From e65c62b1375cbff69fa925787bcdae4b27bffb48 Mon Sep 17 00:00:00 2001 From: Johannes Weiner Date: Fri, 13 Oct 2017 15:58:08 -0700 Subject: [PATCH] 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 commit 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 Cc: Alan Cox Cc: Christoph Hellwig Cc: Dmitry Vyukov Cc: Michal Hocko Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- drivers/tty/tty_ldisc.c | 11 +++++------ 1 file changed, 5 insertions(+), 6 deletions(-) diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c index 2fe216b276e2..84a8ac2a779f 100644 --- a/drivers/tty/tty_ldisc.c +++ b/drivers/tty/tty_ldisc.c @@ -694,10 +694,8 @@ int tty_ldisc_reinit(struct tty_struct *tty, int disc) 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 *tty, bool reinit) 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); }