[PATCH] Bluetooth: RFCOMM: Fix initial port reference race

[Chengfeng Ye]

For RFCOMM_RELEASE_ONHUP devices, rfcomm_tty_install() and
__rfcomm_release_dev() both use RFCOMM_TTY_OWNED to decide who drops the
initial tty_port reference. The release path tests the bit separately
from the install path setting it, so both can drop that reference.

The synchronous hangup does not prevent this race: port->tty is not
assigned until tty_port_open(), after installation. The following
interleaving is possible:

  1. Install and release each obtain a reference with rfcomm_dev_get().
  2. Release observes RFCOMM_TTY_OWNED clear.
  3. Install sets RFCOMM_TTY_OWNED and drops the initial reference.
  4. Release drops the same initial reference again.
  5. TTY cleanup drops its reference and frees the device.
  6. Release performs its final tty_port_put() on the freed port.

KASAN reported:

  BUG: KASAN: slab-use-after-free in tty_port_put+0x22/0x190
  Write of size 4 at addr ffff8881001d3d5c by task poc/92

  Call Trace:
   tty_port_put+0x22/0x190
   rfcomm_dev_ioctl+0x1d4/0x1930
   sock_do_ioctl+0x110/0x260
   sock_ioctl+0x380/0x590
   __x64_sys_ioctl+0x134/0x1c0

  Allocated by task 88:
   rfcomm_dev_ioctl+0x8f6/0x1930
   sock_do_ioctl+0x110/0x260
   sock_ioctl+0x380/0x590
   __x64_sys_ioctl+0x134/0x1c0

  Freed by task 65:
   kfree+0x131/0x3c0
   rfcomm_dev_destruct+0x23b/0x2f0
   release_one_tty+0xc1/0x370
   process_one_work+0x661/0x1090

Use test_and_set_bit() in both paths so that only the caller that changes
RFCOMM_TTY_OWNED from clear to set drops the initial reference. Each path
keeps its own lookup reference until release or TTY cleanup, preserving
the existing callback ordering and error handling.

Fixes: 80ea73378af4 ("Bluetooth: Fix unreleased rfcomm_dev reference")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: LLM
Signed-off-by: Chengfeng Ye <nicoyip.dev@xxxxxxxxx>
---
 net/bluetooth/rfcomm/tty.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/net/bluetooth/rfcomm/tty.c b/net/bluetooth/rfcomm/tty.c
index b2c1060394e6..dc3cdf614def 100644
--- a/net/bluetooth/rfcomm/tty.c
+++ b/net/bluetooth/rfcomm/tty.c
@@ -462,7 +462,7 @@ static int __rfcomm_release_dev(void __user *arg)
 	/* Shut down TTY synchronously before freeing rfcomm_dev */
 	tty_port_tty_vhangup(&dev->port);
 
-	if (!test_bit(RFCOMM_TTY_OWNED, &dev->status))
+	if (!test_and_set_bit(RFCOMM_TTY_OWNED, &dev->status))
 		tty_port_put(&dev->port);
 
 	tty_port_put(&dev->port);
@@ -724,10 +724,9 @@ static int rfcomm_tty_install(struct tty_driver *driver, struct tty_struct *tty)
 	 * when the last process closes the tty. The behaviour is expected by
 	 * userspace.
 	 */
-	if (test_bit(RFCOMM_RELEASE_ONHUP, &dev->flags)) {
-		set_bit(RFCOMM_TTY_OWNED, &dev->status);
+	if (test_bit(RFCOMM_RELEASE_ONHUP, &dev->flags) &&
+	    !test_and_set_bit(RFCOMM_TTY_OWNED, &dev->status))
 		tty_port_put(&dev->port);
-	}
 
 	return 0;
 }
-- 
2.43.0