KVM: x86: change kvm_mmu_page_get_gfn BUG_ON to WARN_ON

Note that in such a case it is quite likely that KVM will BUG_ON
in __pte_list_remove when the VM is closed.  However, there is no
immediate risk of memory corruption in the host so a WARN_ON is
enough and it lets you gather traces for debugging.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

CVE-2018-12207

(cherry picked from commit e9f2a760)
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

arch/x86/kvm/mmu.c

@@ -775,10 +775,16 @@ static gfn_t kvm_mmu_page_get_gfn(struct kvm_mmu_page *sp, int index)
 
 static void kvm_mmu_page_set_gfn(struct kvm_mmu_page *sp, int index, gfn_t gfn)
 {
-	if (sp->role.direct)
-		BUG_ON(gfn != kvm_mmu_page_get_gfn(sp, index));
-	else
+	if (!sp->role.direct) {
 		sp->gfns[index] = gfn;
+		return;
+	}
+
+	if (WARN_ON(gfn != kvm_mmu_page_get_gfn(sp, index)))
+		pr_err_ratelimited("gfn mismatch under direct page %llx "
+				   "(expected %llx, got %llx)\n",
+				   sp->gfn,
+				   kvm_mmu_page_get_gfn(sp, index), gfn);
 }
 
 /*