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

[Liran Alon <liran.alon@oracle.com>]

> On 9 Dec 2019, at 17:28, Nadav Amit <nadav.amit@gmail.com> wrote:
> 
>> On 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.
> 
> Did you use for the matter the “infrastructure” that I added?
> 

No. It’s possible to just change QEMU to run with WHPX.
But it’s true that the “infra” you added for running on Bare-Metal should work as-well.
This is why I wish to change kvm-unit-tests to cpu-unit-tests. :)

-Liran