[PATCH v3 1/4] KVM: x86: Do not allow use of the MSR filter allow flag 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 v3 1/4] KVM: x86: Do not allow use of the MSR filter allow flag in the kernel
Date: Fri, 22 Jul 2022 20:23:00 +0000	[thread overview]
Message-ID: <20220722202303.391709-2-aaronlewis@google.com> (raw)
In-Reply-To: <20220722202303.391709-1-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 ee3896416c68..e6dd76c94d47 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];
-- 
2.37.1.359.gd136c6c3e2-goog

