crypto: adiantum - Drop support for asynchronous xchacha ciphers

This feature isn't useful in practice.  Simplify and streamline the code
in the synchronous case, i.e. the case that actually matters, instead.

For example, by no longer having to support resuming the calculation
after an asynchronous return of the xchacha cipher, we can just keep
more of the state on the stack instead of in the request context.

Link: https://lore.kernel.org/r/20251211011846.8179-10-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

Author: Eric Biggers

crypto/adiantum.c

@@ -83,29 +83,6 @@ struct nhpoly1305_ctx {
 };
 
 struct adiantum_request_ctx {
-
-	/*
-	 * Buffer for right-hand part of data, i.e.
-	 *
-	 *    P_L => P_M => C_M => C_R when encrypting, or
-	 *    C_R => C_M => P_M => P_L when decrypting.
-	 *
-	 * Also used to build the IV for the stream cipher.
-	 */
-	union {
-		u8 bytes[XCHACHA_IV_SIZE];
-		__le32 words[XCHACHA_IV_SIZE / sizeof(__le32)];
-		le128 bignum;	/* interpret as element of Z/(2^{128}Z) */
-	} rbuf;
-
-	bool enc; /* true if encrypting, false if decrypting */
-
-	/*
-	 * The result of the Poly1305 ε-∆U hash function applied to
-	 * (bulk length, tweak)
-	 */
-	le128 header_hash;
-
 	/*
 	 * skcipher sub-request size is unknown at compile-time, so it needs to
 	 * go after the members with known sizes.
@@ -216,7 +193,7 @@ static inline void le128_sub(le128 *r, const le128 *v1, const le128 *v2)
 
 /*
  * Apply the Poly1305 ε-∆U hash function to (bulk length, tweak) and save the
- * result to rctx->header_hash.  This is the calculation
+ * result to @out.  This is the calculation
  *
  *	H_T ← Poly1305_{K_T}(bin_{128}(|L|) || T)
  *
@@ -226,11 +203,10 @@ static inline void le128_sub(le128 *r, const le128 *v1, const le128 *v2)
  * inputs only) taken over the left-hand part (the "bulk") of the message, to
  * give the overall Adiantum hash of the (tweak, left-hand part) pair.
  */
-static void adiantum_hash_header(struct skcipher_request *req)
+static void adiantum_hash_header(struct skcipher_request *req, le128 *out)
 {
 	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
 	const struct adiantum_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
-	struct adiantum_request_ctx *rctx = skcipher_request_ctx(req);
 	const unsigned int bulk_len = req->cryptlen - BLOCKCIPHER_BLOCK_SIZE;
 	struct {
 		__le64 message_bits;
@@ -250,7 +226,7 @@ static void adiantum_hash_header(struct skcipher_request *req)
 	poly1305_core_blocks(&state, &tctx->header_hash_key, req->iv,
 			     TWEAK_SIZE / POLY1305_BLOCK_SIZE, 1);
 
-	poly1305_core_emit(&state, NULL, &rctx->header_hash);
+	poly1305_core_emit(&state, NULL, out);
 }
 
 /* Pass the next NH hash value through Poly1305 */
@@ -389,112 +365,69 @@ static void adiantum_hash_message(struct skcipher_request *req,
 	nhpoly1305_final(&rctx->u.hash_ctx, tctx, out);
 }
 
-/* Continue Adiantum encryption/decryption after the stream cipher step */
-static int adiantum_finish(struct skcipher_request *req)
+static int adiantum_crypt(struct skcipher_request *req, bool enc)
 {
 	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
 	const struct adiantum_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
 	struct adiantum_request_ctx *rctx = skcipher_request_ctx(req);
 	const unsigned int bulk_len = req->cryptlen - BLOCKCIPHER_BLOCK_SIZE;
-	struct scatterlist *dst = req->dst;
-	le128 digest;
-
-	/* If decrypting, decrypt C_M with the block cipher to get P_M */
-	if (!rctx->enc)
-		crypto_cipher_decrypt_one(tctx->blockcipher, rctx->rbuf.bytes,
-					  rctx->rbuf.bytes);
-
+	struct scatterlist *src = req->src, *dst = req->dst;
 	/*
-	 * Second hash step
-	 *	enc: C_R = C_M - H_{K_H}(T, C_L)
-	 *	dec: P_R = P_M - H_{K_H}(T, P_L)
+	 * Buffer for right-hand part of data, i.e.
+	 *
+	 *    P_L => P_M => C_M => C_R when encrypting, or
+	 *    C_R => C_M => P_M => P_L when decrypting.
+	 *
+	 * Also used to build the IV for the stream cipher.
 	 */
-	le128_sub(&rctx->rbuf.bignum, &rctx->rbuf.bignum, &rctx->header_hash);
-	if (dst->length >= req->cryptlen &&
-	    dst->offset + req->cryptlen <= PAGE_SIZE) {
-		/* Fast path for single-page destination */
-		struct page *page = sg_page(dst);
-		void *virt = kmap_local_page(page) + dst->offset;
-
-		nhpoly1305_init(&rctx->u.hash_ctx);
-		nhpoly1305_update(&rctx->u.hash_ctx, tctx, virt, bulk_len);
-		nhpoly1305_final(&rctx->u.hash_ctx, tctx, &digest);
-		le128_sub(&rctx->rbuf.bignum, &rctx->rbuf.bignum, &digest);
-		memcpy(virt + bulk_len, &rctx->rbuf.bignum, sizeof(le128));
-		flush_dcache_page(page);
-		kunmap_local(virt);
-	} else {
-		/* Slow path that works for any destination scatterlist */
-		adiantum_hash_message(req, dst, &digest);
-		le128_sub(&rctx->rbuf.bignum, &rctx->rbuf.bignum, &digest);
-		memcpy_to_sglist(dst, bulk_len, &rctx->rbuf.bignum,
-				 sizeof(le128));
-	}
-	return 0;
-}
-
-static void adiantum_streamcipher_done(void *data, int err)
-{
-	struct skcipher_request *req = data;
-
-	if (!err)
-		err = adiantum_finish(req);
-
-	skcipher_request_complete(req, err);
-}
-
-static int adiantum_crypt(struct skcipher_request *req, bool enc)
-{
-	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
-	const struct adiantum_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
-	struct adiantum_request_ctx *rctx = skcipher_request_ctx(req);
-	const unsigned int bulk_len = req->cryptlen - BLOCKCIPHER_BLOCK_SIZE;
-	struct scatterlist *src = req->src;
+	union {
+		u8 bytes[XCHACHA_IV_SIZE];
+		__le32 words[XCHACHA_IV_SIZE / sizeof(__le32)];
+		le128 bignum; /* interpret as element of Z/(2^{128}Z) */
+	} rbuf;
+	le128 header_hash, msg_hash;
 	unsigned int stream_len;
-	le128 digest;
+	int err;
 
 	if (req->cryptlen < BLOCKCIPHER_BLOCK_SIZE)
 		return -EINVAL;
 
-	rctx->enc = enc;
-
 	/*
 	 * First hash step
 	 *	enc: P_M = P_R + H_{K_H}(T, P_L)
 	 *	dec: C_M = C_R + H_{K_H}(T, C_L)
 	 */
-	adiantum_hash_header(req);
+	adiantum_hash_header(req, &header_hash);
 	if (src->length >= req->cryptlen &&
 	    src->offset + req->cryptlen <= PAGE_SIZE) {
 		/* Fast path for single-page source */
 		void *virt = kmap_local_page(sg_page(src)) + src->offset;
 
 		nhpoly1305_init(&rctx->u.hash_ctx);
 		nhpoly1305_update(&rctx->u.hash_ctx, tctx, virt, bulk_len);
-		nhpoly1305_final(&rctx->u.hash_ctx, tctx, &digest);
-		memcpy(&rctx->rbuf.bignum, virt + bulk_len, sizeof(le128));
+		nhpoly1305_final(&rctx->u.hash_ctx, tctx, &msg_hash);
+		memcpy(&rbuf.bignum, virt + bulk_len, sizeof(le128));
 		kunmap_local(virt);
 	} else {
 		/* Slow path that works for any source scatterlist */
-		adiantum_hash_message(req, src, &digest);
-		memcpy_from_sglist(&rctx->rbuf.bignum, src, bulk_len,
-				   sizeof(le128));
+		adiantum_hash_message(req, src, &msg_hash);
+		memcpy_from_sglist(&rbuf.bignum, src, bulk_len, sizeof(le128));
 	}
-	le128_add(&rctx->rbuf.bignum, &rctx->rbuf.bignum, &rctx->header_hash);
-	le128_add(&rctx->rbuf.bignum, &rctx->rbuf.bignum, &digest);
+	le128_add(&rbuf.bignum, &rbuf.bignum, &header_hash);
+	le128_add(&rbuf.bignum, &rbuf.bignum, &msg_hash);
 
 	/* If encrypting, encrypt P_M with the block cipher to get C_M */
 	if (enc)
-		crypto_cipher_encrypt_one(tctx->blockcipher, rctx->rbuf.bytes,
-					  rctx->rbuf.bytes);
+		crypto_cipher_encrypt_one(tctx->blockcipher, rbuf.bytes,
+					  rbuf.bytes);
 
 	/* Initialize the rest of the XChaCha IV (first part is C_M) */
 	BUILD_BUG_ON(BLOCKCIPHER_BLOCK_SIZE != 16);
 	BUILD_BUG_ON(XCHACHA_IV_SIZE != 32);	/* nonce || stream position */
-	rctx->rbuf.words[4] = cpu_to_le32(1);
-	rctx->rbuf.words[5] = 0;
-	rctx->rbuf.words[6] = 0;
-	rctx->rbuf.words[7] = 0;
+	rbuf.words[4] = cpu_to_le32(1);
+	rbuf.words[5] = 0;
+	rbuf.words[6] = 0;
+	rbuf.words[7] = 0;
 
 	/*
 	 * XChaCha needs to be done on all the data except the last 16 bytes;
@@ -511,12 +444,44 @@ static int adiantum_crypt(struct skcipher_request *req, bool enc)
 
 	skcipher_request_set_tfm(&rctx->u.streamcipher_req, tctx->streamcipher);
 	skcipher_request_set_crypt(&rctx->u.streamcipher_req, req->src,
-				   req->dst, stream_len, &rctx->rbuf);
+				   req->dst, stream_len, &rbuf);
 	skcipher_request_set_callback(&rctx->u.streamcipher_req,
-				      req->base.flags,
-				      adiantum_streamcipher_done, req);
-	return crypto_skcipher_encrypt(&rctx->u.streamcipher_req) ?:
-		adiantum_finish(req);
+				      req->base.flags, NULL, NULL);
+	err = crypto_skcipher_encrypt(&rctx->u.streamcipher_req);
+	if (err)
+		return err;
+
+	/* If decrypting, decrypt C_M with the block cipher to get P_M */
+	if (!enc)
+		crypto_cipher_decrypt_one(tctx->blockcipher, rbuf.bytes,
+					  rbuf.bytes);
+
+	/*
+	 * Second hash step
+	 *	enc: C_R = C_M - H_{K_H}(T, C_L)
+	 *	dec: P_R = P_M - H_{K_H}(T, P_L)
+	 */
+	le128_sub(&rbuf.bignum, &rbuf.bignum, &header_hash);
+	if (dst->length >= req->cryptlen &&
+	    dst->offset + req->cryptlen <= PAGE_SIZE) {
+		/* Fast path for single-page destination */
+		struct page *page = sg_page(dst);
+		void *virt = kmap_local_page(page) + dst->offset;
+
+		nhpoly1305_init(&rctx->u.hash_ctx);
+		nhpoly1305_update(&rctx->u.hash_ctx, tctx, virt, bulk_len);
+		nhpoly1305_final(&rctx->u.hash_ctx, tctx, &msg_hash);
+		le128_sub(&rbuf.bignum, &rbuf.bignum, &msg_hash);
+		memcpy(virt + bulk_len, &rbuf.bignum, sizeof(le128));
+		flush_dcache_page(page);
+		kunmap_local(virt);
+	} else {
+		/* Slow path that works for any destination scatterlist */
+		adiantum_hash_message(req, dst, &msg_hash);
+		le128_sub(&rbuf.bignum, &rbuf.bignum, &msg_hash);
+		memcpy_to_sglist(dst, bulk_len, &rbuf.bignum, sizeof(le128));
+	}
+	return 0;
 }
 
 static int adiantum_encrypt(struct skcipher_request *req)
@@ -624,7 +589,8 @@ static int adiantum_create(struct crypto_template *tmpl, struct rtattr **tb)
 	/* Stream cipher, e.g. "xchacha12" */
 	err = crypto_grab_skcipher(&ictx->streamcipher_spawn,
 				   skcipher_crypto_instance(inst),
-				   crypto_attr_alg_name(tb[1]), 0, mask);
+				   crypto_attr_alg_name(tb[1]), 0,
+				   mask | CRYPTO_ALG_ASYNC /* sync only */);
 	if (err)
 		goto err_free_inst;
 	streamcipher_alg = crypto_spawn_skcipher_alg_common(&ictx->streamcipher_spawn);