Subject: Re: [PATCH v2 21/24] kvm: arm64: Add kvm-arm.protected early kernel parameter

From: Marc Zyngier <[email-masked]>
Date: Mon, 23 Nov 2020 17:30:14 +0000

On Mon, 16 Nov 2020 20:43:15 +0000,
David Brazdil <[email-masked]> wrote:
> 
> Add an early parameter that allows users to opt into protected KVM mode
> when using the nVHE hypervisor. In this mode, guest state will be kept
> private from the host. This will primarily involve enabling stage-2
> address translation for the host, restricting DMA to host memory, and
> filtering host SMCs.
> 
> Capability ARM64_PROTECTED_KVM is set if the param is passed, CONFIG_KVM
> is enabled and the kernel was not booted with VHE.
> 
> Signed-off-by: David Brazdil <[email-masked]>
> ---
>  arch/arm64/include/asm/cpucaps.h |  3 ++-
>  arch/arm64/include/asm/virt.h    |  8 ++++++++
>  arch/arm64/kernel/cpufeature.c   | 29 +++++++++++++++++++++++++++++
>  arch/arm64/kvm/arm.c             | 10 +++++++++-
>  4 files changed, 48 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h
> index e7d98997c09c..ac075f70b2e4 100644
> --- a/arch/arm64/include/asm/cpucaps.h
> +++ b/arch/arm64/include/asm/cpucaps.h
> @@ -66,7 +66,8 @@
>  #define ARM64_HAS_TLB_RANGE			56
>  #define ARM64_MTE				57
>  #define ARM64_WORKAROUND_1508412		58
> +#define ARM64_PROTECTED_KVM			59
>  
> -#define ARM64_NCAPS				59
> +#define ARM64_NCAPS				60
>  
>  #endif /* __ASM_CPUCAPS_H */
> diff --git a/arch/arm64/include/asm/virt.h b/arch/arm64/include/asm/virt.h
> index 6069be50baf9..2fde1186b962 100644
> --- a/arch/arm64/include/asm/virt.h
> +++ b/arch/arm64/include/asm/virt.h
> @@ -97,6 +97,14 @@ static __always_inline bool has_vhe(void)
>  		return cpus_have_final_cap(ARM64_HAS_VIRT_HOST_EXTN);
>  }
>  
> +static __always_inline bool is_protected_kvm_enabled(void)
> +{
> +	if (is_vhe_hyp_code())
> +		return false;
> +	else
> +		return cpus_have_final_cap(ARM64_PROTECTED_KVM);
> +}
> +
>  #endif /* __ASSEMBLY__ */
>  
>  #endif /* ! __ASM__VIRT_H */
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 6f36c4f62f69..dd5bc0f0cf0d 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1709,6 +1709,29 @@ static void cpu_enable_mte(struct arm64_cpu_capabilities const *cap)
>  }
>  #endif /* CONFIG_ARM64_MTE */
>  
> +#ifdef CONFIG_KVM
> +static bool enable_protected_kvm;
> +
> +static bool has_protected_kvm(const struct arm64_cpu_capabilities *entry, int __unused)
> +{
> +	if (!enable_protected_kvm)
> +		return false;
> +
> +	if (is_kernel_in_hyp_mode()) {
> +		pr_warn("Protected KVM not available with VHE\n");
> +		return false;
> +	}
> +
> +	return true;
> +}
> +
> +static int __init early_protected_kvm_cfg(char *buf)
> +{
> +	return strtobool(buf, &enable_protected_kvm);
> +}
> +early_param("kvm-arm.protected", early_protected_kvm_cfg);

Please add some documentation to
Documentation/admin-guide/kernel-parameters.txt.

> +#endif /* CONFIG_KVM */
> +
>  /* Internal helper functions to match cpu capability type */
>  static bool
>  cpucap_late_cpu_optional(const struct arm64_cpu_capabilities *cap)
> @@ -1822,6 +1845,12 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  		.field_pos = ID_AA64PFR0_EL1_SHIFT,
>  		.min_field_value = ID_AA64PFR0_EL1_32BIT_64BIT,
>  	},
> +	{
> +		.desc = "Protected KVM",
> +		.capability = ARM64_PROTECTED_KVM,
> +		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
> +		.matches = has_protected_kvm,
> +	},
>  #endif
>  	{
>  		.desc = "Kernel page table isolation (KPTI)",
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index c76a8e5bd19c..49d2474f2a80 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -1796,6 +1796,12 @@ int kvm_arch_init(void *opaque)
>  		return -ENODEV;
>  	}
>  
> +	/* The PROTECTED_KVM cap should not have been enabled for VHE. */
> +	if (in_hyp_mode && is_protected_kvm_enabled()) {
> +		kvm_pr_unimpl("VHE protected mode unsupported, not initializing\n");
> +		return -ENODEV;

How can this happen? Don't we already take care of this?

> +	}
> +
>  	if (cpus_have_final_cap(ARM64_WORKAROUND_DEVICE_LOAD_ACQUIRE) ||
>  	    cpus_have_final_cap(ARM64_WORKAROUND_1508412))
>  		kvm_info("Guests without required CPU erratum workarounds can deadlock system!\n" \
> @@ -1827,7 +1833,9 @@ int kvm_arch_init(void *opaque)
>  	if (err)
>  		goto out_hyp;
>  
> -	if (in_hyp_mode)
> +	if (is_protected_kvm_enabled())
> +		kvm_info("Protected nVHE mode initialized successfully\n");
> +	else if (in_hyp_mode)
>  		kvm_info("VHE mode initialized successfully\n");
>  	else
>  		kvm_info("Hyp mode initialized successfully\n");
> -- 
> 2.29.2.299.gdc1121823c-goog
> 
> 

Thanks,

	M.

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

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