Bluetooth: L2CAP: Fix stack-out-of-bounds read in l2cap_ecred_conn_req

Syzbot reported a KASAN stack-out-of-bounds read in l2cap_build_cmd()
that is triggered by a malformed Enhanced Credit Based Connection Request.

The vulnerability stems from l2cap_ecred_conn_req(). The function allocates
a local stack buffer (`pdu`) designed to hold a maximum of 5 Source Channel
IDs (SCIDs), totaling 18 bytes. When an attacker sends a request with more
than 5 SCIDs, the function calculates `rsp_len` based on this unvalidated
`cmd_len` before checking if the number of SCIDs exceeds
L2CAP_ECRED_MAX_CID.

If the SCID count is too high, the function correctly jumps to the
`response` label to reject the packet, but `rsp_len` retains the
attacker's oversized value. Consequently, l2cap_send_cmd() is instructed
to read past the end of the 18-byte `pdu` buffer, triggering a
KASAN panic.

Fix this by moving the assignment of `rsp_len` to after the `num_scid`
boundary check. If the packet is rejected, `rsp_len` will safely
remain 0, and the error response will only read the 8-byte base header
from the stack.

Fixes: c28d2bf ("Bluetooth: L2CAP: Fix result of L2CAP_ECRED_CONN_RSP when MTU is too short")
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=b7f3e7d9a596bf6a63e3
Tested-by: [email protected]
Signed-off-by: Minseo Park <[email protected]>
Signed-off-by: Luiz Augusto von Dentz <[email protected]>

Author: Minseo Park

net/bluetooth/l2cap_core.c

@@ -5081,14 +5081,14 @@ static inline int l2cap_ecred_conn_req(struct l2cap_conn *conn,
 	cmd_len -= sizeof(*req);
 	num_scid = cmd_len / sizeof(u16);
 
-	/* Always respond with the same number of scids as in the request */
-	rsp_len = cmd_len;
-
 	if (num_scid > L2CAP_ECRED_MAX_CID) {
 		result = L2CAP_CR_LE_INVALID_PARAMS;
 		goto response;
 	}
 
+	/* Always respond with the same number of scids as in the request */
+	rsp_len = cmd_len;
+
 	mtu  = __le16_to_cpu(req->mtu);
 	mps  = __le16_to_cpu(req->mps);