KVM: MMU: update sp->gfns on pte update path

[Marcelo Tosatti <mtosatti@redhat.com>]

If an emulated pte write modifies the gpa of a present spte, sp->gfns is
not updated, retaining a stale value which later leads to:

rmap_remove: ffff8807d245fff8 0->BUG
------------[ cut here ]------------
kernel BUG at arch/x86/kvm/mmu.c:695!

Fix by updating sp->gfns even if spte was present.

Resolves: https://bugzilla.kernel.org/show_bug.cgi?id=27052
Reported-and-tested-by: Nicolas Prochazka <prochazka.nicolas@gmail.com>
KVM-Stable-Tag.

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index cc1bada..37d0886 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -2054,6 +2054,12 @@ static void mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 		rmap_count = rmap_add(vcpu, sptep, gfn);
 		if (rmap_count > RMAP_RECYCLE_THRESHOLD)
 			rmap_recycle(vcpu, sptep, gfn);
+	} else {
+		struct kvm_mmu_page *sp = page_header(__pa(sptep));
+		int index = sptep - sp->spt;
+
+		if (!sp->role.direct && sp->gfns[index] != gfn)
+			sp->gfns[index] = gfn;
 	}
 	kvm_release_pfn_clean(pfn);
 	if (speculative) {