crypto: authenc - Fix zero-length IV crash

As it is if an algorithm with a zero-length IV is used (e.g.,
NULL encryption) with authenc, authenc may generate an SG entry
of length zero, which will trigger a BUG check in the hash layer.

This patch fixes it by skipping the IV SG generation if the IV
size is zero.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/authenc.c

@@ -158,16 +158,19 @@ static int crypto_authenc_genicv(struct aead_request *req, u8 *iv,
 	dstp = sg_page(dst);
 	vdst = PageHighMem(dstp) ? NULL : page_address(dstp) + dst->offset;
 
+	if (ivsize) {
 		sg_init_table(cipher, 2);
 		sg_set_buf(cipher, iv, ivsize);
 		authenc_chain(cipher, dst, vdst == iv + ivsize);
+		dst = cipher;
+	}
 
 	cryptlen = req->cryptlen + ivsize;
-	hash = crypto_authenc_hash(req, flags, cipher, cryptlen);
+	hash = crypto_authenc_hash(req, flags, dst, cryptlen);
 	if (IS_ERR(hash))
 		return PTR_ERR(hash);
 
-	scatterwalk_map_and_copy(hash, cipher, cryptlen,
+	scatterwalk_map_and_copy(hash, dst, cryptlen,
 				 crypto_aead_authsize(authenc), 1);
 	return 0;
 }
@@ -285,11 +288,14 @@ static int crypto_authenc_iverify(struct aead_request *req, u8 *iv,
 	srcp = sg_page(src);
 	vsrc = PageHighMem(srcp) ? NULL : page_address(srcp) + src->offset;
 
+	if (ivsize) {
 		sg_init_table(cipher, 2);
 		sg_set_buf(cipher, iv, ivsize);
 		authenc_chain(cipher, src, vsrc == iv + ivsize);
+		src = cipher;
+	}
 
-	return crypto_authenc_verify(req, cipher, cryptlen + ivsize);
+	return crypto_authenc_verify(req, src, cryptlen + ivsize);
 }
 
 static int crypto_authenc_decrypt(struct aead_request *req)