[PATCH 16/26] KVM: x86: Add Kconfig-controlled auditing of reverse CPUID lookups

[Sean Christopherson]

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;
-- 
2.24.1