Subject: Re: [PATCH v4 2/4] arm64: cpufeature: Modify address authentication cpufeature to exact

From: Dave Martin <[email-masked]>
Date: Wed, 29 Jul 2020 11:36:48 +0100

On Fri, Jul 10, 2020 at 01:30:08PM +0530, Amit Daniel Kachhap wrote:
> The current address authentication cpufeature levels are set as LOWER_SAFE
> which is not compatible with the different configurations added for Armv8.3
> ptrauth enhancements as the different levels have different behaviour and
> there is no tunable to enable the lower safe versions. This is rectified
> by setting those cpufeature type as EXACT.
> 
> The current cpufeature framework also does not interfere in the booting of
> non-exact secondary cpus but rather marks them as tainted. As a workaround
> this is fixed by replacing the generic match handler with a new handler
> specific to ptrauth.
> 
> After this change, if there is any variation in ptrauth configurations in
> secondary cpus from boot cpu then those mismatched cpus are parked in an
> infinite loop.
> 
> Signed-off-by: Amit Daniel Kachhap <[email-masked]>
> [Suzuki: Introduce new matching function for address authentication]
> Suggested-by: Suzuki K Poulose <[email-masked]>
> ---
> Changes since v3:
>  * Commit logs cleanup.
> 
>  arch/arm64/kernel/cpufeature.c | 56 ++++++++++++++++++++++++++++------
>  1 file changed, 47 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 9fae0efc80c1..8ac8c18f70c9 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -210,9 +210,9 @@ static const struct arm64_ftr_bits ftr_id_aa64isar1[] = {
>  	ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64ISAR1_FCMA_SHIFT, 4, 0),
>  	ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64ISAR1_JSCVT_SHIFT, 4, 0),
>  	ARM64_FTR_BITS(FTR_VISIBLE_IF_IS_ENABLED(CONFIG_ARM64_PTR_AUTH),
> -		       FTR_STRICT, FTR_LOWER_SAFE, ID_AA64ISAR1_API_SHIFT, 4, 0),
> +		       FTR_STRICT, FTR_EXACT, ID_AA64ISAR1_API_SHIFT, 4, 0),
>  	ARM64_FTR_BITS(FTR_VISIBLE_IF_IS_ENABLED(CONFIG_ARM64_PTR_AUTH),
> -		       FTR_STRICT, FTR_LOWER_SAFE, ID_AA64ISAR1_APA_SHIFT, 4, 0),
> +		       FTR_STRICT, FTR_EXACT, ID_AA64ISAR1_APA_SHIFT, 4, 0),
>  	ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64ISAR1_DPB_SHIFT, 4, 0),
>  	ARM64_FTR_END,
>  };
> @@ -1605,11 +1605,49 @@ static void cpu_clear_disr(const struct arm64_cpu_capabilities *__unused)
>  #endif /* CONFIG_ARM64_RAS_EXTN */
>  
>  #ifdef CONFIG_ARM64_PTR_AUTH
> -static bool has_address_auth(const struct arm64_cpu_capabilities *entry,
> -			     int __unused)
> +static bool has_address_auth_cpucap(const struct arm64_cpu_capabilities *entry, int scope)
>  {
> -	return __system_matches_cap(ARM64_HAS_ADDRESS_AUTH_ARCH) ||
> -	       __system_matches_cap(ARM64_HAS_ADDRESS_AUTH_IMP_DEF);
> +	int local_cpu_auth;
> +	static int boot_cpu_auth_arch;
> +	static int boot_cpu_auth_imp_def;

I guess having static variables here is probably safe, provided that
calls to this function are strictly serialised with sufficiently
heavyweight synchronisation.

Might be worth a comment.

Coming up with a cleaner approach using locks or atomics might be
overkill, but other folks might take a stronger view on this.

> +
> +	/* We don't expect to be called with SCOPE_SYSTEM */
> +	WARN_ON(scope == SCOPE_SYSTEM);
> +
> +	local_cpu_auth = cpuid_feature_extract_field(__read_sysreg_by_encoding(entry->sys_reg),
> +						     entry->field_pos, entry->sign);
> +	/*
> +	 * The ptr-auth feature levels are not intercompatible with
> +	 * lower levels. Hence we must match all the CPUs with that
> +	 * of the boot CPU. So cache the level of boot CPU and compare
> +	 * it against the secondary CPUs.
> +	 */
> +	if (scope & SCOPE_BOOT_CPU) {
> +		if (entry->capability == ARM64_HAS_ADDRESS_AUTH_IMP_DEF) {
> +			boot_cpu_auth_imp_def = local_cpu_auth;
> +			return boot_cpu_auth_imp_def >= entry->min_field_value;
> +		} else if (entry->capability == ARM64_HAS_ADDRESS_AUTH_ARCH) {
> +			boot_cpu_auth_arch = local_cpu_auth;
> +			return boot_cpu_auth_arch >= entry->min_field_value;
> +		}
> +	} else if (scope & SCOPE_LOCAL_CPU) {
> +		if (entry->capability == ARM64_HAS_ADDRESS_AUTH_IMP_DEF) {
> +			return (local_cpu_auth >= entry->min_field_value) &&
> +			       (local_cpu_auth == boot_cpu_auth_imp_def);
> +		}
> +		if (entry->capability == ARM64_HAS_ADDRESS_AUTH_ARCH) {
> +			return (local_cpu_auth >= entry->min_field_value) &&
> +			       (local_cpu_auth == boot_cpu_auth_arch);
> +		}
> +	}

This looks complex.  I guess this is something to do with the change to
FTR_EXACT, but won't the cpufeatures code do the right thing anyway with
mismatched features?

I may be missing something here.

> +	return false;
> +}
> +
> +static bool has_address_auth_metacap(const struct arm64_cpu_capabilities *entry,
> +			     int scope)
> +{
> +	return has_address_auth_cpucap(cpu_hwcaps_ptrs[ARM64_HAS_ADDRESS_AUTH_ARCH], scope) ||
> +	       has_address_auth_cpucap(cpu_hwcaps_ptrs[ARM64_HAS_ADDRESS_AUTH_IMP_DEF], scope);
>  }
>  
>  static bool has_generic_auth(const struct arm64_cpu_capabilities *entry,
> @@ -1958,7 +1996,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  		.sign = FTR_UNSIGNED,
>  		.field_pos = ID_AA64ISAR1_APA_SHIFT,
>  		.min_field_value = ID_AA64ISAR1_APA_ARCHITECTED,
> -		.matches = has_cpuid_feature,
> +		.matches = has_address_auth_cpucap,
>  	},
>  	{
>  		.desc = "Address authentication (IMP DEF algorithm)",
> @@ -1968,12 +2006,12 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  		.sign = FTR_UNSIGNED,
>  		.field_pos = ID_AA64ISAR1_API_SHIFT,
>  		.min_field_value = ID_AA64ISAR1_API_IMP_DEF,
> -		.matches = has_cpuid_feature,
> +		.matches = has_address_auth_cpucap,
>  	},
>  	{
>  		.capability = ARM64_HAS_ADDRESS_AUTH,
>  		.type = ARM64_CPUCAP_BOOT_CPU_FEATURE,
> -		.matches = has_address_auth,
> +		.matches = has_address_auth_metacap,
>  	},
>  	{
>  		.desc = "Generic authentication (architected algorithm)",

[...]

Cheers
---Dave

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