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Date:   Wed, 16 Nov 2022 12:30:29 +0100
From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
To:     Robert Elliott <elliott@hpe.com>
Cc:     herbert@gondor.apana.org.au, davem@davemloft.net,
        tim.c.chen@linux.intel.com, ap420073@gmail.com, ardb@kernel.org,
        David.Laight@ACULAB.COM, ebiggers@kernel.org,
        linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 20/24] crypto: x86/ciphers - load based on CPU features
Message-ID: <Y3TJ1e3FzsO5Cae8@zx2c4.com>
References: <20221103042740.6556-1-elliott@hpe.com>
 <20221116041342.3841-1-elliott@hpe.com>
 <20221116041342.3841-21-elliott@hpe.com>
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On Tue, Nov 15, 2022 at 10:13:38PM -0600, Robert Elliott wrote:
> diff --git a/arch/x86/crypto/curve25519-x86_64.c b/arch/x86/crypto/curve25519-x86_64.c
> index d55fa9e9b9e6..ae7536b17bf9 100644
> --- a/arch/x86/crypto/curve25519-x86_64.c
> +++ b/arch/x86/crypto/curve25519-x86_64.c
> @@ -12,7 +12,7 @@
>  #include <linux/kernel.h>
>  #include <linux/module.h>
>  #include <linux/scatterlist.h>
> -
> +#include <asm/cpu_device_id.h>
>  #include <asm/cpufeature.h>
>  #include <asm/processor.h>
>  
> @@ -1697,13 +1697,22 @@ static struct kpp_alg curve25519_alg = {
>  	.max_size		= curve25519_max_size,
>  };
>  
> +static const struct x86_cpu_id module_cpu_ids[] = {
> +	X86_MATCH_FEATURE(X86_FEATURE_ADX, NULL),
> +	{}
> +};
> +MODULE_DEVICE_TABLE(x86cpu, module_cpu_ids);
>  
>  static int __init curve25519_mod_init(void)
>  {
> -	if (boot_cpu_has(X86_FEATURE_BMI2) && boot_cpu_has(X86_FEATURE_ADX))
> -		static_branch_enable(&curve25519_use_bmi2_adx);
> -	else
> -		return 0;
> +	if (!x86_match_cpu(module_cpu_ids))
> +		return -ENODEV;
> +
> +	if (!boot_cpu_has(X86_FEATURE_BMI2))
> +		return -ENODEV;
> +
> +	static_branch_enable(&curve25519_use_bmi2_adx);

Can the user still insmod this? If so, you can't remove the ADX check.
Ditto for rest of patch.