "current" line discipline id.

---
 drivers/tty/tty_io.c |   24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

@@ -2663,6 +2663,28 @@ static int tiocsetd(struct tty_struct *t
 }
 
 /**
+ *	tiocgetd	-	get line discipline
+ *	@tty: tty device
+ *	@p: pointer to user data
+ *
+ *	Retrieves the line discipline id directly from the ldisc.
+ *
+ *	Locking: waits for ldisc reference (in case the line discipline
+ *		is changing or the tty is being hungup)
+ */
+
+static int tiocgetd(struct tty_struct *tty, int __user *p)
+{
+	struct tty_ldisc *ld;
+	int ret;
+
+	ld = tty_ldisc_ref_wait(tty);
+	ret = put_user(ld->ops->num, p);
+	tty_ldisc_deref(ld);
+	return ret;
+}
+
+/**
  *	send_break	-	performed time break
  *	@tty: device to break on
  *	@duration: timeout in mS

@@ -2888,7 +2910,7 @@ long tty_ioctl(struct file *file, unsign
 	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:

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/tty-wait-interruptibly-for-tty-lock-on-reopen.patch queue-4.4/n_tty-fix-unsafe-reference-to-other-ldisc.patch queue-4.4/tty-fix-unsafe-ldisc-reference-via-ioctl-tiocgetd.patch queue-4.4/tty-retry-failed-reopen-if-tty-teardown-in-progress.patch