[dm-devel] [PATCH v7 12/12] crypto: x86/aes-kl - Implement the AES-XTS algorithm

[Chang S. Bae]

On 5/26/2023 12:23 AM, Eric Biggers wrote:
> On Wed, May 24, 2023 at 09:57:17AM -0700, Chang S. Bae wrote:
>> == API Limitation ==
>>
>> The setkey() function transforms an AES key into a handle. But, an
>> extended key is a usual outcome of setkey() in other AES cipher
>> implementations. For this reason, a setkey() failure does not fall
>> back to the other. So, expose AES-KL methods via synchronous
>> interfaces only.
> 
> I don't understand what this paragraph is trying to say.

This text comes with this particular comment as I look back:

 > This basically implies that we cannot expose the cipher interface at
 > all, and so AES-KL can only be used by callers that use the
 > asynchronous interface, which rules out 802.11, s/w kTLS, macsec and
 > kerberos.

https://lore.kernel.org/lkml/CAMj1kXGa4f21eH0mdxd1pQsZMUjUr1Btq+Dgw-gC=O-yYft7xw@mail.gmail.com/

Then, I realize that at that moment the dm-crypt use model was not 
clearly out yet.

This seems to be carried over the versions. But, now, it has XTS only. 
Then, this becomes less relevant which makes confusion I guess.

I think this can go away as claiming the usage clearly now.

> 
>> +/*
>> + * The below wrappers for the encryption/decryption functions
>> + * incorporate the feature availability check:
>> + *
>> + * In the rare event of hardware failure, the wrapping key can be lost
>> + * after wake-up from a deep sleep state. Then, this check helps to
>> + * avoid any subsequent misuse with populating a proper error code.
>> + */
>> +
>> +static inline int aeskl_enc(const void *ctx, u8 *out, const u8 *in)
>> +{
>> +	if (!valid_keylocker())
>> +		return -ENODEV;
>> +
>> +	return __aeskl_enc(ctx, out, in);
>> +}
> 
> Is it not sufficient for the valid_keylocker() check to occur at the top level
> (in xts_encrypt() and xts_decrypt()), which would seem to be a better place to
> do it?  Is this because valid_keylocker() needs to be checked in every
> kernel_fpu_begin() section separately, to avoid a race condition?  If that's
> indeed the reason, can you explain that in the comment?

Maybe something like this:

/*
  * In the event of hardware failure, the wrapping key can be lost
  * from a sleep state. Then, the feature is not usable anymore. This
  * feature state can be found via valid_keylocker().
  *
  * Such disabling could be anywhere preemptible, outside
  * kernel_fpu_begin()/end(). So, to avoid the race condition, check
  * the feature availability on every use in the below wrappers.
  */

> 
>> +static inline int xts_keylen(struct skcipher_request *req, u32 *keylen)
>> +{
>> +	struct aes_xts_ctx *ctx = aes_xts_ctx(crypto_skcipher_reqtfm(req));
>> +
>> +	if (ctx->crypt_ctx.key_length != ctx->tweak_ctx.key_length)
>> +		return -EINVAL;
>> +
>> +	*keylen = ctx->crypt_ctx.key_length;
>> +	return 0;
>> +}
> 
> This is odd.  Why would the key lengths be different here?

I thought it was logical to do such sanity check. But, in practice, they 
are the same.

Looks like this entire crypto code is treated as performance-critical or so.

> 
>> +	err = simd_register_skciphers_compat(aeskl_skciphers, ARRAY_SIZE(aeskl_skciphers),
>> +					     aeskl_simd_skciphers);
>> +	if (err)
>> +		return err;
>> +
>> +	return 0;
> 
> This can be simplified to:
> 
> 	return simd_register_skciphers_compat(aeskl_skciphers,
> 					      ARRAY_SIZE(aeskl_skciphers),
> 					      aeskl_simd_skciphers);

Oh, obviously!

Thanks,
Chang