Bluetooth: Add workaround for broken OS X legacy SMP pairing

OS X version 10.10.2 (and possibly older versions) doesn't support LE
Secure Connections but incorrectly copies all authentication request
bits from a Security Request to its Pairing Request. The result is that
an SC capable initiator (such as BlueZ) will think OS X intends to do SC
when in fact it's incapable of it:

< ACL Data TX: Handle 3585 flags 0x00 dlen 6
      SMP: Security Request (0x0b) len 1
        Authentication requirement: Bonding, No MITM, SC, No Keypresses (0x09)
> ACL Data RX: Handle 3585 flags 0x02 dlen 11
      SMP: Pairing Request (0x01) len 6
        IO capability: KeyboardDisplay (0x04)
        OOB data: Authentication data not present (0x00)
        Authentication requirement: Bonding, No MITM, SC, No Keypresses (0x09)
        Max encryption key size: 16
        Initiator key distribution: EncKey (0x01)
        Responder key distribution: EncKey IdKey Sign (0x07)
< ACL Data TX: Handle 3585 flags 0x00 dlen 11
      SMP: Pairing Response (0x02) len 6
        IO capability: NoInputNoOutput (0x03)
        OOB data: Authentication data not present (0x00)
        Authentication requirement: Bonding, No MITM, SC, No Keypresses (0x09)
        Max encryption key size: 16
        Initiator key distribution: EncKey (0x01)
        Responder key distribution: EncKey Sign (0x05)

The pairing eventually fails when we get an unexpected Pairing Confirm
PDU instead of a Public Key PDU:

> ACL Data RX: Handle 3585 flags 0x02 dlen 21
      SMP: Pairing Confirm (0x03) len 16
        Confim value: bcc3bed31b8f313a78ec3cce32685faf

It is only at this point that we can speculate that the remote doesn't
really support SC. This patch creates a workaround for the just-works
model, however the MITM case is unsolvable because the OS X user has
already been requested to enter a PIN which we're now expected to
randomly generate and show the user (i.e. a chicken-and-egg problem).

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

diff --git a/net/bluetooth/smp.c b/net/bluetooth/smp.c
index 60180b4..6a5afb9 100644 (file)
--- a/net/bluetooth/smp.c
+++ b/net/bluetooth/smp.c
@@ -880,6 +880,12 @@ static int tk_request(struct l2cap_conn *conn, u8 remote_oob, u8 auth,
                return 0;
        }
 
+       /* If this function is used for SC -> legacy fallback we
+        * can only recover the just-works case.
+        */
+       if (test_bit(SMP_FLAG_SC, &smp->flags))
+               return -EINVAL;
+
        /* Not Just Works/Confirm results in MITM Authentication */
        if (smp->method != JUST_CFM) {
                set_bit(SMP_FLAG_MITM_AUTH, &smp->flags);
@@ -1806,6 +1812,13 @@ static u8 smp_cmd_pairing_req(struct l2cap_conn *conn, struct sk_buff *skb)
 
        clear_bit(SMP_FLAG_INITIATOR, &smp->flags);
 
+       /* Strictly speaking we shouldn't allow Pairing Confirm for the
+        * SC case, however some implementations incorrectly copy RFU auth
+        * req bits from our security request, which may create a false
+        * positive SC enablement.
+        */
+       SMP_ALLOW_CMD(smp, SMP_CMD_PAIRING_CONFIRM);
+
        if (test_bit(SMP_FLAG_SC, &smp->flags)) {
                SMP_ALLOW_CMD(smp, SMP_CMD_PUBLIC_KEY);
                /* Clear bits which are generated but not distributed */
@@ -1814,8 +1827,6 @@ static u8 smp_cmd_pairing_req(struct l2cap_conn *conn, struct sk_buff *skb)
                return 0;
        }
 
-       SMP_ALLOW_CMD(smp, SMP_CMD_PAIRING_CONFIRM);
-
        /* Request setup of TK */
        ret = tk_request(conn, 0, auth, rsp.io_capability, req->io_capability);
        if (ret)
@@ -1981,10 +1992,6 @@ static u8 sc_check_confirm(struct smp_chan *smp)
 
        BT_DBG("");
 
-       /* Public Key exchange must happen before any other steps */
-       if (!test_bit(SMP_FLAG_REMOTE_PK, &smp->flags))
-               return SMP_UNSPECIFIED;
-
        if (smp->method == REQ_PASSKEY || smp->method == DSP_PASSKEY)
                return sc_passkey_round(smp, SMP_CMD_PAIRING_CONFIRM);
 
@@ -1997,6 +2004,47 @@ static u8 sc_check_confirm(struct smp_chan *smp)
        return 0;
 }
 
+/* Work-around for some implementations that incorrectly copy RFU bits
+ * from our security request and thereby create the impression that
+ * we're doing SC when in fact the remote doesn't support it.
+ */
+static int fixup_sc_false_positive(struct smp_chan *smp)
+{
+       struct l2cap_conn *conn = smp->conn;
+       struct hci_conn *hcon = conn->hcon;
+       struct hci_dev *hdev = hcon->hdev;
+       struct smp_cmd_pairing *req, *rsp;
+       u8 auth;
+
+       /* The issue is only observed when we're in slave role */
+       if (hcon->out)
+               return SMP_UNSPECIFIED;
+
+       if (hci_dev_test_flag(hdev, HCI_SC_ONLY)) {
+               BT_ERR("Refusing SMP SC -> legacy fallback in SC-only mode");
+               return SMP_UNSPECIFIED;
+       }
+
+       BT_ERR("Trying to fall back to legacy SMP");
+
+       req = (void *) &smp->preq[1];
+       rsp = (void *) &smp->prsp[1];
+
+       /* Rebuild key dist flags which may have been cleared for SC */
+       smp->remote_key_dist = (req->init_key_dist & rsp->resp_key_dist);
+
+       auth = req->auth_req & AUTH_REQ_MASK(hdev);
+
+       if (tk_request(conn, 0, auth, rsp->io_capability, req->io_capability)) {
+               BT_ERR("Failed to fall back to legacy SMP");
+               return SMP_UNSPECIFIED;
+       }
+
+       clear_bit(SMP_FLAG_SC, &smp->flags);
+
+       return 0;
+}
+
 static u8 smp_cmd_pairing_confirm(struct l2cap_conn *conn, struct sk_buff *skb)
 {
        struct l2cap_chan *chan = conn->smp;
@@ -2010,8 +2058,19 @@ static u8 smp_cmd_pairing_confirm(struct l2cap_conn *conn, struct sk_buff *skb)
        memcpy(smp->pcnf, skb->data, sizeof(smp->pcnf));
        skb_pull(skb, sizeof(smp->pcnf));
 
-       if (test_bit(SMP_FLAG_SC, &smp->flags))
-               return sc_check_confirm(smp);
+       if (test_bit(SMP_FLAG_SC, &smp->flags)) {
+               int ret;
+
+               /* Public Key exchange must happen before any other steps */
+               if (test_bit(SMP_FLAG_REMOTE_PK, &smp->flags))
+                       return sc_check_confirm(smp);
+
+               BT_ERR("Unexpected SMP Pairing Confirm");
+
+               ret = fixup_sc_false_positive(smp);
+               if (ret)
+                       return ret;
+       }
 
        if (conn->hcon->out) {
                smp_send_cmd(conn, SMP_CMD_PAIRING_RANDOM, sizeof(smp->prnd),