Re: [PATCH] kvm: nVMX: VMWRITE checks VMCS-link pointer before VMCS field

[Jim Mattson <jmattson@google.com>]

On Thu, Dec 5, 2019 at 1:54 PM Liran Alon <liran.alon@oracle.com> wrote:
>
>
>
> > On 5 Dec 2019, at 23:30, Jim Mattson <jmattson@google.com> wrote:
> >
> > On Thu, Dec 5, 2019 at 5:11 AM Jim Mattson <jmattson@google.com> wrote:
> >>
> >> On Thu, Dec 5, 2019 at 3:46 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
> >>>
> >>> On 04/12/19 22:40, Jim Mattson wrote:
> >>>> According to the SDM, a VMWRITE in VMX non-root operation with an
> >>>> invalid VMCS-link pointer results in VMfailInvalid before the validity
> >>>> of the VMCS field in the secondary source operand is checked.
> >>>>
> >>>> Fixes: 6d894f498f5d1 ("KVM: nVMX: vmread/vmwrite: Use shadow vmcs12 if running L2")
> >>>> Signed-off-by: Jim Mattson <jmattson@google.com>
> >>>> Cc: Liran Alon <liran.alon@oracle.com>
> >>>> ---
> >>>> arch/x86/kvm/vmx/nested.c | 38 +++++++++++++++++++-------------------
> >>>> 1 file changed, 19 insertions(+), 19 deletions(-)
> >>>
> >>> As Vitaly pointed out, the test must be split in two, like this:
> >>
> >> Right. Odd that no kvm-unit-tests noticed.
> >>
> >>> ---------------- 8< -----------------------
> >>> From 3b9d87060e800ffae2bd19da94ede05018066c87 Mon Sep 17 00:00:00 2001
> >>> From: Paolo Bonzini <pbonzini@redhat.com>
> >>> Date: Thu, 5 Dec 2019 12:39:07 +0100
> >>> Subject: [PATCH] kvm: nVMX: VMWRITE checks VMCS-link pointer before VMCS field
> >>>
> >>> According to the SDM, a VMWRITE in VMX non-root operation with an
> >>> invalid VMCS-link pointer results in VMfailInvalid before the validity
> >>> of the VMCS field in the secondary source operand is checked.
> >>>
> >>> While cleaning up handle_vmwrite, make the code of handle_vmread look
> >>> the same, too.
> >>
> >> Okay.
> >>
> >>> Fixes: 6d894f498f5d1 ("KVM: nVMX: vmread/vmwrite: Use shadow vmcs12 if running L2")
> >>> Signed-off-by: Jim Mattson <jmattson@google.com>
> >>> Cc: Liran Alon <liran.alon@oracle.com>
> >>>
> >>> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> >>> index 4aea7d304beb..c080a879b95d 100644
> >>> --- a/arch/x86/kvm/vmx/nested.c
> >>> +++ b/arch/x86/kvm/vmx/nested.c
> >>> @@ -4767,14 +4767,13 @@ static int handle_vmread(struct kvm_vcpu *vcpu)
> >>>        if (to_vmx(vcpu)->nested.current_vmptr == -1ull)
> >>>                return nested_vmx_failInvalid(vcpu);
> >>>
> >>> -       if (!is_guest_mode(vcpu))
> >>> -               vmcs12 = get_vmcs12(vcpu);
> >>> -       else {
> >>> +       vmcs12 = get_vmcs12(vcpu);
> >>> +       if (is_guest_mode(vcpu)) {
> >>>                /*
> >>>                 * When vmcs->vmcs_link_pointer is -1ull, any VMREAD
> >>>                 * to shadowed-field sets the ALU flags for VMfailInvalid.
> >>>                 */
> >>> -               if (get_vmcs12(vcpu)->vmcs_link_pointer == -1ull)
> >>> +               if (vmcs12->vmcs_link_pointer == -1ull)
> >>>                        return nested_vmx_failInvalid(vcpu);
> >>>                vmcs12 = get_shadow_vmcs12(vcpu);
> >>>        }
> >>> @@ -4878,8 +4877,19 @@ static int handle_vmwrite(struct kvm_vcpu *vcpu)
> >>>                }
> >>>        }
> >>>
> >>> +       vmcs12 = get_vmcs12(vcpu);
> >>> +       if (is_guest_mode(vcpu)) {
> >>> +               /*
> >>> +                * When vmcs->vmcs_link_pointer is -1ull, any VMWRITE
> >>> +                * to shadowed-field sets the ALU flags for VMfailInvalid.
> >>> +                */
> >>> +               if (vmcs12->vmcs_link_pointer == -1ull)
> >>> +                       return nested_vmx_failInvalid(vcpu);
> >>> +               vmcs12 = get_shadow_vmcs12(vcpu);
> >>> +       }
> >>>
> >>>        field = kvm_register_readl(vcpu, (((vmx_instruction_info) >> 28) & 0xf));
> >>> +
> >>>        /*
> >>>         * If the vCPU supports "VMWRITE to any supported field in the
> >>>         * VMCS," then the "read-only" fields are actually read/write.
> >>> @@ -4889,24 +4899,12 @@ static int handle_vmwrite(struct kvm_vcpu *vcpu)
> >>>                return nested_vmx_failValid(vcpu,
> >>>                        VMXERR_VMWRITE_READ_ONLY_VMCS_COMPONENT);
> >>>
> >>> -       if (!is_guest_mode(vcpu)) {
> >>> -               vmcs12 = get_vmcs12(vcpu);
> >>> -
> >>> -               /*
> >>> -                * Ensure vmcs12 is up-to-date before any VMWRITE that dirties
> >>> -                * vmcs12, else we may crush a field or consume a stale value.
> >>> -                */
> >>> -               if (!is_shadow_field_rw(field))
> >>> -                       copy_vmcs02_to_vmcs12_rare(vcpu, vmcs12);
> >>> -       } else {
> >>> -               /*
> >>> -                * When vmcs->vmcs_link_pointer is -1ull, any VMWRITE
> >>> -                * to shadowed-field sets the ALU flags for VMfailInvalid.
> >>> -                */
> >>> -               if (get_vmcs12(vcpu)->vmcs_link_pointer == -1ull)
> >>> -                       return nested_vmx_failInvalid(vcpu);
> >>> -               vmcs12 = get_shadow_vmcs12(vcpu);
> >>> -       }
> >>> +       /*
> >>> +        * Ensure vmcs12 is up-to-date before any VMWRITE that dirties
> >>> +        * vmcs12, else we may crush a field or consume a stale value.
> >>> +        */
> >>> +       if (!is_guest_mode(vcpu) && !is_shadow_field_rw(field))
> >>> +               copy_vmcs02_to_vmcs12_rare(vcpu, vmcs12);
> >>>
> >>>        offset = vmcs_field_to_offset(field);
> >>>        if (offset < 0)
> >>>
> >>>
> >>> ... and also, do you have a matching kvm-unit-tests patch?
> >>
> >> I'll put one together, along with a test that shows the current
> >> priority inversion between read-only and unsupported VMCS fields.
> >
> > I can't figure out how to clear IA32_VMX_MISC[bit 29] in qemu, so I'm
> > going to add the test to tools/testing/selftests/kvm instead.
>
> Please don’t.
>
> I wish that we keep clear separation between kvm-unit-tests and self-tests.
> In the sense that kvm-unit-tests tests for correct CPU behaviour semantics
> and self-tests tests for correctness of KVM userspace API.
>
> In the future, I wish to change kvm-unit-tests to cpu-unit-tests. As there is no
> real connection to KVM. It’s a bunch of tests that can be run on top of any CPU
> Implementation (weather vCPU by some hypervisor or bare-metal CPU) and
> test for it’s semantics.
> I have already used this to find semantic issues on Hyper-V vCPU implementation for example.
>
> Regarding your question on how to disable IA32_VMX_MISC in QEMU:
> Paolo have recently created a patch-series for QEMU that can be used to do this.
> (Aimed for QEMU nVMX Live-Migration support)
> See: https://lists.gnu.org/archive/html/qemu-devel/2019-07/msg00711.html
> (You should search for final patch-series version…)
>
> -Liran

Okay. Given the high barrier to entry, I will not be providing a test
at this time.