linux-kernel - Re: [PATCH 1/6] arm64: kpti: move check for non-vulnerable CPUs to a function

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Message-ID: <1c6081ca-8335-759f-3f49-f8fa60b2fb90@arm.com>
Date:   Thu, 13 Dec 2018 09:13:36 +0000
From:   Julien Thierry <julien.thierry@....com>
To:     Jeremy Linton <jeremy.linton@....com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     catalin.marinas@....com, will.deacon@....com, marc.zyngier@....com,
        suzuki.poulose@....com, dave.martin@....com,
        shankerd@...eaurora.org, mark.rutland@....com,
        linux-kernel@...r.kernel.org, ykaukab@...e.de
Subject: Re: [PATCH 1/6] arm64: kpti: move check for non-vulnerable CPUs to a
 function

Hi,

On 06/12/2018 23:44, Jeremy Linton wrote:
> From: Mian Yousaf Kaukab <ykaukab@...e.de>
> 
> Add is_meltdown_safe() which is a whitelist of known safe cores.
> 
> Signed-off-by: Mian Yousaf Kaukab <ykaukab@...e.de>
> [Moved location of function]
> Signed-off-by: Jeremy Linton <jeremy.linton@....com>
> ---
>  arch/arm64/kernel/cpufeature.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index aec5ecb85737..242898395f68 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -908,8 +908,7 @@ has_useable_cnp(const struct arm64_cpu_capabilities *entry, int scope)
>  #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
>  static int __kpti_forced; /* 0: not forced, >0: forced on, <0: forced off */
>  
> -static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
> -				int scope)
> +static bool is_cpu_meltdown_safe(void)
>  {
>  	/* List of CPUs that are not vulnerable and don't need KPTI */
>  	static const struct midr_range kpti_safe_list[] = {
> @@ -917,6 +916,16 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
>  		MIDR_ALL_VERSIONS(MIDR_BRCM_VULCAN),
>  		{ /* sentinel */ }
>  	};
> +	/* Don't force KPTI for CPUs that are not vulnerable */

This is really a nit, but that comment would make more sense where
is_cpu_meltdown_safe() is called since unmap_kernel_at_el0 is the one
deciding whether to apply KPTI, is_cpu_meltdown_safe() just states
whether the core is safe of not.

Otherwise:

Reviewed-by: Julien Thierry <julien.thierry@....com>

Cheers,

Julien

> +	if (is_midr_in_range_list(read_cpuid_id(), kpti_safe_list))
> +		return true;
> +
> +	return false;
> +}
> +
> +static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
> +				int scope)
> +{
>  	char const *str = "command line option";
>  
>  	/*
> @@ -940,8 +949,7 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
>  	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
>  		return true;
>  
> -	/* Don't force KPTI for CPUs that are not vulnerable */
> -	if (is_midr_in_range_list(read_cpuid_id(), kpti_safe_list))
> +	if (is_cpu_meltdown_safe())
>  		return false;
>  
>  	/* Defer to CPU feature registers */
> 

-- 
Julien Thierry