KVM: SVM: Disallow guest from changing userspace's MSR_AMD64_DE_CFG value

Inject a #GP if the guest attempts to change MSR_AMD64_DE_CFG from its
*current* value, not if the guest attempts to write a value other than
KVM's set of supported bits.  As per the comment and the changelog of the
original code, the intent is to effectively make MSR_AMD64_DE_CFG read-
only for the guest.

Opportunistically use a more conventional equality check instead of an
exclusive-OR check to detect attempts to change bits.

Fixes: d1d93fa90f1a ("KVM: SVM: Add MSR-based feature support for serializing LFENCE")
Cc: Tom Lendacky <[email protected]>
Reviewed-by: Tom Lendacky <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>

1 files changed, 7 insertions, 2 deletions

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index d6f252555ab3..eaa41a6a00ee 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3189,8 +3189,13 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
 		if (data & ~msr_entry.data)
 			return 1;
 
-		/* Don't allow the guest to change a bit, #GP */
-		if (!msr->host_initiated && (data ^ msr_entry.data))
+		/*
+		 * Don't let the guest change the host-programmed value.  The
+		 * MSR is very model specific, i.e. contains multiple bits that
+		 * are completely unknown to KVM, and the one bit known to KVM
+		 * is simply a reflection of hardware capabilities.
+		 */
+		if (!msr->host_initiated && data != svm->msr_decfg)
 			return 1;
 
 		svm->msr_decfg = data;