bluetooth: Fix WARNING in tty_set_termios()

tty_set_termios() has the following WARN_ON which can be triggered with a syscall to invoke TIOCSETD __NR_ioctl.

WARN_ON(tty->driver->type == TTY_DRIVER_TYPE_PTY &&

                tty->driver->subtype == PTY_TYPE_MASTER);

Reference: https://syzkaller.appspot.com/bug?id=2410d22f1d8e5984217329dd0884b01d99e3e48d

"The problem started with commit 7721383f4199 ("Bluetooth: hci_uart: Support operational speed during setup") which introduced a new way for how tty_set_termios() could end up being called for a master pty."

Fix hci_uart_tty_open() to check if tty supports set_termios in addition to write and error out, if it doesn't.

Reported-by [email protected]

Cc Johan Hovold <[email protected]>

Cc Marcel Holtmann <[email protected]>

Cc Al Viro <[email protected]>

Signed-off-by Shuah Khan <[email protected]>

---
 drivers/bluetooth/hci_ldisc.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

drivers/bluetooth/hci_ldisc.c +0 -0

index fbf7b4df23ab..087ec2268744 100644

Quoted 28 lines · Shuah Khan

tty_set_termios() has the following WARN_ON which can be triggered with a syscall to invoke TIOCSETD __NR_ioctl.

WARN_ON(tty->driver->type == TTY_DRIVER_TYPE_PTY &&

                 tty->driver->subtype == PTY_TYPE_MASTER);

Reference: https://syzkaller.appspot.com/bug?id=2410d22f1d8e5984217329dd0884b01d99e3e48d

"The problem started with commit 7721383f4199 ("Bluetooth: hci_uart: Support operational speed during setup") which introduced a new way for how tty_set_termios() could end up being called for a master pty."

Fix hci_uart_tty_open() to check if tty supports set_termios in addition to write and error out, if it doesn't.

Reported-by [email protected]

Cc Johan Hovold <[email protected]>

Cc Marcel Holtmann <[email protected]>

Cc Al Viro <[email protected]>

Signed-off-by Shuah Khan <[email protected]>

---
  drivers/bluetooth/hci_ldisc.c | 6 +++---
  1 file changed, 3 insertions(+), 3 deletions(-)

drivers/bluetooth/hci_ldisc.c +0 -0

index fbf7b4df23ab..087ec2268744 100644

drivers/bluetooth/hci_ldisc.c static int hci_uart_tty_open(struct tty_struct *tty) · line 478

Quoted 7 lines

drivers/bluetooth/hci_ldisc.c +2 -2

@@ -474,10 +474,10 @@ static int hci_uart_tty_open(struct tty_struct *tty)
 
  	BT_DBG("tty %p", tty);
 
-	/* Error if the tty has no write op instead of leaving an exploitable
-	 * hole
+	/* Error if the tty has no write/set_termios ops instead of leaving
+	 * an exploitable hole.

Why do you think that a tty driver not implementing set_termios() is exploitable?

Note that tty_set_termios() handles a missing set_termios() callback just fine without dereferencing a NULL-pointer. >

drivers/bluetooth/hci_ldisc.c static int hci_uart_tty_open(struct tty_struct *tty) · line 481

Quoted 4 lines

drivers/bluetooth/hci_ldisc.c +1 -1

  	 */
-	if (tty->ops->write == NULL)
+	if (!tty->ops->write || !tty->ops->set_termios)
  		return -EOPNOTSUPP;

I know Marcel asked you do implement things like this, but what you're really doing is to try to uphold the invariant that tty_set_termios() should never be called for a pty master. The WARN_ON in that function was put in place to make sure line-disciplines use the correct ioctl helpers (and the commit I pointed you to earlier in the paragraph you quote in the commit message broke that invariant).

I'm afraid this is totally obscured by the above change and misleading comment update.