[patch 12/18] tty: fall back to N_NULL if switching to N_TTY fails during hangup

[patch 12/18] tty: fall back to N_NULL if switching to N_TTY fails during hangup

[akpm]

From: Johannes Weiner <hannes@cmpxchg.org>
Subject: tty: fall back to N_NULL if switching to N_TTY fails during hangup

We have seen NULL-pointer dereference crashes in tty->disc_data when the
N_TTY fallback driver failed to open during hangup.  The immediate cause
of this open to fail has been addressed in the preceding patch to
vmalloc(), but this code could be more robust.

As Alan pointed out in 8a8dabf2dd68 ("tty: handle the case where we cannot
restore a line discipline"), the N_TTY driver, historically the safe
fallback that could never fail, can indeed fail, but the surrounding code
is not prepared to handle this.  To avoid crashes he added a new N_NULL
driver to take N_TTY's place as the last resort.

Hook that fallback up to the hangup path.  Update tty_ldisc_reinit() to
reflect the reality that n_tty_open can indeed fail.

Link: http://lkml.kernel.org/r/20171004185959.GC2136@cmpxchg.org
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Alan Cox <alan@llwyncelyn.cymru>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 drivers/tty/tty_ldisc.c |   11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff -puN drivers/tty/tty_ldisc.c~tty-fall-back-to-n_null-if-switching-to-n_tty-fails-during-hangup drivers/tty/tty_ldisc.c
--- a/drivers/tty/tty_ldisc.c~tty-fall-back-to-n_null-if-switching-to-n_tty-fails-during-hangup
+++ a/drivers/tty/tty_ldisc.c
@@ -694,10 +694,8 @@ int tty_ldisc_reinit(struct tty_struct *
 	tty_set_termios_ldisc(tty, disc);
 	retval = tty_ldisc_open(tty, tty->ldisc);
 	if (retval) {
-		if (!WARN_ON(disc == N_TTY)) {
-			tty_ldisc_put(tty->ldisc);
-			tty->ldisc = NULL;
-		}
+		tty_ldisc_put(tty->ldisc);
+		tty->ldisc = NULL;
 	}
 	return retval;
 }
@@ -752,8 +750,9 @@ void tty_ldisc_hangup(struct tty_struct
 
 	if (tty->ldisc) {
 		if (reinit) {
-			if (tty_ldisc_reinit(tty, tty->termios.c_line) < 0)
-				tty_ldisc_reinit(tty, N_TTY);
+			if (tty_ldisc_reinit(tty, tty->termios.c_line) < 0 &&
+			    tty_ldisc_reinit(tty, N_TTY) < 0)
+				WARN_ON(tty_ldisc_reinit(tty, N_NULL) < 0);
 		} else
 			tty_ldisc_kill(tty);
 	}
_