Bluetooth: RFCOMM: take rfcomm_mutex for the deferred setup accept

Overview

In the Linux kernel, the following vulnerability has been resolved:

Bluetooth: RFCOMM: take rfcomm_mutex for the deferred setup accept

rfcomm_sock_recvmsg() completes a deferred setup by calling rfcomm_dlc_accept() without holding any RFCOMM lock:

if (test_and_clear_bit(RFCOMM_DEFER_SETUP, &d->flags)) {
	rfcomm_dlc_accept(d);
	return 0;
}

and rfcomm_dlc_accept() dereferences the session on its first line:

struct sock *sk = d->session->sock->sk;

Every other path that touches d->session runs under rfcomm_mutex: rfcomm_dlc_open(), rfcomm_dlc_close(), rfcomm_dlc_exists(), rfcomm_dlc_send_rpn(), and the RFCOMM thread through rfcomm_process_sessions(). rfcomm_connect_ind() is even documented as "called under rfcomm_lock()". This call site is the only one that skips it.

The RFCOMM_DEFER_SETUP bit looks like it serialises the accept against teardown, since __rfcomm_dlc_close() returns early when it wins the test_and_clear. But rfcomm_recv_disc() forces the state first:

d->state = BT_CLOSED;
__rfcomm_dlc_close(d, err);

and the early return only covers BT_CONNECT, BT_CONFIG, BT_OPEN and BT_CONNECT2. With the state already BT_CLOSED that switch does not match, the bit is never consulted, and __rfcomm_dlc_close() falls through to rfcomm_dlc_unlink(), which sets d->session = NULL.

So a remote DISC on a deferred dlc clears the session while leaving RFCOMM_DEFER_SETUP set. The next recvmsg() then passes the test_and_clear and dereferences a NULL session. No timing window is needed: once the DISC has been processed, the dereference is unconditional.

Give rfcomm_dlc_accept() the same shape as rfcomm_dlc_open() and rfcomm_dlc_close(): an exported wrapper that takes rfcomm_mutex and re-checks the session, around a __rfcomm_dlc_accept() that the two in-core callers, which already hold the mutex, keep using.

Reproduced on a KASAN + PROVE_LOCKING kernel with a BR/EDR peer emulated over /dev/vhci: the peer brings up an ACL link, opens L2CAP on the RFCOMM PSM, starts a session, opens a dlc on a channel bound with BT_DEFER_SETUP, and sends DISC after the socket is accepted. recv() on the accepted socket then hits:

Oops: general protection fault KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017] RIP: 0010:rfcomm_dlc_accept+0x54/0x350 Call Trace: rfcomm_sock_recvmsg+0x1cd/0x230 sock_recvmsg+0x166/0x1c0 __sys_recvfrom+0x20d/0x300

0x10 is the offset of sock in struct rfcomm_session. With this patch the same run completes with recv() returning 0 and no report, and lockdep stays quiet, confirming rfcomm_mutex is still taken before lock_sock on this path as it is on the thread side.

Affected

• Linux >= bb23c0ab824653be4aa7dfca15b07b3059717004 < d8d686dd5662a7c4745e4515f1237a9f3b7df181
• Linux >= bb23c0ab824653be4aa7dfca15b07b3059717004 < eb71d5a1ea8ff2683e394b48ae3cd676037ab4c2
• Linux >= bb23c0ab824653be4aa7dfca15b07b3059717004 < 56f0aa75c7640e46397ef73bea251fcbef9150c0
• Linux >= bb23c0ab824653be4aa7dfca15b07b3059717004 < 362726c9c6e56eea4262109183e49868c39ccd3a
• Linux >= bb23c0ab824653be4aa7dfca15b07b3059717004 < 825b95561d7b7c393df9e7bc295451aaeadc3d18
• Linux >= bb23c0ab824653be4aa7dfca15b07b3059717004 < d4b1a13b1eff2e80925c7368ffdeaaa50cba93df
• Linux >= bb23c0ab824653be4aa7dfca15b07b3059717004 < 355bfd57ca4ca881c6eb03ca813b440a094b1f44
• Linux >= bb23c0ab824653be4aa7dfca15b07b3059717004 < b405c2f96ae2e37375105881890f7738833b1d62
• Linux >= bb23c0ab824653be4aa7dfca15b07b3059717004 < 43a556b2fd43f2df6dded59c2e26560a27874c24
• Linux 2.6.30

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.