4.14-stable review patch. If anyone has any objections, please let me know.
------------------ From: Tetsuo Handa <[email protected]> commit 598c2d41ff44889dd8eced4f117403e472158d85 upstream. syzbot is reporting crashes [1] triggered by memory allocation failure at tty_ldisc_get() from tty_ldisc_restore(). While syzbot stops at WARN_ON() due to panic_on_warn == true, panic_on_warn == false will after all trigger an OOPS by dereferencing old->ops->num if IS_ERR(old) == true. We can simplify tty_ldisc_restore() as three calls (old->ops->num, N_TTY, N_NULL) to tty_ldisc_failto() in addition to avoiding possible error pointer dereference. If someone reports kernel panic triggered by forcing all memory allocations for tty_ldisc_restore() to fail, we can consider adding __GFP_NOFAIL for tty_ldisc_restore() case. [1] https://syzkaller.appspot.com/bug?id=6ac359c61e71d22e06db7f8f88243feb11d927e7 Reported-by: [email protected] Signed-off-by: Tetsuo Handa <[email protected]> Cc: Greg Kroah-Hartman <[email protected]> Cc: Jiri Slaby <[email protected]> Cc: Dmitry Vyukov <[email protected]> Cc: Johannes Weiner <[email protected]> Cc: Alan Cox <[email protected]> Cc: Christoph Hellwig <[email protected]> Cc: Michal Hocko <[email protected]> Cc: stable <[email protected]> Signed-off-by: Greg Kroah-Hartman <[email protected]> --- drivers/tty/tty_ldisc.c | 13 +++++-------- 1 file changed, 5 insertions(+), 8 deletions(-) --- a/drivers/tty/tty_ldisc.c +++ b/drivers/tty/tty_ldisc.c @@ -526,19 +526,16 @@ static int tty_ldisc_failto(struct tty_s static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old) { /* There is an outstanding reference here so this is safe */ - old = tty_ldisc_get(tty, old->ops->num); - WARN_ON(IS_ERR(old)); - tty->ldisc = old; - tty_set_termios_ldisc(tty, old->ops->num); - if (tty_ldisc_open(tty, old) < 0) { - tty_ldisc_put(old); + if (tty_ldisc_failto(tty, old->ops->num) < 0) { + const char *name = tty_name(tty); + + pr_warn("Falling back ldisc for %s.\n", name); /* The traditional behaviour is to fall back to N_TTY, we want to avoid falling back to N_NULL unless we have no choice to avoid the risk of breaking anything */ if (tty_ldisc_failto(tty, N_TTY) < 0 && tty_ldisc_failto(tty, N_NULL) < 0) - panic("Couldn't open N_NULL ldisc for %s.", - tty_name(tty)); + panic("Couldn't open N_NULL ldisc for %s.", name); } }
