Re: [RFC PATCH] arm64: Enable data independent timing (DIT) in the kernel

[Eric Biggers]

Hi Ard,

On Thu, Oct 27, 2022 at 01:27:41PM +0200, Ard Biesheuvel wrote:
> Given that running privileged code with DIT disabled on a CPU that
> implements support for it may result in a side channel that exposes
> privileged data to unprivileged user space processes, let's enable DIT
> while running in the kernel if supported by all CPUs.

This patch looks good to me, though I'm not an expert in low-level arm64 stuff.
It's a bit unfortunate that we have to manually create the .inst to enable DIT
instead of just using the assembler.  But it looks like there's a reason for it
(it's done for PAN et al. too), and based on the manual it looks correct.

Two nits below:

> diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
> index 7d301700d1a9..18e065f5130c 100644
> --- a/arch/arm64/include/asm/sysreg.h
> +++ b/arch/arm64/include/asm/sysreg.h
> @@ -94,15 +94,18 @@
>  #define PSTATE_PAN			pstate_field(0, 4)
>  #define PSTATE_UAO			pstate_field(0, 3)
>  #define PSTATE_SSBS			pstate_field(3, 1)
> +#define PSTATE_DIT			pstate_field(3, 2)
>  #define PSTATE_TCO			pstate_field(3, 4)
>  
>  #define SET_PSTATE_PAN(x)		__emit_inst(0xd500401f | PSTATE_PAN | ((!!x) << PSTATE_Imm_shift))
>  #define SET_PSTATE_UAO(x)		__emit_inst(0xd500401f | PSTATE_UAO | ((!!x) << PSTATE_Imm_shift))
>  #define SET_PSTATE_SSBS(x)		__emit_inst(0xd500401f | PSTATE_SSBS | ((!!x) << PSTATE_Imm_shift))
> +#define SET_PSTATE_DIT(x)		__emit_inst(0xd500401f | PSTATE_DIT | ((!!x) << PSTATE_Imm_shift))
>  #define SET_PSTATE_TCO(x)		__emit_inst(0xd500401f | PSTATE_TCO | ((!!x) << PSTATE_Imm_shift))
>  
>  #define set_pstate_pan(x)		asm volatile(SET_PSTATE_PAN(x))
>  #define set_pstate_uao(x)		asm volatile(SET_PSTATE_UAO(x))
> +#define set_pstate_dit(x)		asm volatile(SET_PSTATE_DIT(x))
>  #define set_pstate_ssbs(x)		asm volatile(SET_PSTATE_SSBS(x))

To keep the order consistent, set_pstate_dit() should be defined after
set_pstate_ssbs(), not before.

>  /* Internal helper functions to match cpu capability type */
>  static bool
>  cpucap_late_cpu_optional(const struct arm64_cpu_capabilities *cap)
> @@ -2640,6 +2645,17 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  		.matches = has_cpuid_feature,
>  		.cpu_enable = cpu_trap_el0_impdef,
>  	},
> +	{
> +		.desc = "Data independent timing control (DIT)",
> +		.capability = ARM64_HAS_DIT,
> +		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
> +		.matches = has_cpuid_feature,
> +		.sys_reg = SYS_ID_AA64PFR0_EL1,
> +		.field_pos = ID_AA64PFR0_EL1_DIT_SHIFT,
> +		.field_width = 4,
> +		.min_field_value = 1,
> +		.cpu_enable = cpu_enable_dit,
> +	},

The other entries in this array are explicit about '.sign = FTR_UNSIGNED'
(even though FTR_UNSIGNED is defined to false, so it's the default value).

- Eric