[PATCH] KVM: SVM: Limit PFERR_NESTED_GUEST_PAGE error_code check to L1 guest

[Brijesh Singh]

Commit: 1472775 (kvm: svm: Add support for additional SVM NPF error codes)
added new error code to aid nested page fault handling. The commit
unprotect (kvm_mmu_unprotect_page) the page when we get a NFP due to
guest page table walk where the page was marked RO.

Paolo highlighted a use case,  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 table. If such a page is accessed by L2 while walking page tables
it can cause a nested page fault (page table walks are write accessed).
However, after kvm_mmu_unprotect_page we may get another page fault, and
again in an endless stream.

To cover this use case, we qualify the new error_code check with
vcpu->arch.mmu_direct_map so that the error_code check would run on L1
guest, and not the L2 guest. This would restrict it avoid hitting the above
use case.

Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: "Radim KrÄmÃÅ" <rkrcmar@xxxxxxxxxx>
Cc: Thomas Lendacky <thomas.lendacky@xxxxxxx>
Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
---

See http://marc.info/?l=kvm&m=150153155519373&w=2 for detail discussion on the use case and code flow.

 arch/x86/kvm/mmu.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 9b1dd11..4aaa4aa 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -4839,7 +4839,8 @@ int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t cr2, u64 error_code,
 	 * Note: AMD only (since it supports the PFERR_GUEST_PAGE_MASK used
 	 *       in PFERR_NEXT_GUEST_PAGE)
 	 */
-	if (error_code == PFERR_NESTED_GUEST_PAGE) {
+	if (vcpu->arch.mmu.direct_map &&
+		(error_code == PFERR_NESTED_GUEST_PAGE)) {
 		kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(cr2));
 		return 1;
 	}
-- 
2.9.4