linux-kernel - [PATCH v2] Bluetooth: Add more enc key size check

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Message-ID: <ZXbWcZPq_2zIbqEO@alexlu>
Date:   Mon, 11 Dec 2023 17:29:21 +0800
From:   Alex Lu <alex_lu@...lsil.com.cn>
To:     Marcel Holtmann <marcel@...tmann.org>,
        Johan Hedberg <johan.hedberg@...il.com>,
        Luiz Augusto von Dentz <luiz.dentz@...il.com>,
        <linux-bluetooth@...r.kernel.org>, <linux-kernel@...r.kernel.org>
CC:     Max Chou <max.chou@...ltek.com>, Karen Hsu <karenhsu@...ltek.com>
Subject: [PATCH v2] Bluetooth: Add more enc key size check

From: Alex Lu <alex_lu@...lsil.com.cn>

When we are slave role and receives l2cap conn req when encryption has
started, we should check the enc key size to avoid KNOB attack or BLUFFS
attack.
>From SIG recommendation, implementations are advised to reject
service-level connections on an encrypted baseband link with key
strengths below 7 octets.

The btmon log below shows the case that lacks enc key size check.

> HCI Event: Connect Request (0x04) plen 10
        Address: BB:22:33:44:55:99 (OUI BB-22-33)
        Class: 0x480104
          Major class: Computer (desktop, notebook, PDA, organizers)
          Minor class: Desktop workstation
          Capturing (Scanner, Microphone)
          Telephony (Cordless telephony, Modem, Headset)
        Link type: ACL (0x01)
< HCI Command: Accept Connection Request (0x01|0x0009) plen 7
        Address: BB:22:33:44:55:99 (OUI BB-22-33)
        Role: Peripheral (0x01)
> HCI Event: Command Status (0x0f) plen 4
      Accept Connection Request (0x01|0x0009) ncmd 2
        Status: Success (0x00)
> HCI Event: Connect Complete (0x03) plen 11
        Status: Success (0x00)
        Handle: 1
        Address: BB:22:33:44:55:99 (OUI BB-22-33)
        Link type: ACL (0x01)
        Encryption: Disabled (0x00)
...

> HCI Event: Encryption Change (0x08) plen 4
        Status: Success (0x00)
        Handle: 1 Address: BB:22:33:44:55:99 (OUI BB-22-33)
        Encryption: Enabled with E0 (0x01)
< HCI Command: Read Encryption Key Size (0x05|0x0008) plen 2
        Handle: 1 Address: BB:22:33:44:55:99 (OUI BB-22-33)
> HCI Event: Command Complete (0x0e) plen 7
      Read Encryption Key Size (0x05|0x0008) ncmd 2
        Status: Success (0x00)
        Handle: 1 Address: BB:22:33:44:55:99 (OUI BB-22-33)
        Key size: 6
...

// We should check the enc key size
> ACL Data RX: Handle 1 flags 0x02 dlen 12
      L2CAP: Connection Request (0x02) ident 3 len 4
        PSM: 25 (0x0019)
        Source CID: 64
< ACL Data TX: Handle 1 flags 0x00 dlen 16
      L2CAP: Connection Response (0x03) ident 3 len 8
        Destination CID: 64
        Source CID: 64
        Result: Connection pending (0x0001)
        Status: Authorization pending (0x0002)
> HCI Event: Number of Completed Packets (0x13) plen 5
        Num handles: 1
        Handle: 1 Address: BB:22:33:44:55:99 (OUI BB-22-33)
        Count: 1
        #35: len 16 (25 Kb/s)
        Latency: 5 msec (2-7 msec ~4 msec)
< ACL Data TX: Handle 1 flags 0x00 dlen 16
      L2CAP: Connection Response (0x03) ident 3 len 8
        Destination CID: 64
        Source CID: 64
        Result: Connection successful (0x0000)
        Status: No further information available (0x0000)

Signed-off-by: Alex Lu <alex_lu@...lsil.com.cn>
Signed-off-by: Max Chou <max.chou@...ltek.com>
---
Changes in v2:
  - Fix compiling issue reported by sparse

 net/bluetooth/l2cap_core.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index 17ca13e8c044..a9809fbe6b00 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -1669,7 +1669,13 @@ static void l2cap_conn_start(struct l2cap_conn *conn)
 			rsp.dcid = cpu_to_le16(chan->scid);
 
 			if (l2cap_chan_check_security(chan, false)) {
-				if (test_bit(FLAG_DEFER_SETUP, &chan->flags)) {
+				if (!l2cap_check_enc_key_size(conn->hcon)) {
+					l2cap_state_change(chan, BT_DISCONN);
+					__set_chan_timer(chan,
+							 L2CAP_DISC_TIMEOUT);
+					rsp.result = cpu_to_le16(L2CAP_CR_SEC_BLOCK);
+					rsp.status = cpu_to_le16(L2CAP_CS_NO_INFO);
+				} else if (test_bit(FLAG_DEFER_SETUP, &chan->flags)) {
 					rsp.result = cpu_to_le16(L2CAP_CR_PEND);
 					rsp.status = cpu_to_le16(L2CAP_CS_AUTHOR_PEND);
 					chan->ops->defer(chan);
@@ -4202,7 +4208,15 @@ static struct l2cap_chan *l2cap_connect(struct l2cap_conn *conn,
 
 	if (conn->info_state & L2CAP_INFO_FEAT_MASK_REQ_DONE) {
 		if (l2cap_chan_check_security(chan, false)) {
-			if (test_bit(FLAG_DEFER_SETUP, &chan->flags)) {
+			/* As slave role, we should check the enc key size when
+			 * l2cap conn req is received.
+			 */
+			if (!l2cap_check_enc_key_size(conn->hcon)) {
+				l2cap_state_change(chan, BT_DISCONN);
+				__set_chan_timer(chan, L2CAP_DISC_TIMEOUT);
+				result = L2CAP_CR_SEC_BLOCK;
+				status = L2CAP_CS_NO_INFO;
+			} else if (test_bit(FLAG_DEFER_SETUP, &chan->flags)) {
 				l2cap_state_change(chan, BT_CONNECT2);
 				result = L2CAP_CR_PEND;
 				status = L2CAP_CS_AUTHOR_PEND;
-- 
2.39.2