Re: [PATCH 29/61] KVM: x86: Add Kconfig-controlled auditing of reverse CPUID lookups

[Vitaly Kuznetsov]

Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes:

> Add WARNs in the low level __cpuid_entry_get_reg() to assert that the
> function and index of the CPUID entry and reverse CPUID entry match.
> Wrap the WARNs in a new Kconfig, KVM_CPUID_AUDIT, as the checks add
> almost no value in a production environment, i.e. will only detect
> blatant KVM bugs and fatal hardware errors.  Add a Kconfig instead of
> simply wrapping the WARNs with an off-by-default #ifdef so that syzbot
> and other automated testing can enable the auditing.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> ---
>  arch/x86/kvm/Kconfig | 10 ++++++++++
>  arch/x86/kvm/cpuid.h |  5 +++++
>  2 files changed, 15 insertions(+)
>
> diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
> index 840e12583b85..bbbc3258358e 100644
> --- a/arch/x86/kvm/Kconfig
> +++ b/arch/x86/kvm/Kconfig
> @@ -96,6 +96,16 @@ config KVM_MMU_AUDIT
>  	 This option adds a R/W kVM module parameter 'mmu_audit', which allows
>  	 auditing of KVM MMU events at runtime.
>  
> +config KVM_CPUID_AUDIT
> +	bool "Audit KVM reverse CPUID lookups"
> +	depends on KVM
> +	help
> +	 This option enables runtime checking of reverse CPUID lookups in KVM
> +	 to verify the function and index of the referenced X86_FEATURE_* match
> +	 the function and index of the CPUID entry being accessed.
> +
> +	 If unsure, say N.
> +
>  # OK, it's a little counter-intuitive to do this, but it puts it neatly under
>  # the virtualization menu.
>  source "drivers/vhost/Kconfig"
> diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
> index 51f19eade5a0..41ff94a7d3e0 100644
> --- a/arch/x86/kvm/cpuid.h
> +++ b/arch/x86/kvm/cpuid.h
> @@ -98,6 +98,11 @@ static __always_inline struct cpuid_reg x86_feature_cpuid(unsigned x86_feature)
>  static __always_inline u32 *__cpuid_entry_get_reg(struct kvm_cpuid_entry2 *entry,
>  						  const struct cpuid_reg *cpuid)
>  {
> +#ifdef CONFIG_KVM_CPUID_AUDIT
> +	WARN_ON_ONCE(entry->function != cpuid->function);
> +	WARN_ON_ONCE(entry->index != cpuid->index);
> +#endif
> +
>  	switch (cpuid->reg) {
>  	case CPUID_EAX:
>  		return &entry->eax;

Honestly, I was thinking we should BUG_ON() and even in production builds
but not everyone around is so rebellious I guess, so

Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

-- 
Vitaly