Re: [PATCH v2 1/3] kvm: svm: Add support for additional SVM NPF error codes

Paolo Bonzini
2017-07-31T20:05:46Z

> > There can be different cases where an L0->L2 shadow nested page table is
> > marked read only, in particular when a page is read only in L1's nested
> > page tables. If such a page is accessed by L2 while walking page tables
> > it will cause a nested page fault (page table walks are write accesses).
> > However, after kvm_mmu_unprotect_page you will get another page fault,
> > and again in an endless stream.
> >
> > Instead, emulation would have caused a nested page fault vmexit, I think.
>
> If possible could you please give me some pointer on how to create this use
> case so that we can get definitive answer.
>
> Looking at the code path is giving me indication that the new code
> (the kvm_mmu_unprotect_page call) only happens if vcpu->arch.mmu_page_fault()
> returns an indication that the instruction should be emulated. I would not
> expect that to be the case scenario you described above since L1 making a page
> read-only (this is a page table for L2) is an error and should result in #NPF
> being injected into L1.

The flow is:

hardware walks page table; L2 page table points to read only memory
-> pf_interception (code =
-> kvm_handle_page_fault (need_unprotect = false)
-> kvm_mmu_page_fault
-> paging64_page_fault (for example)
-> try_async_pf
map_writable set to false
-> paging64_fetch(write_fault = true, map_writable = false, prefault = false)
-> mmu_set_spte(speculative = false, host_writable = false, write_fault = true)
-> set_spte
mmu_need_write_protect returns true
return true
write_fault == true -> set emulate = true
return true
return true
return true
emulate

Without this patch, emulation would have called

..._gva_to_gpa_nested
-> translate_nested_gpa
-> paging64_gva_to_gpa
-> paging64_walk_addr
-> paging64_walk_addr_generic
set fault (nested_page_fault=true)

and then:

kvm_propagate_fault
-> nested_svm_inject_npf_exit

> It's bit hard for me to visualize the code flow and
> figure out exactly how that would happen, but I just tried booting nested
> virtualization and it seem to be working okay.

I don't expect the above to happen when booting a normal guest (usual L1
guests hardly have readonly mappings).

> Is there a kvm-unit-test which I can run to trigger this scenario ? thanks

No, there isn't.

Paolo

> -Brijesh
>