[RFC PATCH v3 07/19] KVM: x86: mmu: tweak fast path for emulation of access to nested NPT pages

[Maxim Levitsky <mlevitsk@redhat.com>]

If a non leaf mmu page is write tracked externally for some reason,
which can in theory happen if it was used for nested avic physid page
before, then this code will enter an endless loop of page faults because
unprotecting the mmu page will not remove write tracking, nor will the
write tracker callback be called, because there is no mmu page at
this address.

Fix this by only invoking the fast path if we succeeded in zapping the
mmu page.

Fixes: 147277540bbc5 ("kvm: svm: Add support for additional SVM NPF error codes")
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
 arch/x86/kvm/mmu/mmu.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 633a3138d68e1..8f77d41e7fd80 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5341,8 +5341,8 @@ int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 error_code,
 	 */
 	if (vcpu->arch.mmu->root_role.direct &&
 	    (error_code & PFERR_NESTED_GUEST_PAGE) == PFERR_NESTED_GUEST_PAGE) {
-		kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(cr2_or_gpa));
-		return 1;
+		if (kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(cr2_or_gpa)))
+			return 1;
 	}
 
 	/*
-- 
2.26.3