[PATCH v2] Bluetooth: RFCOMM: fix NULL dereference of dlc->session in RFCOMM_CONNINFO

[Hui Peng <benquike@gmail.com>]

The RFCOMM_CONNINFO getsockopt handler accepts a socket that is not
connected as long as deferred setup is enabled:

	if (sk->sk_state != BT_CONNECTED &&
				!rfcomm_pi(sk)->dlc->defer_setup) {
		err = -ENOTCONN;
		break;
	}
	l2cap_sk = rfcomm_pi(sk)->dlc->session->sock->sk;

dlc->defer_setup is set in rfcomm_sock_init() when rfcomm_connect_ind()
creates a child socket for an incoming connection on a listening socket
that has BT_DEFER_SETUP enabled. It is never cleared afterwards. The
session, however, can go away underneath it.

rfcomm_recv_disc() forces the dlc state before tearing it down:

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

The RFCOMM_DEFER_SETUP early return in __rfcomm_dlc_close() only covers
BT_CONNECT, BT_CONFIG, BT_OPEN and BT_CONNECT2, so with the state
already BT_CLOSED that switch does not match and the function falls
through to rfcomm_dlc_unlink(), which sets d->session = NULL, while
d->defer_setup stays 1.

A getsockopt(SOL_RFCOMM, RFCOMM_CONNINFO) on the accepted socket after
that point therefore skips the -ENOTCONN path -- sk->sk_state is
BT_CLOSED, but dlc->defer_setup is still set -- and dereferences the
NULL session. No race is needed: once the DISC has been processed, the
dereference is unconditional.

Reproduced on a KASAN kernel under QEMU 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 and sends SABM for a channel bound with
BT_DEFER_SETUP, and sends DISC for that dlci after the socket has been
accepted. getsockopt(SOL_RFCOMM, RFCOMM_CONNINFO) on the accepted
socket then hits:

 Oops: general protection fault, probably for non-canonical address
 0xdffffc0000000002: 0000 [#1] SMP KASAN PTI
 KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
 CPU: 1 UID: 0 PID: 150 Comm: init Tainted: G B 7.3.0-rc3-g5dd1818b15d9
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)
 RIP: 0010:rfcomm_sock_getsockopt+0x529/0x780
 Call Trace:
  <TASK>
  do_sock_getsockopt+0x3ad/0x7d0
  __sys_getsockopt+0x10e/0x1b0
  __x64_sys_getsockopt+0xc2/0x160
  do_syscall_64+0xda/0x4b0
  entry_SYSCALL_64_after_hwframe+0x77/0x7f
  </TASK>

0x10 is the offset of sock in struct rfcomm_session;
rfcomm_sock_getsockopt_old() is inlined into rfcomm_sock_getsockopt().

Commit 43a556b2fd43 ("Bluetooth: RFCOMM: take rfcomm_mutex for the
deferred setup accept") fixed the same "a remote DISC clears the session
while deferred setup is still flagged" problem in rfcomm_dlc_accept();
this is the remaining instance of it, in the getsockopt path.

Deferred setup only leaves a socket usable here once it has reached
BT_CONNECT2, so restrict the exception to that state and check that a
session is actually present before following it.

Fixes: bb23c0ab8246 ("Bluetooth: Add support for deferring RFCOMM connection setup")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@gmail.com>

---
v2: Corrected the changelog. v1 claimed BT_DEFER_SETUP on a listening
socket stores into dlc->defer_setup; it does not - it sets
BT_SK_DEFER_SETUP in bt_sk(sk)->flags, and dlc->defer_setup is only set
on the child socket created by rfcomm_connect_ind(). The three-line
"reproducer" in v1 (socket/listen/setsockopt/getsockopt) consequently
could not have triggered anything and has been dropped. The real
trigger is a remote DISC unlinking the session of an already accepted
deferred-setup dlc, which is what the reproducer actually did. Also
added the Fixes: and Cc: stable tags. Apologies for the noise on v1.

Behaviour change worth noting: deferred-setup sockets whose session has
gone away now get -ENOTCONN instead of crashing, and deferred-setup
sockets outside BT_CONNECT2 also get -ENOTCONN.

 net/bluetooth/rfcomm/sock.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/net/bluetooth/rfcomm/sock.c
+++ b/net/bluetooth/rfcomm/sock.c
@@ -786,8 +786,10 @@ static int rfcomm_sock_getsockopt_old(st
 		break;
 
 	case RFCOMM_CONNINFO:
-		if (sk->sk_state != BT_CONNECTED &&
-					!rfcomm_pi(sk)->dlc->defer_setup) {
+		if ((sk->sk_state != BT_CONNECTED &&
+		     !(sk->sk_state == BT_CONNECT2 &&
+		       rfcomm_pi(sk)->dlc->defer_setup)) ||
+		    !rfcomm_pi(sk)->dlc->session) {
 			err = -ENOTCONN;
 			break;
 		}
-- 
2.43.0