Patch "KVM: SVM: Limit PFERR_NESTED_GUEST_PAGE error_code check to L1 guest" has been added to the 4.12-stable tree

This is a note to let you know that I've just added the patch titled

KVM: SVM: Limit PFERR_NESTED_GUEST_PAGE error_code check to L1 guest

to the 4.12-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
kvm-svm-limit-pferr_nested_guest_page-error_code-check-to-l1-guest.patch
and it can be found in the queue-4.12 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Brijesh Singh <[email protected]>
Date: Mon, 7 Aug 2017 14:11:30 -0500
Subject: KVM: SVM: Limit PFERR_NESTED_GUEST_PAGE error_code check to L1 guest
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From: Brijesh Singh <[email protected]>

commit 64531a3b70b17c8d3e77f2e49e5e1bb70f571266 upstream.

Commit 147277540bbc ("kvm: svm: Add support for additional SVM NPF error codes", 2016-11-23) added a new error code to aid nested page fault handling. The commit unprotects (kvm_mmu_unprotect_page) the page when we get a NPF due to guest page table walk where the page was marked RO.

However, if an L0->L2 shadow nested page table can also be marked read-only 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 accesses). 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 avoids hitting the above scenario.

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

@@ -4759,7 +4759,8 @@ int kvm_mmu_page_fault(struct kvm_vcpu *
 	 * 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;
 	}

Patches currently in stable-queue which might be from [email protected] are

queue-4.12/kvm-svm-limit-pferr_nested_guest_page-error_code-check-to-l1-guest.patch