Re: [PATCH] arm64: cpufeature: make sure cpu_mitigations_off then kpti off

[Will Deacon]

On Thu, Jun 16, 2022 at 08:35:38PM +0800, Liu Song wrote:
> From: Liu Song <liusong@xxxxxxxxxxxxxxxxx>
> 
> If kaslr is enabled, kpti cannot be turned off even if "mitigations=off",
> adjust the code order to ensure that kpti is off when "mitigations=off".
> 
> Signed-off-by: Liu Song <liusong@xxxxxxxxxxxxxxxxx>
> ---
>  arch/arm64/kernel/cpufeature.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 42ea2bd..27d4850 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1616,6 +1616,11 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
>  		__kpti_forced = -1;
>  	}
>  
> +	if (cpu_mitigations_off() && !__kpti_forced) {
> +		str = "mitigations=off";
> +		__kpti_forced = -1;
> +	}
> +
>  	/* Useful for KASLR robustness */
>  	if (kaslr_requires_kpti()) {
>  		if (!__kpti_forced) {
> @@ -1624,11 +1629,6 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
>  		}
>  	}
>  
> -	if (cpu_mitigations_off() && !__kpti_forced) {
> -		str = "mitigations=off";
> -		__kpti_forced = -1;
> -	}

The current behaviour is intentional: KASLR is trivial to bypass when kpti
is disabled.

Will