[PATCH] Bluetooth: RFCOMM: Fix NULL tty_dev dereference in rfcomm_dev_shutdown

[Palla Raghunath <raghunathpalla.0209@gmail.com>]

syzbot hit a NULL pointer dereference in rfcomm_dev_shutdown() when an
RFCOMM tty is closed.

rfcomm_dev_add() puts the new device on rfcomm_dev_list first and only
then registers the tty device and saves it in dev->tty_dev. If that
registration fails, dev->tty_dev is never set at all. The device can be
looked up in the meantime, so an open() of /dev/rfcommN gets through
rfcomm_tty_install() and attaches to it. When that tty is closed again,
rfcomm_dev_shutdown() uses dev->tty_dev without checking it:

  Oops: general protection fault, probably for non-canonical address
  KASAN: null-ptr-deref in range [0x0000000000000040-0x0000000000000047]
  RIP: 0010:rfcomm_dev_shutdown+0x48/0xb0 net/bluetooth/rfcomm/tty.c:131
  Call Trace:
   tty_port_shutdown+0x1bf/0x220 drivers/tty/tty_port.c:372
   tty_port_close+0x4e/0x150 drivers/tty/tty_port.c:705
   tty_release+0x359/0x1670 drivers/tty/tty_io.c:1718
   __fput+0x418/0xa50 fs/file_table.c:512

rfcomm_dev_destruct() already copes with a NULL dev->tty_dev. Add the
same check to rfcomm_dev_shutdown(), so the tty device is only moved
when it was actually registered.

Fixes: cad348a17e17 ("Bluetooth: Implement .activate, .shutdown and .carrier_raised methods")
Reported-by: syzbot+4a807caf7680652d53b6@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=4a807caf7680652d53b6
Cc: stable@vger.kernel.org
Signed-off-by: Palla Raghunath <raghunathpalla.0209@gmail.com>
---
I reproduced this on next-20261001 with a small program that creates and
releases rfcomm0 in a loop while two other threads keep opening and
closing /dev/rfcomm0, with a vhci controller up so the open gets as far
as rfcomm_dev_activate(). It crashed within about ten seconds every time.
With this patch applied it ran for five minutes without any problems.

 net/bluetooth/rfcomm/tty.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/bluetooth/rfcomm/tty.c b/net/bluetooth/rfcomm/tty.c
index dc3cdf614def..198c7dd1a95b 100644
--- a/net/bluetooth/rfcomm/tty.c
+++ b/net/bluetooth/rfcomm/tty.c
@@ -128,7 +128,7 @@ static void rfcomm_dev_shutdown(struct tty_port *port)
 {
 	struct rfcomm_dev *dev = container_of(port, struct rfcomm_dev, port);
 
-	if (dev->tty_dev->parent)
+	if (dev->tty_dev && dev->tty_dev->parent)
 		device_move(dev->tty_dev, NULL, DPM_ORDER_DEV_LAST);
 
 	/* close the dlc */
-- 
2.34.1