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 <alan@linux.intel.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 30 insertions, 14 deletions

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
@@ -492,6 +492,29 @@ static void tty_ldisc_close(struct tty_struct *tty, struct tty_ldisc *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
  *	@old: previous ldisc
@@ -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));
 	}
 }