userns: limit the maximum depth of user_namespace->parent chain

Ensure that user_namespace->parent chain can't grow too much.
Currently we use the hardroded 32 as limit.

Reported-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index b6b215f13b45..14105c26a836 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -23,6 +23,7 @@ struct user_namespace {
         struct uid_gid_map      projid_map;
         atomic_t                count;
         struct user_namespace   *parent;
+        int                     level;
         kuid_t                  owner;
         kgid_t                  group;
         unsigned int            proc_inum;