Re: [PATCH v2 05/13] crypto: simd - Update `walksize` in simd skcipher

Jerry Shih
2023-11-28T05:38:45Z

On Nov 28, 2023, at 11:58, Eric Biggers <ebiggers@kernel.org> wrote:
> On Mon, Nov 27, 2023 at 03:06:55PM +0800, Jerry Shih wrote:
>> The `walksize` assignment is missed in simd skcipher.
>>
>> Signed-off-by: Jerry Shih <jerry.shih@sifive.com>
>> ---
>> crypto/cryptd.c | 1 +
>> crypto/simd.c | 1 +
>> 2 files changed, 2 insertions(+)
>>
>> diff --git a/crypto/cryptd.c b/crypto/cryptd.c
>> index bbcc368b6a55..253d13504ccb 100644
>> --- a/crypto/cryptd.c
>> +++ b/crypto/cryptd.c
>> @@ -405,6 +405,7 @@ static int cryptd_create_skcipher(struct crypto_template *tmpl,
>> (alg->base.cra_flags & CRYPTO_ALG_INTERNAL);
>> inst->alg.ivsize = crypto_skcipher_alg_ivsize(alg);
>> inst->alg.chunksize = crypto_skcipher_alg_chunksize(alg);
>> + inst->alg.walksize = crypto_skcipher_alg_walksize(alg);
>> inst->alg.min_keysize = crypto_skcipher_alg_min_keysize(alg);
>> inst->alg.max_keysize = crypto_skcipher_alg_max_keysize(alg);
>>
>> diff --git a/crypto/simd.c b/crypto/simd.c
>> index edaa479a1ec5..ea0caabf90f1 100644
>> --- a/crypto/simd.c
>> +++ b/crypto/simd.c
>> @@ -181,6 +181,7 @@ struct simd_skcipher_alg *simd_skcipher_create_compat(const char *algname,
>>
>> alg->ivsize = ialg->ivsize;
>> alg->chunksize = ialg->chunksize;
>> + alg->walksize = ialg->walksize;
>> alg->min_keysize = ialg->min_keysize;
>> alg->max_keysize = ialg->max_keysize;
>
> What are the consequences of this bug? I wonder if it actually matters? The
> "inner" algorithm is the one that actually gets used for the "walk", right?
>
> - Eric

Without this, we might still use chunksize or cra_blocksize as the walksize
even though we setup with the larger walksize.

Here is the code for the walksize default value:
static int skcipher_prepare_alg(struct skcipher_alg *alg)
{
...
if (!alg->chunksize)
alg->chunksize = base->cra_blocksize;
if (!alg->walksize)
alg->walksize = alg->chunksize;

And we already have the bigger walksize for x86 aes-xts.
.base = {
.cra_name = "__xts(aes)",
...
},
.walksize = 2 * AES_BLOCK_SIZE,

The x86 aes-xts only uses one `walk` to handle the tail elements. It assumes
that the walksize contains 2 aes blocks. If walksize is not set correctly, maybe
some tail elements is not processed in simd-cipher mode for x86 aes-xts.

-Jerry