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

Paolo Bonzini
2017-07-27T16:36:36Z

On 23/11/2016 18:01, Brijesh Singh wrote:
>
> + /*
> + * Before emulating the instruction, check if the error code
> + * was due to a RO violation while translating the guest page.
> + * This can occur when using nested virtualization with nested
> + * paging in both guests. If true, we simply unprotect the page
> + * and resume the guest.
> + *
> + * Note: AMD only (since it supports the PFERR_GUEST_PAGE_MASK used
> + * in PFERR_NEXT_GUEST_PAGE)
> + */
> + if (error_code == PFERR_NESTED_GUEST_PAGE) {
> + kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(cr2));
> + return 1;
> + }

What happens if L1 is mapping some memory that is read only in L0? That
is, the L1 nested page tables make it read-write, but the L0 shadow
nested page tables make it read-only.

Accessing it would cause an NPF, and then my guess is that the L1 guest
would loop on the failing instruction instead of just dropping the write.

Paolo