Re: [PATCH v5 29/39] arm64: Add helpers to probe local CPU for PAC and BTI support

[Marc Zyngier <maz@kernel.org>]

On Fri, 24 Nov 2023 10:19:09 +0000,
Ard Biesheuvel <ardb@google.com> wrote:
> 
> From: Ard Biesheuvel <ardb@kernel.org>
> 
> Add some helpers that will be used by the early kernel mapping code to
> check feature support on the local CPU. This permits the early kernel
> mapping to be created with the right attributes, removing the need for
> tearing it down and recreating it.
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  arch/arm64/include/asm/cpufeature.h | 44 ++++++++++++++++++++
>  1 file changed, 44 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 99c01417e544..301d4d2211d5 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -921,6 +921,50 @@ static inline bool kaslr_disabled_cmdline(void)
>  u32 get_kvm_ipa_limit(void);
>  void dump_cpu_features(void);
>  
> +static inline bool cpu_has_bti(void)
> +{
> +	u64 pfr1;
> +
> +	if (!IS_ENABLED(CONFIG_ARM64_BTI))
> +		return false;
> +
> +	pfr1 = read_cpuid(ID_AA64PFR1_EL1);
> +	pfr1 &= ~id_aa64pfr1_override.mask;
> +	pfr1 |= id_aa64pfr1_override.val;

There is a potential gotcha here. If the override failed because a
filter rejected it, val will be set to full ones for the size of the
failed override field, and the corresponding mask will be set to 0
(see match_options()).

A safer pattern would be:

	pfr1 |= id_aa64pfr1_override.val & id_aa64pfr1_override.mask;

although we currently don't have such a filter for BTI nor PAC, so the
code is OK for now.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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