Re: [PATCH] kvm/x86/mmu: use the correct inherited permissions to get shadow page

[Paolo Bonzini <pbonzini@redhat.com>]

On 30/11/20 18:41, Sean Christopherson wrote:
>>
>> pmd1 and pmd2 point to the same pte table, so:
>> ptr1 and ptr3 points to the same page.
>> ptr2 and ptr4 points to the same page.
>>
>>    The guess read-accesses to ptr1 first. So the hypervisor gets the
>> shadow pte page table with role.access=u-- among other things.
>>     (Note the shadowed pmd1's access is uwx)
>>
>>    And then the guest write-accesses to ptr2, and the hypervisor
>> set up shadow page for ptr2.
>>     (Note the hypervisor silencely accepts the role.access=u--
>>      shadow pte page table in FNAME(fetch))
>>
>>    After that, the guess read-accesses to ptr3, the hypervisor
>> reused the same shadow pte page table as above.
>>
>>    At last, the guest writes to ptr4 without vmexit nor pagefault,
>> Which should cause vmexit as the guest expects.
>
> Hmm, yes, KVM would incorrectly handle this scenario.  But, the proposed patch
> would not address the issue as KVM always maps non-leaf shadow pages with full
> access permissions.

Can we have a testcase in kvm-unit-tests?  It's okay of course if it 
only fails with ept=0.

Paolo