linux-kernel - [PATCH] pre-shared passcode: secure pairing for "no keyboard, no display" devices

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Message-ID: <20190213224859.GA7151@amd>
Date:   Wed, 13 Feb 2019 23:48:59 +0100
From:   Pavel Machek <pavel@....cz>
To:     marcel@...tmann.org, johan.hedberg@...il.com,
        linux-bluetooth@...r.kernel.org,
        kernel list <linux-kernel@...r.kernel.org>
Subject: [PATCH] pre-shared passcode: secure pairing for "no keyboard, no
 display" devices

Hi!

Currently, "no keyboard, no display" devices can be paired, but
pairing is not secure against active attacker.

Can we do better? Not for the first pairing; but for the next ones --
yes, I believe we can.

BLE device in this case has internal storage, and Linux running
there. From factory, random 6-digit number is stored in the
flash. Legitimate user knows the number, and system is manipulated so
that pairing passkey will be this pre-shared passkey. After pairing,
user is allowed to change it.

[Or maybe passkey is 000000 from the factory; this is still win for
the user, as long as he can change the key to something random in a
secure cave.]

Fortunately, kernel support for this is rather easy; patch is attached
below.

Does someone see a security issue with proposal above?

What would be suitable interface for setting pre-shared passkey?
Module parameter is really easy.

Signed-off-by: Pavel Machek <pavel@...x.de>

diff --git a/net/bluetooth/smp.c b/net/bluetooth/smp.c
index 621146d..7a2b06595 100644
--- a/net/bluetooth/smp.c
+++ b/net/bluetooth/smp.c
@@ -2674,6 +2674,11 @@ static u8 sc_select_method(struct smp_chan *smp)
 	return method;
 }
 
+static int preshared_passkey = -1;
+
+module_param(preshared_passkey, int, 0600);
+MODULE_PARM_DESC(preshared_passkey, "Preshared passkey for device w/o keyboard or display");
+
 static int smp_cmd_public_key(struct l2cap_conn *conn, struct sk_buff *skb)
 {
 	struct smp_cmd_public_key *key = (void *) skb->data;
@@ -2752,9 +2757,11 @@ static int smp_cmd_public_key(struct l2cap_conn *conn, struct sk_buff *skb)
 		set_bit(SMP_FLAG_DEBUG_KEY, &smp->flags);
 
 	if (smp->method == DSP_PASSKEY) {
 		get_random_bytes(&hcon->passkey_notify,
 				 sizeof(hcon->passkey_notify));
+		if (preshared_passkey != -1)
+			hcon->passkey_notify = preshared_passkey;
 		hcon->passkey_notify %= 1000000;
 		hcon->passkey_entered = 0;
 		smp->passkey_round = 0;
 		if (mgmt_user_passkey_notify(hdev, &hcon->dst, hcon->type,




-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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