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SubjectRe: [PATCH 1/4] KVM: X86: Fix tlb flush for tdp in kvm_invalidate_pcid()

On 2021/10/21 22:52, Sean Christopherson wrote:
> On Thu, Oct 21, 2021, Lai Jiangshan wrote:
>> On 2021/10/21 02:26, Sean Christopherson wrote:
>>> On Wed, Oct 20, 2021, Lai Jiangshan wrote:
>>>> On 2021/10/19 23:25, Sean Christopherson wrote:
>>>> I just read some interception policy in vmx.c, if EPT=1 but vmx_need_pf_intercept()
>>>> return true for some reasons/configs, #PF is intercepted. But CR3 write is not
>>>> intercepted, which means there will be an EPT fault _after_ (IIUC) the CR3 write if
>>>> the GPA of the new CR3 exceeds the guest maxphyaddr limit. And kvm queues a fault to
>>>> the guest which is also _after_ the CR3 write, but the guest expects the fault before
>>>> the write.
>>>> IIUC, it can be fixed by intercepting CR3 write or reversing the CR3 write in EPT
>>>> violation handler.
>>> KVM implicitly does the latter by emulating the faulting instruction.
>>> static int handle_ept_violation(struct kvm_vcpu *vcpu)
>>> {
>>> ...
>>> /*
>>> * Check that the GPA doesn't exceed physical memory limits, as that is
>>> * a guest page fault. We have to emulate the instruction here, because
>>> * if the illegal address is that of a paging structure, then
>>> * EPT_VIOLATION_ACC_WRITE bit is set. Alternatively, if supported we
>>> * would also use advanced VM-exit information for EPT violations to
>>> * reconstruct the page fault error code.
>>> */
>>> if (unlikely(allow_smaller_maxphyaddr && kvm_vcpu_is_illegal_gpa(vcpu, gpa)))
>>> return kvm_emulate_instruction(vcpu, 0);
>>> return kvm_mmu_page_fault(vcpu, gpa, error_code, NULL, 0);
>>> }
>>> and injecting a #GP when kvm_set_cr3() fails.
>> I think the EPT violation happens *after* the cr3 write. So the instruction to be
>> emulated is not "cr3 write". The emulation will queue fault into guest though,
>> recursive EPT violation happens since the cr3 exceeds maxphyaddr limit.
> Doh, you're correct. I think my mind wandered into thinking about what would
> happen with PDPTRs and forgot to get back to normal MOV CR3.
> So yeah, the only way to correctly handle this would be to intercept CR3 loads.
> I'm guessing that would have a noticeable impact on guest performance.

I think we can detect it in handle_ept_violation() via checking the cr3 value,
and make it triple-fault if it is the case, so that the VMM can exit. I don't
think any OS would use the reserved bit in CR3 and the corresponding #GP.

> Paolo, I'll leave this one for you to decide, we have pretty much written off
> allow_smaller_maxphyaddr :-)

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