mac802154: llsec: reject frames shorter than the authentication tag

mac802154: llsec: reject frames shorter than the authentication tag

llsec_do_decrypt_auth() computes the associated-data length for the
AEAD request as

	assoclen += datalen - authlen;

where datalen is the number of bytes after the MAC header and authlen
(4, 8 or 16) is the length of the authentication tag. Nothing verifies
that the frame actually carries at least authlen payload bytes. A
secured frame whose payload is shorter than the tag makes
datalen - authlen negative; assoclen is then passed to
aead_request_set_ad() as an unsigned value close to 4 GiB, so
crypto_aead_decrypt() walks far off the end of the scatterlist that
only spans the real frame.

The frame is fully attacker-controlled and reaches this path from any
IEEE 802.15.4 peer in radio range. Reject frames whose payload is
shorter than the authentication tag before the subtraction.

Dynamically reproduced on a KASAN kernel as a general-protection-fault
in the AEAD scatterwalk, and the fix confirmed.

Fixes: 4c14a2fb5d14 ("mac802154: add llsec decryption method")
Cc: stable@vger.kernel.org
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Doruk Tan Ozturk <doruk@0sec.ai>
Link: https://patch.msgid.link/20260716193423.32498-1-doruk@0sec.ai
Signed-off-by: Jakub Kicinski <kuba@kernel.org>