On Thu, Aug 03, 2023, Yang Weijiang wrote: > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index 0b9033551d8c..5d6d6fa33e5b 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -3780,10 +3780,12 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info) > * IA32_XSS[bit 8]. Guests have to use RDMSR/WRMSR rather than > * XSAVES/XRSTORS to save/restore PT MSRs. > */ > - if (data & ~kvm_caps.supported_xss) > + if (data & ~vcpu->arch.guest_supported_xss) Hmm, this is arguably wrong for userspace-initiated writes, as it would prevent userspace from restoring MSRs before CPUID. And it would make the handling of MSR_IA32_XSS writes inconsistent just within this case statement. The initial "can this MSR be written at all" check would *not* honor guest CPUID for host writes, but then the per-bit check *would* honor guest CPUID for host writes. But if we exempt host writes, then we'll end up with another mess, as exempting host writes for MSR_KVM_GUEST_SSP would let the guest coerce KVM into writing an illegal value by modifying SMRAM while in SMM. Blech. If we can get away with it, i.e. not break userspace, I think my preference is to enforce guest CPUID for host accesses to XSS, XFD, XFD_ERR, etc. I'm 99% certain we can make that change, because there are many, many MSRs that do NOT exempt host writes, i.e. the only way this would be a breaking change is if userspace is writing things like XSS before KVM_SET_CPUID2, but other MSRs after KVM_SET_CPUID2. I'm pretty sure I've advocated for the exact opposite in the past, i.e. argued that KVM's ABI is to not enforce ordering between KVM_SET_CPUID2 and KVM_SET_MSR. But this is becoming untenable, juggling the dependencies in KVM is complex and is going to result in a nasty bug at some point. For this series, lets just tighten the rules for XSS, i.e. drop the host_initated exemption. And in a parallel/separate series, try to do a wholesale cleanup of all the cases that essentially allow userspace to do KVM_SET_MSR before KVM_SET_CPUID2.
