[PATCH] Bluetooth: L2CAP: Reject signalling responses in invalid states

[Siwei Zhang <oss@fourdim.xyz>]

A peer can send configuration messages while an
incoming channel is in BT_CONNECT2, before the
security or deferred accept gate has completed. A
configuration response can then promote the channel
to BT_CONNECTED. Configuration responses can also
act on channels that are closing, and a late connection
response can move a disconnecting channel back to
BT_CONFIG.

Accept connection responses only while waiting in
BT_CONNECT. Answer configuration requests in
BT_CONNECT2, but defer our own request until
BT_CONFIG so configuration cannot complete before
the gate. Ignore configuration responses outside
BT_CONFIG and BT_CONNECTED, and only complete
configuration in BT_CONFIG.

Assisted-By: Claude:claude-opus-5-5
Signed-off-by: Siwei Zhang <oss@fourdim.xyz>
---
 net/bluetooth/l2cap_core.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index aaa2a1cd489a..d3450a90b4d2 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -4284,6 +4284,10 @@ static int l2cap_connect_create_rsp(struct l2cap_conn *conn,
 
 	l2cap_chan_lock(chan);
 
+	/* Ignore a late response after the channel has left BT_CONNECT. */
+	if (chan->state != BT_CONNECT)
+		goto done;
+
 	switch (result) {
 	case L2CAP_CR_SUCCESS:
 		if (__l2cap_get_chan_by_dcid(conn, dcid)) {
@@ -4313,6 +4317,7 @@ static int l2cap_connect_create_rsp(struct l2cap_conn *conn,
 		break;
 	}
 
+done:
 	l2cap_chan_unlock(chan);
 	l2cap_chan_put(chan);
 
@@ -4445,7 +4450,13 @@ static inline int l2cap_config_req(struct l2cap_conn *conn,
 		goto unlock;
 	}
 
-	if (!test_and_set_bit(CONF_REQ_SENT, &chan->conf_state)) {
+	/* A channel in BT_CONNECT2 has not passed security or userspace
+	 * accept yet. Answer the peer's request, but hold our own until
+	 * the channel reaches BT_CONFIG: the CONNECT2 -> CONFIG sites send
+	 * it. Connected channels may also need to reconfigure.
+	 */
+	if ((chan->state == BT_CONFIG || chan->state == BT_CONNECTED) &&
+	    !test_and_set_bit(CONF_REQ_SENT, &chan->conf_state)) {
 		u8 buf[64];
 		l2cap_send_cmd(conn, l2cap_get_ident(conn), L2CAP_CONF_REQ,
 			       l2cap_build_conf_req(chan, buf, sizeof(buf)), buf);
@@ -4493,6 +4504,10 @@ static inline int l2cap_config_rsp(struct l2cap_conn *conn,
 	if (!chan)
 		return 0;
 
+	/* Connected channels may also receive reconfiguration responses. */
+	if (chan->state != BT_CONFIG && chan->state != BT_CONNECTED)
+		goto done;
+
 	switch (result) {
 	case L2CAP_CONF_SUCCESS:
 		l2cap_conf_rfc_get(chan, rsp->data, len);
-- 
2.55.0