Revert "tty: don't panic on OOM in tty_set_ldisc()"

This reverts commit 5362544b as it is
reported to cause a reproducable crash.

Fixes: 5362544b ("tty: don't panic on OOM in tty_set_ldisc()")
Reported-by: Vegard Nossum <vegard.nossum@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: <syzkaller@googlegroups.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

drivers/tty/tty_ldisc.c

@@ -491,6 +491,41 @@ static void tty_ldisc_close(struct tty_struct *tty, struct tty_ldisc *ld)
 	tty_ldisc_debug(tty, "%p: closed\n", ld);
 }
 
+/**
+ *	tty_ldisc_restore	-	helper for tty ldisc change
+ *	@tty: tty to recover
+ *	@old: previous ldisc
+ *
+ *	Restore the previous line discipline or N_TTY when a line discipline
+ *	change fails due to an open error
+ */
+
+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));
+	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);
+	}
+}
+
 /**
  *	tty_set_ldisc		-	set line discipline
  *	@tty: the terminal to set
@@ -504,7 +539,12 @@ static void tty_ldisc_close(struct tty_struct *tty, struct tty_ldisc *ld)
 
 int tty_set_ldisc(struct tty_struct *tty, int disc)
 {
-	int retval, old_disc;
+	int retval;
+	struct tty_ldisc *old_ldisc, *new_ldisc;
+
+	new_ldisc = tty_ldisc_get(tty, disc);
+	if (IS_ERR(new_ldisc))
+		return PTR_ERR(new_ldisc);
 
 	tty_lock(tty);
 	retval = tty_ldisc_lock(tty, 5 * HZ);
@@ -517,8 +557,7 @@ int tty_set_ldisc(struct tty_struct *tty, int disc)
 	}
 
 	/* Check the no-op case */
-	old_disc = tty->ldisc->ops->num;
-	if (old_disc == disc)
+	if (tty->ldisc->ops->num == disc)
 		goto out;
 
 	if (test_bit(TTY_HUPPED, &tty->flags)) {
@@ -527,25 +566,34 @@ int tty_set_ldisc(struct tty_struct *tty, int disc)
 		goto out;
 	}
 
-	retval = tty_ldisc_reinit(tty, disc);
+	old_ldisc = tty->ldisc;
+
+	/* Shutdown the old discipline. */
+	tty_ldisc_close(tty, old_ldisc);
+
+	/* Now set up the new line discipline. */
+	tty->ldisc = new_ldisc;
+	tty_set_termios_ldisc(tty, disc);
+
+	retval = tty_ldisc_open(tty, new_ldisc);
 	if (retval < 0) {
 		/* Back to the old one or N_TTY if we can't */
-		if (tty_ldisc_reinit(tty, old_disc) < 0) {
-			pr_err("tty: TIOCSETD failed, reinitializing N_TTY\n");
-			if (tty_ldisc_reinit(tty, N_TTY) < 0) {
-				/* At this point we have tty->ldisc == NULL. */
-				pr_err("tty: reinitializing N_TTY failed\n");
-			}
-		}
+		tty_ldisc_put(new_ldisc);
+		tty_ldisc_restore(tty, old_ldisc);
 	}
 
-	if (tty->ldisc && tty->ldisc->ops->num != old_disc &&
-	    tty->ops->set_ldisc) {
+	if (tty->ldisc->ops->num != old_ldisc->ops->num && tty->ops->set_ldisc) {
 		down_read(&tty->termios_rwsem);
 		tty->ops->set_ldisc(tty);
 		up_read(&tty->termios_rwsem);
 	}
 
+	/* At this point we hold a reference to the new ldisc and a
+	   reference to the old ldisc, or we hold two references to
+	   the old ldisc (if it was restored as part of error cleanup
+	   above). In either case, releasing a single reference from
+	   the old ldisc is correct. */
+	new_ldisc = old_ldisc;
 out:
 	tty_ldisc_unlock(tty);
 
@@ -553,6 +601,7 @@ int tty_set_ldisc(struct tty_struct *tty, int disc)
 	   already running */
 	tty_buffer_restart_work(tty->port);
 err:
+	tty_ldisc_put(new_ldisc);	/* drop the extra reference */
 	tty_unlock(tty);
 	return retval;
 }
@@ -613,8 +662,10 @@ int tty_ldisc_reinit(struct tty_struct *tty, int disc)
 	int retval;
 
 	ld = tty_ldisc_get(tty, disc);
-	if (IS_ERR(ld))
+	if (IS_ERR(ld)) {
+		BUG_ON(disc == N_TTY);
 		return PTR_ERR(ld);
+	}
 
 	if (tty->ldisc) {
 		tty_ldisc_close(tty, tty->ldisc);
@@ -626,8 +677,10 @@ 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) {
-		tty_ldisc_put(tty->ldisc);
-		tty->ldisc = NULL;
+		if (!WARN_ON(disc == N_TTY)) {
+			tty_ldisc_put(tty->ldisc);
+			tty->ldisc = NULL;
+		}
 	}
 	return retval;
 }