crypto: ctr - add helper for performing a CTR encryption walk

Add a static inline helper modeled after crypto_cbc_encrypt_walk()
that can be reused for SIMD algorithms that need to implement a
non-SIMD fallback for performing CTR encryption.
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

include/crypto/ctr.h

@@ -8,8 +8,58 @@
 #ifndef _CRYPTO_CTR_H
 #define _CRYPTO_CTR_H
 
+#include <crypto/algapi.h>
+#include <crypto/internal/skcipher.h>
+#include <linux/string.h>
+#include <linux/types.h>
+
 #define CTR_RFC3686_NONCE_SIZE 4
 #define CTR_RFC3686_IV_SIZE 8
 #define CTR_RFC3686_BLOCK_SIZE 16
 
+static inline int crypto_ctr_encrypt_walk(struct skcipher_request *req,
+					  void (*fn)(struct crypto_skcipher *,
+						     const u8 *, u8 *))
+{
+	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
+	int blocksize = crypto_skcipher_chunksize(tfm);
+	u8 buf[MAX_CIPHER_BLOCKSIZE];
+	struct skcipher_walk walk;
+	int err;
+
+	/* avoid integer division due to variable blocksize parameter */
+	if (WARN_ON_ONCE(!is_power_of_2(blocksize)))
+		return -EINVAL;
+
+	err = skcipher_walk_virt(&walk, req, false);
+
+	while (walk.nbytes > 0) {
+		u8 *dst = walk.dst.virt.addr;
+		u8 *src = walk.src.virt.addr;
+		int nbytes = walk.nbytes;
+		int tail = 0;
+
+		if (nbytes < walk.total) {
+			tail = walk.nbytes & (blocksize - 1);
+			nbytes -= tail;
+		}
+
+		do {
+			int bsize = min(nbytes, blocksize);
+
+			fn(tfm, walk.iv, buf);
+
+			crypto_xor_cpy(dst, src, buf, bsize);
+			crypto_inc(walk.iv, blocksize);
+
+			dst += bsize;
+			src += bsize;
+			nbytes -= bsize;
+		} while (nbytes > 0);
+
+		err = skcipher_walk_done(&walk, tail);
+	}
+	return err;
+}
+
 #endif  /* _CRYPTO_CTR_H */