KVM: x86: synthesize CPUID bits only if CPU capability is set

KVM incorrectly synthesizes CPUID bits for KVM-only leaves, as the
following branch in kvm_cpu_cap_init() is never taken:

    if (leaf < NCAPINTS)
        kvm_cpu_caps[leaf] &= kernel_cpu_caps[leaf];

This means that bits set via SYNTHESIZED_F() for KVM-only leaves are
unconditionally set. This for example can cause issues for SEV-SNP
guests running on Family 19h CPUs, as TSA_SQ_NO and TSA_L1_NO are
always enabled by KVM in 80000021[ECX]. When userspace issues a
SNP_LAUNCH_UPDATE command to update the CPUID page for the guest, SNP
firmware will explicitly reject the command if the page sets sets these
bits on vulnerable CPUs.

To fix this, check in SYNTHESIZED_F() that the corresponding X86
capability is set before adding it to to kvm_cpu_cap_features.

Fixes: 31272ab ("KVM: SVM: Advertise TSA CPUID bits to guests")
Link: https://lore.kernel.org/all/20260208164233.30405-1-clopez@suse.de/
Signed-off-by: Carlos López <clopez@suse.de>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Link: https://patch.msgid.link/20260209153108.70667-2-clopez@suse.de
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Author: 00xc

arch/x86/kvm/cpuid.c

@@ -776,7 +776,10 @@ do {									\
 #define SYNTHESIZED_F(name)					\
 ({								\
 	kvm_cpu_cap_synthesized |= feature_bit(name);		\
-	F(name);						\
+								\
+	BUILD_BUG_ON(X86_FEATURE_##name >= MAX_CPU_FEATURES);	\
+	if (boot_cpu_has(X86_FEATURE_##name))			\
+		F(name);					\
 })
 
 /*