kvm: x86: Host feature SSBD doesn't imply guest feature SPEC_CTRL_SSBD

commit 396d2e8 upstream.

The host reports support for the synthetic feature X86_FEATURE_SSBD
when any of the three following hardware features are set:
  CPUID.(EAX=7,ECX=0):EDX.SSBD[bit 31]
  CPUID.80000008H:EBX.AMD_SSBD[bit 24]
  CPUID.80000008H:EBX.VIRT_SSBD[bit 25]

Either of the first two hardware features implies the existence of the
IA32_SPEC_CTRL MSR, but CPUID.80000008H:EBX.VIRT_SSBD[bit 25] does
not. Therefore, CPUID.(EAX=7,ECX=0):EDX.SSBD[bit 31] should only be
set in the guest if CPUID.(EAX=7,ECX=0):EDX.SSBD[bit 31] or
CPUID.80000008H:EBX.AMD_SSBD[bit 24] is set on the host.

Fixes: 0c54914 ("KVM: x86: use Intel speculation bugs and features as derived in generic x86 code")
Signed-off-by: Jim Mattson <[email protected]>
Reviewed-by: Jacob Xu <[email protected]>
Reviewed-by: Peter Shier <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Reported-by: Eric Biggers <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
[bwh: Backported to 4.x: adjust indentation]
Signed-off-by: Ben Hutchings <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>

Author: jsmattsonjr

arch/x86/kvm/cpuid.c

@@ -479,7 +479,8 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
 				entry->edx |= F(SPEC_CTRL);
 			if (boot_cpu_has(X86_FEATURE_STIBP))
 				entry->edx |= F(INTEL_STIBP);
-			if (boot_cpu_has(X86_FEATURE_SSBD))
+			if (boot_cpu_has(X86_FEATURE_SPEC_CTRL_SSBD) ||
+			    boot_cpu_has(X86_FEATURE_AMD_SSBD))
 				entry->edx |= F(SPEC_CTRL_SSBD);
 			/*
 			 * We emulate ARCH_CAPABILITIES in software even