[PATCH] tty: add the option to have a tty reject a new ldisc

[Linus Torvalds]

.. and use it to limit the virtual terminals to just N_TTY.  They are
kind of special, and in particular, the "con_write()" routine violates
the "writes cannot sleep" rule that some ldiscs rely on.

This avoids the

   BUG: sleeping function called from invalid context at kernel/printk/printk.c:2659

when N_GSM has been attached to a virtual console, and gsmld_write()
calls con_write() while holding a spinlock, and con_write() then tries
to get the console lock.

Tested-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Cc: Jiri Slaby <jirislaby@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Daniel Starke <daniel.starke@xxxxxxxxxxx>
Reported-by: syzbot <syzbot+dbac96d8e73b61aa559c@xxxxxxxxxxxxxxxxxxxxxxxxx>
Closes: https://syzkaller.appspot.com/bug?extid=dbac96d8e73b61aa559c
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
---
 drivers/tty/tty_ldisc.c    |  6 ++++++
 drivers/tty/vt/vt.c        | 10 ++++++++++
 include/linux/tty_driver.h |  8 ++++++++
 3 files changed, 24 insertions(+)

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index 3f68e213df1f..d80e9d4c974b 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -545,6 +545,12 @@ int tty_set_ldisc(struct tty_struct *tty, int disc)
 		goto out;
 	}
 
+	if (tty->ops->ldisc_ok) {
+		retval = tty->ops->ldisc_ok(tty, disc);
+		if (retval)
+			goto out;
+	}
+
 	old_ldisc = tty->ldisc;
 
 	/* Shutdown the old discipline. */
diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 9b5b98dfc8b4..cd87e3d1291e 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -3576,6 +3576,15 @@ static void con_cleanup(struct tty_struct *tty)
 	tty_port_put(&vc->port);
 }
 
+/*
+ * We can't deal with anything but the N_TTY ldisc,
+ * because we can sleep in our write() routine.
+ */
+static int con_ldisc_ok(struct tty_struct *tty, int ldisc)
+{
+	return ldisc == N_TTY ? 0 : -EINVAL;
+}
+
 static int default_color           = 7; /* white */
 static int default_italic_color    = 2; // green (ASCII)
 static int default_underline_color = 3; // cyan (ASCII)
@@ -3695,6 +3704,7 @@ static const struct tty_operations con_ops = {
 	.resize = vt_resize,
 	.shutdown = con_shutdown,
 	.cleanup = con_cleanup,
+	.ldisc_ok = con_ldisc_ok,
 };
 
 static struct cdev vc0_cdev;
diff --git a/include/linux/tty_driver.h b/include/linux/tty_driver.h
index 7372124fbf90..dd4b31ce6d5d 100644
--- a/include/linux/tty_driver.h
+++ b/include/linux/tty_driver.h
@@ -154,6 +154,13 @@ struct serial_struct;
  *
  *	Optional. Called under the @tty->termios_rwsem. May sleep.
  *
+ * @ldisc_ok: ``int ()(struct tty_struct *tty, int ldisc)``
+ *
+ *	This routine allows the @tty driver to decide if it can deal
+ *	with a particular @ldisc.
+ *
+ *	Optional. Called under the @tty->ldisc_sem and @tty->termios_rwsem.
+ *
  * @set_ldisc: ``void ()(struct tty_struct *tty)``
  *
  *	This routine allows the @tty driver to be notified when the device's
@@ -372,6 +379,7 @@ struct tty_operations {
 	void (*hangup)(struct tty_struct *tty);
 	int (*break_ctl)(struct tty_struct *tty, int state);
 	void (*flush_buffer)(struct tty_struct *tty);
+	int (*ldisc_ok)(struct tty_struct *tty, int ldisc);
 	void (*set_ldisc)(struct tty_struct *tty);
 	void (*wait_until_sent)(struct tty_struct *tty, int timeout);
 	void (*send_xchar)(struct tty_struct *tty, u8 ch);
-- 
2.44.0.330.g4d18c88175