KVM: x86: Disallow the use of KVM_MSR_FILTER_DEFAULT_ALLOW in the kernel

Protect the kernel from using the flag KVM_MSR_FILTER_DEFAULT_ALLOW.
Its value is 0, and using it incorrectly could have unintended
consequences. E.g. prevent someone in the kernel from writing something
like this.

if (filter.flags & KVM_MSR_FILTER_DEFAULT_ALLOW)
        <allow the MSR>

and getting confused when it doesn't work.

It would be more ideal to remove this flag altogether, but userspace
may already be using it, so protecting the kernel is all that can
reasonably be done at this point.
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220921151525.904162-2-aaronlewis@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/uapi/asm/kvm.h

@@ -222,7 +222,9 @@ struct kvm_msr_filter_range {
 
 #define KVM_MSR_FILTER_MAX_RANGES 16
 struct kvm_msr_filter {
+#ifndef __KERNEL__
 #define KVM_MSR_FILTER_DEFAULT_ALLOW (0 << 0)
+#endif
 #define KVM_MSR_FILTER_DEFAULT_DENY  (1 << 0)
 	__u32 flags;
 	struct kvm_msr_filter_range ranges[KVM_MSR_FILTER_MAX_RANGES];