From: Johan Hedberg <johan.hedberg@xxxxxxxxx>
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@xxxxxxxxx>
---
net/bluetooth/smp.c | 75 +++++++++++++++++++++++++++++++++++++++++++++++------
1 file changed, 67 insertions(+), 8 deletions(-)
diff --git a/net/bluetooth/smp.c b/net/bluetooth/smp.c
index 9155840068cf..dc27bad535f2 100644
--- a/net/bluetooth/smp.c
+++ b/net/bluetooth/smp.c
@@ -818,6 +818,12 @@ 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);
@@ -1737,6 +1743,13 @@ 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 */
@@ -1745,8 +1758,6 @@ 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)
@@ -1885,10 +1896,6 @@ 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);
@@ -1901,6 +1908,47 @@ 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;
@@ -1914,8 +1962,19 @@ 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),
--
2.1.0
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