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>

diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index b6b215f..14105c2 100644 (file)
--- 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;

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 6e50a44..9064b91 100644 (file)
--- 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;