| Message ID | 20241211172952.1477605-1-seanjc@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | KVM: SVM: Allow guest writes to set MSR_AMD64_DE_CFG bits | expand |
On 12/11/24 11:29, Sean Christopherson wrote: > 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 > Signed-off-by: Sean Christopherson <seanjc@google.com> Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com> > --- > arch/x86/kvm/svm/svm.c | 9 --------- > 1 file changed, 9 deletions(-) > > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c > index dd15cc635655..21dacd312779 100644 > --- a/arch/x86/kvm/svm/svm.c > +++ b/arch/x86/kvm/svm/svm.c > @@ -3201,15 +3201,6 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr) > 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; > } > > base-commit: fac04efc5c793dccbd07e2d59af9f90b7fc0dca4
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index dd15cc635655..21dacd312779 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -3201,15 +3201,6 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr) 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; }
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 Signed-off-by: Sean Christopherson <seanjc@google.com> --- arch/x86/kvm/svm/svm.c | 9 --------- 1 file changed, 9 deletions(-) base-commit: fac04efc5c793dccbd07e2d59af9f90b7fc0dca4