[PATCH v4 1/5] KVM: x86: Disallow the use of KVM_MSR_FILTER_DEFAULT_ALLOW in the kernel

From: Aaron Lewis <aaronlewis@google.com>
To: kvm@vger.kernel.org
Cc: pbonzini@redhat.com, jmattson@google.com, seanjc@google.com,
	Aaron Lewis <aaronlewis@google.com>
Subject: [PATCH v4 1/5] KVM: x86: Disallow the use of KVM_MSR_FILTER_DEFAULT_ALLOW in the kernel
Date: Wed, 21 Sep 2022 15:15:21 +0000	[thread overview]
Message-ID: <20220921151525.904162-2-aaronlewis@google.com> (raw)
In-Reply-To: <20220921151525.904162-1-aaronlewis@google.com>

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>
---

Google's VMM is already using this flag, so we *know* that dropping the
flag entirely will break userspace.  All we can do at this point is
prevent the kernel from using it.

 arch/x86/include/uapi/asm/kvm.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index 46de10a809ec..73ad693aa653 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/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];
-- 
2.37.3.968.ga6b4b080e4-goog

