[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: Herbert Xu <[email protected]>
---
crypto/cbc.c | 18 ++++++++++--------
1 files changed, 10 insertions(+), 8 deletions(-)
diff --git a/crypto/cbc.c b/crypto/cbc.c
index b013d6f..6affff8 100644
--- a/crypto/cbc.c
+++ b/crypto/cbc.c
@@ -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;