crypto: authencesn - reject too-short AAD (assoclen<8) to match ESP/ESN spec

authencesn assumes an ESP/ESN-formatted AAD. When assoclen is shorter than
the minimum expected length, crypto_authenc_esn_decrypt() can advance past
the end of the destination scatterlist and trigger a NULL pointer dereference
in scatterwalk_map_and_copy(), leading to a kernel panic (DoS).

Add a minimum AAD length check to fail fast on invalid inputs.

Fixes: 104880a ("crypto: authencesn - Convert to new AEAD interface")
Reported-By: Taeyang Lee <0wn@theori.io>
Signed-off-by: Taeyang Lee <0wn@theori.io>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Author: 5unkn0wn-Theori

crypto/authencesn.c

@@ -169,6 +169,9 @@ static int crypto_authenc_esn_encrypt(struct aead_request *req)
 	struct scatterlist *src, *dst;
 	int err;
 
+	if (assoclen < 8)
+		return -EINVAL;
+
 	sg_init_table(areq_ctx->src, 2);
 	src = scatterwalk_ffwd(areq_ctx->src, req->src, assoclen);
 	dst = src;
@@ -256,6 +259,9 @@ static int crypto_authenc_esn_decrypt(struct aead_request *req)
 	u32 tmp[2];
 	int err;
 
+	if (assoclen < 8)
+		return -EINVAL;
+
 	cryptlen -= authsize;
 
 	if (req->src != dst)