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Commit 50b6544e authored by Herbert Xu's avatar Herbert Xu
Browse files

[CRYPTO] cbc: Require block size to be a power of 2 



All common block ciphers have a block size that's a power of 2.  In fact,
all of our block ciphers obey this rule.

If we require this then CBC can be optimised to avoid an expensive divide
on in-place decryption.

I've also changed the saving of the first IV in the in-place decryption
case to the last IV because that lets us use walk->iv (which is already
aligned) for the xor operation where alignment is required.

Signed-off-by: default avatarHerbert Xu <herbert@gondor.apana.org.au>
parent 3c7f076d

crypto/cbc.c

+10 −8
Original line number Diff line number Diff line
@@ -14,6 +14,7 @@ 
#include <linux/err.h>

#include <linux/init.h>

#include <linux/kernel.h>

#include <linux/log2.h>

#include <linux/module.h>

#include <linux/scatterlist.h>

#include <linux/slab.h>
@@ -143,17 +144,13 @@ static int crypto_cbc_decrypt_inplace(struct blkcipher_desc *desc, 
	void (*fn)(struct crypto_tfm *, u8 *, const u8 *) =

		crypto_cipher_alg(tfm)->cia_decrypt;

	int bsize = crypto_cipher_blocksize(tfm);

	unsigned long alignmask = crypto_cipher_alignmask(tfm);

	unsigned int nbytes = walk->nbytes;

	u8 *src = walk->src.virt.addr;

	u8 stack[bsize + alignmask];

	u8 *first_iv = (u8 *)ALIGN((unsigned long)stack, alignmask + 1);



	memcpy(first_iv, walk->iv, bsize);

	u8 last_iv[bsize];



	/* Start of the last block. */

	src += nbytes - nbytes % bsize - bsize;

	memcpy(walk->iv, src, bsize);

	src += nbytes - (nbytes & (bsize - 1)) - bsize;

	memcpy(last_iv, src, bsize);



	for (;;) {

		fn(crypto_cipher_tfm(tfm), src, src);
@@ -163,7 +160,8 @@ static int crypto_cbc_decrypt_inplace(struct blkcipher_desc *desc, 
		src -= bsize;

	}



	crypto_xor(src, first_iv, bsize);

	crypto_xor(src, walk->iv, bsize);

	memcpy(walk->iv, last_iv, bsize);



	return nbytes;

}
@@ -228,6 +226,10 @@ static struct crypto_instance *crypto_cbc_alloc(struct rtattr **tb) 
	if (IS_ERR(alg))

		return ERR_PTR(PTR_ERR(alg));



	inst = ERR_PTR(-EINVAL);

	if (!is_power_of_2(alg->cra_blocksize))

		goto out_put_alg;



	inst = crypto_alloc_instance("cbc", alg);

	if (IS_ERR(inst))

		goto out_put_alg;