From 8a8dabf2dd68caff842d38057097c23bc514ea6e Mon Sep 17 00:00:00 2001 From: Alan Cox Date: Fri, 2 Jun 2017 13:49:30 +0100 Subject: tty: handle the case where we cannot restore a line discipline Historically the N_TTY driver could never fail but this has become broken over time. Rather than trying to rewrite half the ldisc layer to fix the breakage introduce a second level of fallback with an N_NULL ldisc which cannot fail, and thus restore the guarantees required by the ldisc layer. We still try and fail to N_TTY first. It's much more useful to find yourself back in your old ldisc (first attempt) or in N_TTY (second attempt), and while I'm not aware of any code out there that makes those assumptions it's good to drive(r) defensively. Signed-off-by: Alan Cox Reported-by: Dmitry Vyukov Tested-by: Dmitry Vyukov Signed-off-by: Greg Kroah-Hartman --- drivers/tty/tty_ldisc.c | 44 ++++++++++++++++++++++++++++++-------------- 1 file changed, 30 insertions(+), 14 deletions(-) (limited to 'drivers/tty/tty_ldisc.c') diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c index e4603b09863a..4a04567d9aef 100644 --- a/drivers/tty/tty_ldisc.c +++ b/drivers/tty/tty_ldisc.c @@ -491,6 +491,29 @@ static void tty_ldisc_close(struct tty_struct *tty, struct tty_ldisc *ld) tty_ldisc_debug(tty, "%p: closed\n", ld); } +/** + * tty_ldisc_failto - helper for ldisc failback + * @tty: tty to open the ldisc on + * @ld: ldisc we are trying to fail back to + * + * Helper to try and recover a tty when switching back to the old + * ldisc fails and we need something attached. + */ + +static int tty_ldisc_failto(struct tty_struct *tty, int ld) +{ + struct tty_ldisc *disc = tty_ldisc_get(tty, ld); + int r; + + if (IS_ERR(disc)) + return PTR_ERR(disc); + tty->ldisc = disc; + tty_set_termios_ldisc(tty, ld); + if ((r = tty_ldisc_open(tty, disc)) < 0) + tty_ldisc_put(disc); + return r; +} + /** * tty_ldisc_restore - helper for tty ldisc change * @tty: tty to recover @@ -502,9 +525,6 @@ static void tty_ldisc_close(struct tty_struct *tty, struct tty_ldisc *ld) static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old) { - struct tty_ldisc *new_ldisc; - int r; - /* There is an outstanding reference here so this is safe */ old = tty_ldisc_get(tty, old->ops->num); WARN_ON(IS_ERR(old)); @@ -512,17 +532,13 @@ static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old) tty_set_termios_ldisc(tty, old->ops->num); if (tty_ldisc_open(tty, old) < 0) { tty_ldisc_put(old); - /* This driver is always present */ - new_ldisc = tty_ldisc_get(tty, N_TTY); - if (IS_ERR(new_ldisc)) - panic("n_tty: get"); - tty->ldisc = new_ldisc; - tty_set_termios_ldisc(tty, N_TTY); - r = tty_ldisc_open(tty, new_ldisc); - if (r < 0) - panic("Couldn't open N_TTY ldisc for " - "%s --- error %d.", - tty_name(tty), r); + /* 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)); } } -- cgit v1.2.3