[PATCH v5 1/3] Bluetooth: SMP: force responder MITM requirements before building the pairing response

[Luiz Augusto von Dentz <luiz.dentz@gmail.com>]

From: Oleh Konko <security@1seal.org>

smp_cmd_pairing_req() currently builds the pairing response from the
initiator auth_req before enforcing the local BT_SECURITY_HIGH
requirement. If the initiator omits SMP_AUTH_MITM, the response can
also omit it even though the local side still requires MITM.

tk_request() then sees an auth value without SMP_AUTH_MITM and may
select JUST_CFM, making method selection inconsistent with the pairing
policy the responder already enforces.

When the local side requires HIGH security, first verify that MITM can
be achieved from the IO capabilities and then force SMP_AUTH_MITM in the
response before build_pairing_cmd(). This keeps the responder auth bits
and later method selection aligned.

Fixes: 2b64d153a0cc ("Bluetooth: Add MITM mechanism to LE-SMP")
Cc: stable@vger.kernel.org
Suggested-by: Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Signed-off-by: Oleh Konko <security@1seal.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
---
v5: Address the comments on
https://sashiko.dev/#/patchset/bt-smp-v4-ea63d24bfcd1416f9da279190fab15fc%401seal.org?patch=14762

 net/bluetooth/smp.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/bluetooth/smp.c b/net/bluetooth/smp.c
index 485e3468bd26..deb8dd244b77 100644
--- a/net/bluetooth/smp.c
+++ b/net/bluetooth/smp.c
@@ -1826,7 +1826,7 @@ static u8 smp_cmd_pairing_req(struct l2cap_conn *conn, struct sk_buff *skb)
 	if (sec_level > conn->hcon->pending_sec_level)
 		conn->hcon->pending_sec_level = sec_level;
 
-	/* If we need MITM check that it can be achieved */
+	/* If we need MITM check that it can be achieved. */
 	if (conn->hcon->pending_sec_level >= BT_SECURITY_HIGH) {
 		u8 method;
 
@@ -1834,6 +1834,10 @@ static u8 smp_cmd_pairing_req(struct l2cap_conn *conn, struct sk_buff *skb)
 					 req->io_capability);
 		if (method == JUST_WORKS || method == JUST_CFM)
 			return SMP_AUTH_REQUIREMENTS;
+
+		/* Force MITM bit if it isn't set by the initiator. */
+		auth |= SMP_AUTH_MITM;
+		rsp.auth_req |= SMP_AUTH_MITM;
 	}
 
 	key_size = min(req->max_key_size, rsp.max_key_size);
-- 
2.53.0