[PATCH v2] Bluetooth: Check for encryption key size on connect

[Archie Pusaka <apusaka@google.com>]

From: Archie Pusaka <apusaka@chromium.org>

When receiving connection, we only check whether the link has been
encrypted, but not the encryption key size of the link.

This patch adds check for encryption key size, and reject L2CAP
connection which size is below the specified threshold (default 7)
with security block.

Here is some btmon trace.
@ MGMT Event: New Link Key (0x0009) plen 26    {0x0001} [hci0] 5.847722
        Store hint: No (0x00)
        BR/EDR Address: 38:00:25:F7:F1:B0 (OUI 38-00-25)
        Key type: Unauthenticated Combination key from P-192 (0x04)
        Link key: 7bf2f68c81305d63a6b0ee2c5a7a34bc
        PIN length: 0
> HCI Event: Encryption Change (0x08) plen 4        #29 [hci0] 5.871537
        Status: Success (0x00)
        Handle: 256
        Encryption: Enabled with E0 (0x01)
< HCI Command: Read Encryp... (0x05|0x0008) plen 2  #30 [hci0] 5.871609
        Handle: 256
> HCI Event: Command Complete (0x0e) plen 7         #31 [hci0] 5.872524
      Read Encryption Key Size (0x05|0x0008) ncmd 1
        Status: Success (0x00)
        Handle: 256
        Key size: 3

////// WITHOUT PATCH //////
> ACL Data RX: Handle 256 flags 0x02 dlen 12        #42 [hci0] 5.895023
      L2CAP: Connection Request (0x02) ident 3 len 4
        PSM: 4097 (0x1001)
        Source CID: 64
< ACL Data TX: Handle 256 flags 0x00 dlen 16        #43 [hci0] 5.895213
      L2CAP: Connection Response (0x03) ident 3 len 8
        Destination CID: 64
        Source CID: 64
        Result: Connection successful (0x0000)
        Status: No further information available (0x0000)

////// WITH PATCH //////
> ACL Data RX: Handle 256 flags 0x02 dlen 12        #42 [hci0] 4.887024
      L2CAP: Connection Request (0x02) ident 3 len 4
        PSM: 4097 (0x1001)
        Source CID: 64
< ACL Data TX: Handle 256 flags 0x00 dlen 16        #43 [hci0] 4.887127
      L2CAP: Connection Response (0x03) ident 3 len 8
        Destination CID: 0
        Source CID: 64
        Result: Connection refused - security block (0x0003)
        Status: No further information available (0x0000)

Signed-off-by: Archie Pusaka <apusaka@chromium.org>
Reviewed-by: Alain Michaud <alainm@chromium.org>

---
Btw, it looks like the patch sent by Alex Lu with the title
[PATCH] Bluetooth: Fix the vulnerable issue on enc key size
also solves the exact same issue.

Changes in v2:
* Add btmon trace to the commit message

 net/bluetooth/l2cap_core.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index ade83e224567..b4fc0ad38aaa 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -4101,7 +4101,8 @@ static struct l2cap_chan *l2cap_connect(struct l2cap_conn *conn,
 
 	/* Check if the ACL is secure enough (if not SDP) */
 	if (psm != cpu_to_le16(L2CAP_PSM_SDP) &&
-	    !hci_conn_check_link_mode(conn->hcon)) {
+	    (!hci_conn_check_link_mode(conn->hcon) ||
+	    !l2cap_check_enc_key_size(conn->hcon))) {
 		conn->disc_reason = HCI_ERROR_AUTH_FAILURE;
 		result = L2CAP_CR_SEC_BLOCK;
 		goto response;
-- 
2.28.0.681.g6f77f65b4e-goog