[PATCH] 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;

BT_DEFER_SETUP can be set on a listening socket, and it is stored in
rfcomm_pi(sk)->dlc->defer_setup while dlc->session is still NULL. A
local user can therefore create an RFCOMM socket, call listen(), set
BT_DEFER_SETUP, and then call getsockopt(SOL_RFCOMM, RFCOMM_CONNINFO)
to skip the -ENOTCONN path and dereference the NULL session:

 Oops: general protection fault, probably for non-canonical address
 0xdffffc0000000002
 KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
 CPU: 1 UID: 0 PID: 150 Comm: init Tainted: G B 7.3.0-rc3-g5dd1818b15d9 #1
 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>

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.

Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@gmail.com>
---
No Fixes: tag: I could not pin the change that introduced this with
confidence, so I left it out rather than guess.

Reproduced on Linux 7.3.0-rc3 (5dd1818b15d9) with KASAN under QEMU with
a virtual HCI device:

  s = socket(AF_BLUETOOTH, SOCK_STREAM, BTPROTO_RFCOMM);
  bind(s, ...); listen(s, 1);
  setsockopt(s, SOL_BLUETOOTH, BT_DEFER_SETUP, &one, 4);
  getsockopt(s, SOL_RFCOMM, RFCOMM_CONNINFO, &ci, &len);   /* boom */

With this patch the getsockopt() returns -ENOTCONN instead.

 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