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

[Nadav Amit <nadav.amit@gmail.com>]

> On Dec 9, 2019, at 2:58 PM, Liran Alon <liran.alon@oracle.com> wrote:
> 
> 
> 
>> 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. :)

Thanks for the explanation. I may give QEMU+WHPX a try just to see how many
bugs it reports.