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    SubjectRe: [PATCH 1/2] KVM: X86: Move ignore_msrs handling upper the stack
    On Thu, Jun 25, 2020 at 09:25:40AM -0700, Sean Christopherson wrote:
    > On Thu, Jun 25, 2020 at 10:09:13AM +0200, Paolo Bonzini wrote:
    > > On 25/06/20 08:15, Sean Christopherson wrote:
    > > > IMO, kvm_cpuid() is simply buggy. If KVM attempts to access a non-existent
    > > > MSR then it darn well should warn.
    > > >
    > > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
    > > > index 8a294f9747aa..7ef7283011d6 100644
    > > > --- a/arch/x86/kvm/cpuid.c
    > > > +++ b/arch/x86/kvm/cpuid.c
    > > > @@ -1013,7 +1013,8 @@ bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
    > > > *ebx = entry->ebx;
    > > > *ecx = entry->ecx;
    > > > *edx = entry->edx;
    > > > - if (function == 7 && index == 0) {
    > > > + if (function == 7 && index == 0 && (*ebx | (F(RTM) | F(HLE))) &&
    > > > + (vcpu->arch.arch_capabilities & ARCH_CAP_TSX_CTRL_MSR)) {
    > > > u64 data;
    > > > if (!__kvm_get_msr(vcpu, MSR_IA32_TSX_CTRL, &data, true) &&
    > > > (data & TSX_CTRL_CPUID_CLEAR))
    > > >
    > >
    > > That works too, but I disagree that warning is the correct behavior
    > > here. It certainly should warn as long as kvm_get_msr blindly returns
    > > zero. However, for a guest it's fine to access a potentially
    > > non-existent MSR if you're ready to trap the #GP, and the point of this
    > > series is to let cpuid.c or any other KVM code do the same.
    >
    > I get the "what" of the change, and even the "why" to some extent, but I
    > dislike the idea of supporting/encouraging blind reads/writes to MSRs.
    > Blind writes are just asking for problems, and suppressing warnings on reads
    > is almost guaranteed to be suppressing a KVM bug.
    >
    > Case in point, looking at the TSX thing again, I actually think the fix
    > should be:
    >
    > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
    > index 5eb618dbf211..64322446e590 100644
    > --- a/arch/x86/kvm/cpuid.c
    > +++ b/arch/x86/kvm/cpuid.c
    > @@ -1013,9 +1013,9 @@ bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
    > *ebx = entry->ebx;
    > *ecx = entry->ecx;
    > *edx = entry->edx;
    > - if (function == 7 && index == 0) {
    > + if (function == 7 && index == 0 && (*ebx | (F(RTM) | F(HLE))) {
    > u64 data;
    > - if (!__kvm_get_msr(vcpu, MSR_IA32_TSX_CTRL, &data, true) &&
    > + if (!kvm_get_msr(vcpu, MSR_IA32_TSX_CTRL, &data) &&
    > (data & TSX_CTRL_CPUID_CLEAR))
    > *ebx &= ~(F(RTM) | F(HLE));
    > }
    >
    >
    > On VMX, MSR_IA32_TSX_CTRL will be added to the so called shared MSR array
    > regardless of whether or not it is being advertised to userspace (this is
    > a bug in its own right). Using the host_initiated variant means KVM will
    > incorrectly bypass VMX's ARCH_CAP_TSX_CTRL_MSR check, i.e. incorrectly
    > clear the bits if userspace is being weird and stuffed MSR_IA32_TSX_CTRL
    > without advertising it to the guest.

    Argh, belatedly realized that MSR_IA32_TSX_CTRL needs to be swapped even
    when ARCH_CAP_TSX_CTRL_MSR isn't exposed to the guest, but if and only if
    if TSX is disabled in the host _and_ enabled in the guest. So triggering
    setup_msrs() on ARCH_CAP_TSX_CTRL_MSR is insufficient, but I believe we can
    and should redo setup_msrs() during vmx_cpuid_update(). I'm pretty sure
    that's needed for MSR_TSC_AUX+RDTSCP as well. I suspect RDTSCP is broken
    on 32-bit guests, but no has noticed because Linux only employs RDTSCP on
    64-bit kernels, and 32-bit guests are exactly common in the first place.

    I'll check the above to confirm and prep some patches if RDTSCP is indeed
    busted.

    > In short, the whole MSR_IA32_TSX_CTRL implementation seems messy and this
    > is just papering over that mess. The correct fix is to invoke setup_msrs()
    > on writes to MSR_IA32_ARCH_CAPABILITIES, filtering MSR_IA32_TSX_CTRL out of
    > shared MSRs when it's not advertised, and change kvm_cpuid() to use the
    > unpriveleged variant.
    >
    > TSC_CTRL aside, if we insist on pointing a gun at our foot at some point,
    > this should be a dedicated flavor of MSR access, e.g. msr_data.kvm_initiated,
    > so that it at least requires intentionally loading the gun.

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