tty: Fix unsafe ldisc reference via ioctl(TIOCGETD)

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.

Cc: <stable@vger.kernel.org>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 5c17c861a357e9458001f021a7afa7aab9937439)

Orabug: 23205576
CVE: CVE-2016-0723

Signed-off-by: Manjunath Govindashetty <manjunath.govindashetty@oracle.com>

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index e5695467598f9b68629738586c5ed9285d8384ae..01fbc22a23b58daa194fced8a5f689a010c094eb 100644 (file)
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -2642,6 +2642,28 @@ static int tiocsetd(struct tty_struct *tty, int __user *p)
        return ret;
 }
 
+/**
+ *     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
@@ -2868,7 +2890,7 @@ 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: