6.12-stable review patch. If anyone has any objections, please let me know.
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From: Sean Christopherson seanjc@google.com
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@mailbox.org Closes: https://lore.kernel.org/all/52914da7-a97b-45ad-86a0-affdf8266c61@mailbox.org Cc: Tom Lendacky thomas.lendacky@amd.com Cc: stable@vger.kernel.org Reviewed-by: Tom Lendacky thomas.lendacky@amd.com Link: https://lore.kernel.org/r/20241211172952.1477605-1-seanjc@google.com Signed-off-by: Sean Christopherson seanjc@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- 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; }