KVM: x86: Add BUILD_BUG_ON() to detect bad usage of "scattered" flags

Add a compile-time assert in the SF() macro to detect improper usage,
i.e. to detect passing in an X86_FEATURE_* flag that isn't actually
scattered by the kernel.  Upcoming feature flags will be 100% KVM-only
and will have X86_FEATURE_* macros that point at a kvm_only_cpuid_leafs
word, not a kernel-defined word.  Using SF() and thus boot_cpu_has() for
such feature flags would access memory beyond x86_capability[NCAPINTS]
and at best incorrectly hide a feature, and at worst leak kernel state to
userspace.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221125125845.1182922-2-jiaxi.chen@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/cpuid.c

@@ -65,7 +65,13 @@ u32 xstate_required_size(u64 xstate_bv, bool compacted)
 #define KVM_X86_FEATURE_AMD_PSFD	(13*32+28) /* Predictive Store Forwarding Disable */
 
 #define F feature_bit
-#define SF(name) (boot_cpu_has(X86_FEATURE_##name) ? F(name) : 0)
+
+/* Scattered Flag - For features that are scattered by cpufeatures.h. */
+#define SF(name)						\
+({								\
+	BUILD_BUG_ON(X86_FEATURE_##name >= MAX_CPU_FEATURES);	\
+	(boot_cpu_has(X86_FEATURE_##name) ? F(name) : 0);	\
+})
 
 /*
  * Magic value used by KVM when querying userspace-provided CPUID entries and