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

[Eric Biggers <ebiggers@kernel.org>]

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.

> +/*
> + * 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?

> +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?

> +	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);

- Eric