um: Ensure that a stub page cannot get unmapped

Trinity discovered an execution path such that a task
can unmap his stub page.
Reported-by: NToralf Förster <toralf.foerster@gmx.de>
Signed-off-by: NRichard Weinberger <richard@nod.at>

arch/um/kernel/tlb.c

@@ -124,6 +124,9 @@ static int add_munmap(unsigned long addr, unsigned long len,
 	struct host_vm_op *last;
 	int ret = 0;
 
+	if ((addr >= STUB_START) && (addr < STUB_END))
+		return -EINVAL;
+
 	if (hvc->index != 0) {
 		last = &hvc->ops[hvc->index - 1];
 		if ((last->type == MUNMAP) &&