Subject: Re: [PATCH 1/2] arm64: Support execute-only permissions with Enhanced PAN

From: Catalin Marinas <[email-masked]>
Date: Wed, 2 Dec 2020 18:23:03 +0000

On Thu, Nov 19, 2020 at 01:39:52PM +0000, Vladimir Murzin wrote:
> diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
> index 174817b..19147b6 100644
> --- a/arch/arm64/include/asm/sysreg.h
> +++ b/arch/arm64/include/asm/sysreg.h
> @@ -579,6 +579,7 @@
>  #endif
>  
>  /* SCTLR_EL1 specific flags. */
> +#define SCTLR_EL1_EPAN		(BIT(57))
>  #define SCTLR_EL1_ATA0		(BIT(42))
>  
>  #define SCTLR_EL1_TCF0_SHIFT	38
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index dcc165b..540245c 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1602,6 +1602,14 @@ static void cpu_enable_pan(const struct arm64_cpu_capabilities *__unused)
>  }
>  #endif /* CONFIG_ARM64_PAN */
>  
> +#ifdef CONFIG_ARM64_EPAN
> +static void cpu_enable_epan(const struct arm64_cpu_capabilities *__unused)
> +{
> +	sysreg_clear_set(sctlr_el1, 0, SCTLR_EL1_EPAN);
> +	local_flush_tlb_all();
> +}
> +#endif /* CONFIG_ARM64_EPAN */

Thinking about this, can we not set the SCTLR_EL1.EPAN bit in proc.S
directly, regardless of whether the system supports it or not (it should
be write-ignored)? It would go in INIT_SCTLR_EL1_MMU_ON. We use the
cpufeature entry only for detection, not enabling.

-- 
Catalin

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