KVM: x86: hyper-v: Make Hyper-V emulation enablement conditional

Hyper-V emulation is enabled in KVM unconditionally. This is bad at least
from security standpoint as it is an extra attack surface. Ideally, there
should be a per-VM capability explicitly enabled by VMM but currently it
is not the case and we can't mandate one without breaking backwards
compatibility. We can, however, check guest visible CPUIDs and only enable
Hyper-V emulation when "Hv#1" interface was exposed in
HYPERV_CPUID_INTERFACE.

Note, VMMs are free to act in any sequence they like, e.g. they can try
to set MSRs first and CPUIDs later so we still need to allow the host
to read/write Hyper-V specific MSRs unconditionally.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20210126134816.1880136-14-vkuznets@redhat.com>
[Add selftest vcpu_set_hv_cpuid API to avoid breaking xen_vmcall_test. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 2 insertions, 38 deletions

diff --git a/tools/testing/selftests/kvm/x86_64/evmcs_test.c b/tools/testing/selftests/kvm/x86_64/evmcs_test.c
index 39a3cb2bd103..ca22ee6d19cb 100644
--- a/tools/testing/selftests/kvm/x86_64/evmcs_test.c
+++ b/tools/testing/selftests/kvm/x86_64/evmcs_test.c
@@ -78,42 +78,6 @@ void guest_code(struct vmx_pages *vmx_pages)
 	GUEST_ASSERT(vmlaunch());
 }
 
-struct kvm_cpuid2 *guest_get_cpuid(void)
-{
-	static struct kvm_cpuid2 *cpuid_full;
-	struct kvm_cpuid2 *cpuid_sys, *cpuid_hv;
-	int i, nent = 0;
-
-	if (cpuid_full)
-		return cpuid_full;
-
-	cpuid_sys = kvm_get_supported_cpuid();
-	cpuid_hv = kvm_get_supported_hv_cpuid();
-
-	cpuid_full = malloc(sizeof(*cpuid_full) +
-			    (cpuid_sys->nent + cpuid_hv->nent) *
-			    sizeof(struct kvm_cpuid_entry2));
-	if (!cpuid_full) {
-		perror("malloc");
-		abort();
-	}
-
-	/* Need to skip KVM CPUID leaves 0x400000xx */
-	for (i = 0; i < cpuid_sys->nent; i++) {
-		if (cpuid_sys->entries[i].function >= 0x40000000 &&
-		    cpuid_sys->entries[i].function < 0x40000100)
-			continue;
-		cpuid_full->entries[nent] = cpuid_sys->entries[i];
-		nent++;
-	}
-
-	memcpy(&cpuid_full->entries[nent], cpuid_hv->entries,
-	       cpuid_hv->nent * sizeof(struct kvm_cpuid_entry2));
-	cpuid_full->nent = nent + cpuid_hv->nent;
-
-	return cpuid_full;
-}
-
 int main(int argc, char *argv[])
 {
 	vm_vaddr_t vmx_pages_gva = 0;
@@ -135,7 +99,7 @@ int main(int argc, char *argv[])
 		exit(KSFT_SKIP);
 	}
 
-	vcpu_set_cpuid(vm, VCPU_ID, guest_get_cpuid());
+	vcpu_set_hv_cpuid(vm, VCPU_ID);
 	vcpu_enable_evmcs(vm, VCPU_ID);
 
 	run = vcpu_state(vm, VCPU_ID);
@@ -179,7 +143,7 @@ int main(int argc, char *argv[])
 		/* Restore state in a new VM.  */
 		kvm_vm_restart(vm, O_RDWR);
 		vm_vcpu_add(vm, VCPU_ID);
-		vcpu_set_cpuid(vm, VCPU_ID, guest_get_cpuid());
+		vcpu_set_hv_cpuid(vm, VCPU_ID);
 		vcpu_enable_evmcs(vm, VCPU_ID);
 		vcpu_load_state(vm, VCPU_ID, state);
 		run = vcpu_state(vm, VCPU_ID);