6.12-stable review patch. If anyone has any objections, please let me know.
------------------
From: Sean Christopherson <seanjc@xxxxxxxxxx>
commit 4d5163cba43fe96902165606fa54e1aecbbb32de upstream.
Drop KVM's arbitrary behavior of making DE_CFG.LFENCE_SERIALIZE read-only
for the guest, as rejecting writes can lead to guest crashes, e.g. Windows
in particular doesn't gracefully handle unexpected #GPs on the WRMSR, and
nothing in the AMD manuals suggests that LFENCE_SERIALIZE is read-only _if
it exists_.
KVM only allows LFENCE_SERIALIZE to be set, by the guest or host, if the
underlying CPU has X86_FEATURE_LFENCE_RDTSC, i.e. if LFENCE is guaranteed
to be serializing. So if the guest sets LFENCE_SERIALIZE, KVM will provide
the desired/correct behavior without any additional action (the guest's
value is never stuffed into hardware). And having LFENCE be serializing
even when it's not _required_ to be is a-ok from a functional perspective.
Fixes: 74a0e79df68a ("KVM: SVM: Disallow guest from changing userspace's MSR_AMD64_DE_CFG value")
Fixes: d1d93fa90f1a ("KVM: SVM: Add MSR-based feature support for serializing LFENCE")
Reported-by: Simon Pilkington <simonp.git@xxxxxxxxxxx>
Closes: https://lore.kernel.org/all/52914da7-a97b-45ad-86a0-affdf8266c61@xxxxxxxxxxx
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
Link: https://lore.kernel.org/r/20241211172952.1477605-1-seanjc@xxxxxxxxxx
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/x86/kvm/svm/svm.c | 9 ---------
1 file changed, 9 deletions(-)
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3199,15 +3199,6 @@ static int svm_set_msr(struct kvm_vcpu *
if (data & ~supported_de_cfg)
return 1;
- /*
- * 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;
break;
}
