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

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Mon, Feb 24, 2020 at 02:54:38PM +0100, Vitaly Kuznetsov wrote:
> Sean Christopherson <sean.j.christopherson@intel.com> 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@intel.com>
> > ---
> >  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

LOL.  It's a waste of cycles for something that will "never" be hit, i.e.
we _really_ dropped the ball if a bug of this natures makes it into a
kernel release.
 
> Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> 
> -- 
> Vitaly
>