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: Toralf Förster <toralf.foerster@gmx.de>
Signed-off-by: Richard Weinberger <richard@nod.at>

1 files changed, 3 insertions, 0 deletions

diff --git a/arch/um/kernel/tlb.c b/arch/um/kernel/tlb.c
index 9472079471bb..1fc619e5dfe9 100644
--- a/arch/um/kernel/tlb.c
+++ b/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) &&