tty: fix close/hangup race

We can get a situation where a hangup occurs during or after a close. In
that case the ldisc gets disposed of by the close and the hangup then
explodes.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 15 insertions, 10 deletions

diff --git a/drivers/char/tty_ldisc.c b/drivers/char/tty_ldisc.c
index 0ef0dc97ba20..acd76b767d4c 100644
--- a/drivers/char/tty_ldisc.c
+++ b/drivers/char/tty_ldisc.c
@@ -790,17 +790,20 @@ void tty_ldisc_hangup(struct tty_struct *tty)
          * N_TTY.
          */
         if (tty->driver->flags & TTY_DRIVER_RESET_TERMIOS) {
-                /* Avoid racing set_ldisc */
+                /* Avoid racing set_ldisc or tty_ldisc_release */
                 mutex_lock(&tty->ldisc_mutex);
-                /* Switch back to N_TTY */
-                tty_ldisc_halt(tty);
-                tty_ldisc_wait_idle(tty);
-                tty_ldisc_reinit(tty);
-                /* At this point we have a closed ldisc and we want to
-                   reopen it. We could defer this to the next open but
-                   it means auditing a lot of other paths so this is a FIXME */
-                WARN_ON(tty_ldisc_open(tty, tty->ldisc));
-                tty_ldisc_enable(tty);
+                if (tty->ldisc) {       /* Not yet closed */
+                        /* Switch back to N_TTY */
+                        tty_ldisc_halt(tty);
+                        tty_ldisc_wait_idle(tty);
+                        tty_ldisc_reinit(tty);
+                        /* At this point we have a closed ldisc and we want to
+                           reopen it. We could defer this to the next open but
+                           it means auditing a lot of other paths so this is
+                           a FIXME */
+                        WARN_ON(tty_ldisc_open(tty, tty->ldisc));
+                        tty_ldisc_enable(tty);
+                }
                 mutex_unlock(&tty->ldisc_mutex);
                 tty_reset_termios(tty);
         }
@@ -865,6 +868,7 @@ void tty_ldisc_release(struct tty_struct *tty, struct tty_struct *o_tty)
 
         tty_ldisc_wait_idle(tty);
 
+        mutex_lock(&tty->ldisc_mutex);
         /*
          * Now kill off the ldisc
          */
@@ -875,6 +879,7 @@ void tty_ldisc_release(struct tty_struct *tty, struct tty_struct *o_tty)
 
         /* Ensure the next open requests the N_TTY ldisc */
         tty_set_termios_ldisc(tty, N_TTY);
+        mutex_unlock(&tty->ldisc_mutex);
 
         /* This will need doing differently if we need to lock */
         if (o_tty)