kvm: x86: Exclude unpermitted xfeatures at KVM_GET_SUPPORTED_CPUID

KVM_GET_SUPPORTED_CPUID should not include any dynamic xstates in
CPUID[0xD] if they have not been requested with prctl. Otherwise
a process which directly passes KVM_GET_SUPPORTED_CPUID to
KVM_SET_CPUID2 would now fail even if it doesn't intend to use a
dynamically enabled feature. Userspace must know that prctl is
required and allocate >4K xstate buffer before setting any dynamic
bit.
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jing Liu <jing2.liu@intel.com>
Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Message-Id: <20220105123532.12586-5-yang.zhong@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/api.rst

@@ -1687,6 +1687,10 @@ userspace capabilities, and with user requirements (for example, the
 user may wish to constrain cpuid to emulate older hardware, or for
 feature consistency across a cluster).
 
+Dynamically-enabled feature bits need to be requested with
+``arch_prctl()`` before calling this ioctl. Feature bits that have not
+been requested are excluded from the result.
+
 Note that certain capabilities, such as KVM_CAP_X86_DISABLE_EXITS, may
 expose cpuid features (e.g. MONITOR) which are not supported by kvm in
 its default configuration. If userspace enables such capabilities, it

arch/x86/kvm/cpuid.c

@@ -815,11 +815,13 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 				goto out;
 		}
 		break;
-	case 0xd:
-		entry->eax &= supported_xcr0;
+	case 0xd: {
+		u64 guest_perm = xstate_get_guest_group_perm();
+
+		entry->eax &= supported_xcr0 & guest_perm;
 		entry->ebx = xstate_required_size(supported_xcr0, false);
 		entry->ecx = entry->ebx;
-		entry->edx &= supported_xcr0 >> 32;
+		entry->edx &= (supported_xcr0 & guest_perm) >> 32;
 		if (!supported_xcr0)
 			break;
 
@@ -866,6 +868,7 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 			entry->edx = 0;
 		}
 		break;
+	}
 	case 0x12:
 		/* Intel SGX */
 		if (!kvm_cpu_cap_has(X86_FEATURE_SGX)) {