[PATCH] KVM: PPC: Handle non-present PTEs in kvmppc_book3s_hv_page_fault()

[David Gibson]

Since cd758a9b57ee "KVM: PPC: Book3S HV: Use __gfn_to_pfn_memslot in HPT
page fault handler", it's been possible in fairly rare circumstances to
load a non-present PTE in kvmppc_book3s_hv_page_fault() when running a
guest on a POWER8 host.

Because that case wasn't checked for, we could misinterpret the non-present
PTE as being a cache-inhibited PTE.  That could mismatch with the
corresponding hash PTE, which would cause the function to fail with -EFAULT
a little further down.  That would propagate up to the KVM_RUN ioctl()
generally causing the KVM userspace (usually qemu) to fall over.

This addresses the problem by catching that case and returning to the guest
instead.

Fixes: cd758a9b57ee "KVM: PPC: Book3S HV: Use __gfn_to_pfn_memslot in HPT page fault handler"
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1820402
Suggested-by: Paul Mackerras <paulus@xxxxxxxxx>
Signed-off-by: David Gibson <david@xxxxxxxxxxxxxxxxxxxxx>
---
 arch/powerpc/kvm/book3s_64_mmu_hv.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
index 6404df613ea3..394fca8e630a 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
@@ -616,6 +616,11 @@ int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,
 	}
 	pte = *ptep;
 	local_irq_enable();
+	if (!pte_present(pte)) {
+		if (page)
+			put_page(page);
+		return RESUME_GUEST;
+	}
 	hpa = pte_pfn(pte) << PAGE_SHIFT;
 	pte_size = PAGE_SIZE;
 	if (shift)
-- 
2.25.2