Integer underflow in IEEE 802.15.4 frame decryption leads to out-of-bounds read and write

ieee802154_decipher_data_frame() in subsys/net/l2/ieee802154/ieee802154_frame.c computed payload_len = net_pkt_get_len(pkt) - ll_hdr_len - authtag_len without first checking that the received frame is at least ll_hdr_len + authtag_len bytes long. All three variables are uint8_t, so a frame whose payload is shorter than the configured authentication tag makes the subtraction wrap around to a large value (up to 255). The wrapped length is passed unchanged to ieee802154_decrypt_auth() and on to the CCM operation as cipher_pkt.in_len/out_buf_max, with apkt->tag pointing at frame + ll_hdr_len + payload_len. Because the receive buffer is allocated to the exact length of the frame received from the radio driver, the crypto layer then reads several hundred bytes past the end of the packet buffer and writes the same number of decrypted bytes back over it in place. The frame's authentication tag is only verified after this processing has taken place, so no key material, association or prior authentication is needed — a single crafted short frame from any device in radio range is sufficient. Frame validation in ieee802154_validate_frame() does not prevent it: a data frame is accepted with a one-byte payload. The result is an out-of-bounds read and an out-of-bounds write of up to roughly 240 bytes into the adjacent network-buffer pool, corrupting other packets or allocator metadata and typically faulting the target. The out-of-bounds content is not attacker-chosen (it is ciphertext XOR keystream over out-of-bounds memory) and the frame is dropped when tag verification fails, so the primary impact is memory corruption and denial of service rather than information disclosure. Exposure is limited to configurations that enable the experimental CONFIG_NET_L2_IEEE802154_SECURITY option, select a crypto device via CONFIG_NET_L2_IEEE802154_SECURITY_CRYPTO_DEV_NAME, and have established a security session with a level other than IEEE802154_SECURITY_LEVEL_NONE; with security disabled or at level NONE the tag length is zero and no underflow occurs. The fix rejects frames shorter than ll_hdr_len + authtag_len before the subtraction, and adds the matching guard on the transmit side in ieee802154_create_data_frame().