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

[<gregkh@linuxfoundation.org>]

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 <stable@vger.kernel.org> know about it.


>From 64531a3b70b17c8d3e77f2e49e5e1bb70f571266 Mon Sep 17 00:00:00 2001
From: Brijesh Singh <brijesh.singh@amd.com>
Date: Mon, 7 Aug 2017 14:11:30 -0500
Subject: KVM: SVM: Limit PFERR_NESTED_GUEST_PAGE error_code check to L1 guest
MIME-Version: 1.0
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Content-Transfer-Encoding: 8bit

From: Brijesh Singh <brijesh.singh@amd.com>

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.

Fixes: 147277540bbc54119172481c8ef6d930cc9fbfc2
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Thomas Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -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 brijesh.singh@amd.com are

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