[RFC PATCH] Bluetooth: L2CAP: tolerate zero-ident LE credit indications

[Kyle Pepper <cochcoder@proton.me>]

A nonconforming ASHA peer sends identifier-zero credit indications
that are discarded, reducing usable transmit credits until audio stalls.

Tolerate only exact-size credit indications for a connected LE CoC.
Preserve the channel reference, lock and credit-overflow checks and do
not send a response with identifier zero. Other zero-ID commands and
BR/EDR signaling remain unchanged.

Core Vol 3 Part A section 4 forbids identifier zero. This is an
interoperability RFC requiring maintainer agreement, not a conformance
fix or a claim of automatic stream recovery.

Assisted-by: OpenCode:gpt-6.1-sol
Signed-off-by: Kyle Pepper <cochcoder@proton.me>
---
Submission review notes (not part of the commit message):
Core specification forbids identifier zero. This is a narrow opcode
interoperability exception, not a conformance fix. N8 captures contain
zero-ID credit indications. Native malformed/overflow/teardown and
lockdep/KASAN coverage and exception policy remain review gates.

 net/bluetooth/l2cap_core.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index 49a998804..1a0868437 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -5328,6 +5328,12 @@ static inline int l2cap_le_credits(struct l2cap_conn *conn,
 
 	l2cap_chan_lock(chan);
 
+	/* Tolerate identifier-zero credits only for a live LE CoC. */
+	if (!cmd->ident &&
+	    (!credits || chan->state != BT_CONNECTED ||
+	     chan->mode != L2CAP_MODE_LE_FLOWCTL))
+		goto unlock;
+
 	max_credits = LE_FLOWCTL_MAX_CREDITS - chan->tx_credits;
 	if (credits > max_credits) {
 		BT_ERR("LE credits overflow");
@@ -5923,11 +5929,21 @@ static inline void l2cap_le_sig_channel(struct l2cap_conn *conn,
 
 	BT_DBG("code 0x%2.2x len %d id 0x%2.2x", cmd->code, len, cmd->ident);
 
-	if (len != skb->len || !cmd->ident) {
+	if (len != skb->len) {
 		BT_DBG("corrupted command");
 		goto drop;
 	}
 
+	if (!cmd->ident) {
+		if (cmd->code != L2CAP_LE_CREDITS ||
+		    len != sizeof(struct l2cap_le_credits))
+			goto drop;
+
+		/* Credits have no response or identifier-matched transaction. */
+		l2cap_le_credits(conn, cmd, len, skb->data);
+		goto drop;
+	}
+
 	err = l2cap_le_sig_cmd(conn, cmd, len, skb->data);
 	if (err) {
 		struct l2cap_cmd_rej_unk rej;

base-commit: 964430c038967fd06643e866f68115664e1a1bd1