[PATCH] Bluetooth: SMP: Fail if the X coordinate of remote and local public keys are identical

[Chun-Yi Lee <joeyli.kernel@gmail.com>]

The 6d19628f539f ("Bluetooth: SMP: Fail if remote and local public keys
are identical") patch be introduced since v5.13. Looks this patch can
handle the CVE-2021-37577 issue. However, it only checks if the full
64-byte public key (X and Y coordinates) is identical, which is
insufficient.

In CVE-2021-37577:
Bluetooth LE and BR/EDR Secure Connections pairing and Secure Simple
Pairing using the Passkey entry protocol in Bluetooth Core
Specifications 2.1 through 5.3 may permit an unauthenticated
man-in-the-middle attacker to identify the Passkey used during pairing
by reflection of a crafted public key with the same X coordinate as the
offered public key and by reflection of the authentication evidence of
the initiating device, potentially permitting this attacker to complete
authenticated pairing with the responding device using the correct
Passkey for the pairing session. This is a related issue to
CVE-2020-26558.

And base on spec, the remote and local public key should not have
the same X coordinate:

Bluetooth Core Specification Version: v5.4
(7.1 PHASE 1: PUBLIC KEY EXCHANGE)
"If the two public keys have the same X coordinate and neither is the
debug key (see [Vol 4] Part E, Section 7.6.4), each device should fail
the pairing process."

Bluetooth Core Specification Version: v6.0
(7.1 Phase 1: Public key exchange)
"If the two public keys have the same X coordinate and neither is the
debug key (see [Vol 4] Part E, Section 7.6.4), each device shall fail
the pairing process."

Update the check in smp_cmd_public_key() to only compare the 32-byte X
coordinate of the public keys, making the validation strictly compliant
with the specification and effectively mitigating CVE-2021-37577.

Fixes: 6d19628f539f ("Bluetooth: SMP: Fail if remote and local public keys are identical")
Link: https://nvd.nist.gov/vuln/detail/cve-2021-37577
Signed-off-by: Chun-Yi Lee <jlee@suse.com>
---
 net/bluetooth/smp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/bluetooth/smp.c b/net/bluetooth/smp.c
index 2df303f6a38f..80128b31e631 100644
--- a/net/bluetooth/smp.c
+++ b/net/bluetooth/smp.c
@@ -2734,7 +2734,7 @@ static int smp_cmd_public_key(struct l2cap_conn *conn, struct sk_buff *skb)
 	 * not in use.
 	 */
 	if (!test_bit(SMP_FLAG_DEBUG_KEY, &smp->flags) &&
-	    !crypto_memneq(key, smp->local_pk, 64)) {
+	    !crypto_memneq(key, smp->local_pk, 32)) {
 		bt_dev_err(hdev, "Remote and local public keys are identical");
 		return SMP_DHKEY_CHECK_FAILED;
 	}
-- 
2.51.0