[PATCH v2] Bluetooth: L2CAP: reject BR/EDR signaling packets over MTUsig

[Michael Bommarito <michael.bommarito@gmail.com>]

net/bluetooth/l2cap_core.c:l2cap_sig_channel() accepts BR/EDR
signaling packets up to the channel MTU and dispatches each command
without enforcing the signaling MTU (MTUsig). A Bluetooth BR/EDR peer
within radio range can send a fixed-channel CID 0x0001 packet that is
larger than MTUsig and contains many L2CAP_ECHO_REQ commands before
pairing.

In a real-radio stock-kernel run, one 681-byte signaling
packet containing 168 zero-length ECHO_REQ commands made the target
transmit 168 ECHO_RSP frames over about 220 ms.

Define Linux's BR/EDR signaling MTU as the spec minimum of 48 bytes and
reject larger signaling packets before dispatching their commands. When
the over-MTUsig packet contains a request command, send one
L2CAP_COMMAND_REJECT_RSP with L2CAP_REJ_MTU_EXCEEDED and the first
request identifier; packets for which no valid request command is found
are dropped.

Cc: stable@vger.kernel.org
Suggested-by: Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Link: https://lore.kernel.org/r/20260518002800.1361430-1-michael.bommarito@gmail.com
Assisted-by: Claude:claude-opus-4-7
Assisted-by: Codex:gpt-5-5-xhigh
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
I reproduced the stock behavior with a real-radio BR/EDR ACL link and a
harness that sends a single fixed-channel signaling packet containing
packed zero-length ECHO_REQ commands. The patched code builds for
net/bluetooth/l2cap_core.o on x86_64 defconfig. There are no in-tree
Bluetooth selftests that reference l2cap_sig_channel(), L2CAP_SIG_MTU,
or L2CAP_ECHO_REQ.

The unrestricted BR/EDR signaling parser and ECHO_REQ response path both
trace to the initial git import; no later introducing commit is
available for a Fixes tag.

Changes in v2:
- Replace the per-PDU echo-count cap with the MTUsig direction from
  review.
- Reject the whole over-MTUsig signaling packet with one
  L2CAP_REJ_MTU_EXCEEDED command reject.
- Add L2CAP_SIG_MTU and drop over-MTUsig packets when no valid request
  command identifier is found.

v1: https://lore.kernel.org/r/20260518002800.1361430-1-michael.bommarito@gmail.com
---
 include/net/bluetooth/l2cap.h |  1 +
 net/bluetooth/l2cap_core.c    | 60 +++++++++++++++++++++++++++++++++++
 2 files changed, 61 insertions(+)

diff --git a/include/net/bluetooth/l2cap.h b/include/net/bluetooth/l2cap.h
index 5172afee54943..e0a1f2293679a 100644
--- a/include/net/bluetooth/l2cap.h
+++ b/include/net/bluetooth/l2cap.h
@@ -33,6 +33,7 @@
 /* L2CAP defaults */
 #define L2CAP_DEFAULT_MTU		672
 #define L2CAP_DEFAULT_MIN_MTU		48
+#define L2CAP_SIG_MTU			48	/* BR/EDR signaling MTU */
 #define L2CAP_DEFAULT_FLUSH_TO		0xFFFF
 #define L2CAP_EFS_DEFAULT_FLUSH_TO	0xFFFFFFFF
 #define L2CAP_DEFAULT_TX_WINDOW		63
diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index 77dec104a9c36..5417e3cb0636d 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -5626,6 +5626,55 @@ static inline void l2cap_sig_send_rej(struct l2cap_conn *conn, u16 ident)
 	l2cap_send_cmd(conn, ident, L2CAP_COMMAND_REJ, sizeof(rej), &rej);
 }
 
+static bool l2cap_sig_cmd_is_req(u8 code)
+{
+	switch (code) {
+	case L2CAP_CONN_REQ:
+	case L2CAP_CONF_REQ:
+	case L2CAP_DISCONN_REQ:
+	case L2CAP_ECHO_REQ:
+	case L2CAP_INFO_REQ:
+	case L2CAP_CONN_PARAM_UPDATE_REQ:
+	case L2CAP_LE_CONN_REQ:
+	case L2CAP_ECRED_CONN_REQ:
+	case L2CAP_ECRED_RECONF_REQ:
+		return true;
+	}
+
+	return false;
+}
+
+static u8 l2cap_sig_first_req_ident(const struct sk_buff *skb)
+{
+	const u8 *data = skb->data;
+	unsigned int len = skb->len;
+
+	while (len >= L2CAP_CMD_HDR_SIZE) {
+		const struct l2cap_cmd_hdr *cmd = (const void *)data;
+		u16 cmd_len = le16_to_cpu(cmd->len);
+
+		if (cmd->ident && l2cap_sig_cmd_is_req(cmd->code))
+			return cmd->ident;
+
+		if (cmd_len > len - L2CAP_CMD_HDR_SIZE)
+			break;
+
+		data += L2CAP_CMD_HDR_SIZE + cmd_len;
+		len -= L2CAP_CMD_HDR_SIZE + cmd_len;
+	}
+
+	return 0;
+}
+
+static inline void l2cap_sig_send_mtu_rej(struct l2cap_conn *conn, u8 ident)
+{
+	struct l2cap_cmd_rej_mtu rej;
+
+	rej.reason = cpu_to_le16(L2CAP_REJ_MTU_EXCEEDED);
+	rej.max_mtu = cpu_to_le16(L2CAP_SIG_MTU);
+	l2cap_send_cmd(conn, ident, L2CAP_COMMAND_REJ, sizeof(rej), &rej);
+}
+
 static inline void l2cap_sig_channel(struct l2cap_conn *conn,
 				     struct sk_buff *skb)
 {
@@ -5638,6 +5687,17 @@ static inline void l2cap_sig_channel(struct l2cap_conn *conn,
 	if (hcon->type != ACL_LINK)
 		goto drop;
 
+	if (skb->len > L2CAP_SIG_MTU) {
+		u8 ident = l2cap_sig_first_req_ident(skb);
+
+		BT_DBG("signaling packet exceeds MTU");
+
+		if (ident)
+			l2cap_sig_send_mtu_rej(conn, ident);
+
+		goto drop;
+	}
+
 	while (skb->len >= L2CAP_CMD_HDR_SIZE) {
 		u16 len;
 
-- 
2.53.0