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 <thomas.lendacky@amd.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20240802181935.292540-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index d6f252555ab3ff0c68ead0ab03013ec8782364ab..eaa41a6a00ee6addab990f25fc1de8f96b509495 100644 (file)
--- 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;