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, 4 insertions, 0 deletions

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 6e50a44610ee..9064b919a406 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -62,6 +62,9 @@ int create_user_ns(struct cred *new)
         kgid_t group = new->egid;
         int ret;
 
+        if (parent_ns->level > 32)
+                return -EUSERS;
+
         /*
          * Verify that we can not violate the policy of which files
          * may be accessed that is specified by the root directory,
@@ -92,6 +95,7 @@ int create_user_ns(struct cred *new)
         atomic_set(&ns->count, 1);
         /* Leave the new->user_ns reference with the new user namespace. */
         ns->parent = parent_ns;
+        ns->level = parent_ns->level + 1;
         ns->owner = owner;
         ns->group = group;