diff --git a/include/net/bluetooth/l2cap.h b/include/net/bluetooth/l2cap.h
index 5172afee5494..e0a1f2293679 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 45b175399e8d..c4ccfbda9d78 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -5643,6 +5643,15 @@ 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 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)
 {
@@ -5655,6 +5664,43 @@ static inline void l2cap_sig_channel(struct l2cap_conn *conn,
 	if (hcon->type != ACL_LINK)
 		goto drop;
 
+	/*
+	 * Bluetooth Core v5.4, Vol 3, Part A, Section 4: the BR/EDR
+	 * signaling channel has a fixed signaling MTU (MTUsig) whose
+	 * minimum and default is 48 octets.  Section 4.1 says that on
+	 * an MTUExceeded command reject the identifier "shall match
+	 * the first request command in the L2CAP packet" and that
+	 * packets containing only response commands "shall be
+	 * silently discarded".
+	 *
+	 * Linux intentionally deviates from that prescription:
+	 *
+	 *   1. Silently discarding desynchronizes the peer.  The
+	 *      remote stack never learns its responses were dropped,
+	 *      so any state machine waiting on a paired response
+	 *      stalls until its own timer fires.
+	 *
+	 *   2. Locating "the first request command" requires walking
+	 *      command headers past MTUsig, i.e. processing bytes
+	 *      from a packet we have already decided is too large to
+	 *      process.
+	 *
+	 * Reject every over-MTUsig signaling packet with one
+	 * L2CAP_REJ_MTU_EXCEEDED command reject.  The reject's
+	 * reason field is what tells the peer that the whole packet
+	 * was discarded; the identifier value is informational, so
+	 * we use the identifier from the first command header, a
+	 * single fixed-offset byte read.
+	 */
+	if (skb->len > L2CAP_SIG_MTU) {
+		u8 ident = skb->data[1];
+
+		BT_DBG("signaling packet exceeds MTU: %u > %u",
+		       skb->len, L2CAP_SIG_MTU);
+		l2cap_sig_send_mtu_rej(conn, ident);
+		goto drop;
+	}
+
 	while (skb->len >= L2CAP_CMD_HDR_SIZE) {
 		u16 len;