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Date:   Mon, 11 Dec 2023 16:29:12 +0800
From:   Alex Lu <alex_lu@realsil.com.cn>
To:     Marcel Holtmann <marcel@holtmann.org>,
        Johan Hedberg <johan.hedberg@gmail.com>,
        Luiz Augusto von Dentz <luiz.dentz@gmail.com>,
        <linux-bluetooth@vger.kernel.org>, <linux-kernel@vger.kernel.org>
CC:     Max Chou <max.chou@realtek.com>, Karen Hsu <karenhsu@realtek.com>
Subject: [PATCH] Bluetooth: Add more enc key size check
Message-ID: <ZXbIWMZyZIYyetff@alexlu>
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From: Alex Lu <alex_lu@realsil.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@realsil.com.cn>
Signed-off-by: Max Chou <max.chou@realtek.com>
---
 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..b0f31d798132 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 = L2CAP_CR_SEC_BLOCK;
+					rsp.status = 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