Re: [PATCH 3/5] mmu: don't set the present bit unconditionally

Bandan Das
2016-06-28T17:30:58Z

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 28/06/2016 06:32, Bandan Das wrote:
>> + bool execonly = !(context->guest_rsvd_check.bad_mt_xwr &
>> + (1ull << VMX_EPT_EXECUTABLE_MASK));
>>
>> if (set_mmio_spte(vcpu, sptep, gfn, pfn, pte_access))
>> return 0;
>>
>> - spte = PT_PRESENT_MASK;
>> + if (!execonly)
>> + spte |= PT_PRESENT_MASK;
>
> This needs a comment:
>
> /*
> * There are two cases in which execonly is false: 1) for
> * non-EPT page tables, in which case we need to set the
> * P bit; 2) for EPT page tables where an X-- page table
> * entry is invalid, in which case we need to force the R
> * bit of the page table entry to 1.
> */

I think this should be: 2) for EPT page tables where an X-- page
table entry is invalid and a EPT misconfig is injected to the guest
before we reach here.

> BUILD_BUG_ON(PT_PRESENT_MASK != VMX_EPT_READABLE_MASK);
> if (!execonly)
> spte |= PT_PRESENT_MASK;
>
>
>> if (!speculative)
>> spte |= shadow_accessed_mask;
>>
>> if (enable_ept) {
>> - kvm_mmu_set_mask_ptes(0ull,
>> + kvm_mmu_set_mask_ptes(PT_PRESENT_MASK,
>
> This should be VMX_EPT_READABLE_MASK.
>
>> @@ -2531,6 +2535,7 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
>> else
>> spte |= shadow_nx_mask;
>>
>> + /* In the EPT case, shadow_user_mask is PT_PRESENT_MASK */
>
> I don't think this comment is necessary, but it's better to add one in
> FNAME(gpte_access).
>
> /*
> * In the EPT case, a page table can be executable but not
> * readable (on some processors). Therefore, set_spte does not
> * automatically set bit 0 if execute-only is supported.
> * Instead, since EPT page tables do not have a U bit, we
> * repurpose ACC_USER_MASK to signify readability. Likewise,
> * when EPT is in use shadow_user_mask is set to
> * VMX_EPT_READABLE_MASK.
> */
>
>
> Thanks,
>
> Paolo
>
>> if (pte_access & ACC_USER_MASK)
>> spte |= shadow_user_mask;
>
>
> Paolo
>
>> (enable_ept_ad_bits) ? VMX_EPT_ACCESS_BIT : 0ull,
>> (enable_ept_ad_bits) ? VMX_EPT_DIRTY_BIT : 0ull,
>> 0ull, VMX_EPT_EXECUTABLE_MASK);
>
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