[PATCH v3 2/2] Bluetooth: SMP: derive legacy responder STK authentication from MITM state

[Oleh Konko <security@1seal.org>]

The legacy responder path in smp_random() currently labels the stored
STK as authenticated whenever pending_sec_level is BT_SECURITY_HIGH.
That reflects what the local service requested, not what the pairing
flow actually achieved.

For Just Works/Confirm legacy pairing, SMP_FLAG_MITM_AUTH stays clear
and the resulting STK should remain unauthenticated even if the local
side requested HIGH security. Use the established MITM state when
storing the responder STK so the key metadata matches the pairing result.

This also keeps the legacy path aligned with the Secure Connections code,
which already treats JUST_WORKS/JUST_CFM as unauthenticated.

Fixes: fff3490f4781 ("Bluetooth: Fix setting correct authentication information for SMP STK")
Cc: stable@vger.kernel.org
Signed-off-by: Oleh Konko <security@1seal.org>
---
 net/bluetooth/smp.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/net/bluetooth/smp.c b/net/bluetooth/smp.c
index a9fb9b513d6..0e20497988e 100644
--- a/net/bluetooth/smp.c
+++ b/net/bluetooth/smp.c
@@ -1019,10 +1019,7 @@ static u8 smp_random(struct smp_chan *smp)
 
 		smp_s1(smp->tk, smp->prnd, smp->rrnd, stk);
 
-		if (hcon->pending_sec_level == BT_SECURITY_HIGH)
-			auth = 1;
-		else
-			auth = 0;
+		auth = test_bit(SMP_FLAG_MITM_AUTH, &smp->flags) ? 1 : 0;
 
 		/* Even though there's no _RESPONDER suffix this is the
 		 * responder STK we're adding for later lookup (the initiator
-- 
2.50.0