userns: Don't allow unprivileged creation of gid mappings

commit be7c6dba2332cef0677fbabb606e279ae76652c3 upstream.

As any gid mapping will allow and must allow for backwards
compatibility dropping groups don't allow any gid mappings to be
established without CAP_SETGID in the parent user namespace.

For a small class of applications this change breaks userspace
and removes useful functionality.  This small class of applications
includes tools/testing/selftests/mount/unprivilged-remount-test.c

Most of the removed functionality will be added back with the addition
of a one way knob to disable setgroups.  Once setgroups is disabled
setting the gid_map becomes as safe as setting the uid_map.

For more common applications that set the uid_map and the gid_map
with privilege this change will have no affect.

This is part of a fix for CVE-2014-8989.

Reviewed-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 048bb7b641a9a1965846cc370c750b7ab249b322..a5809c42a1b31ffebb30b557aae3db50cad106f4 100644 (file)
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -808,11 +808,6 @@ static bool new_idmap_permitted(const struct file *file,
                        if (uid_eq(uid, file->f_cred->fsuid))
                                return true;
                }
-               else if (cap_setid == CAP_SETGID) {
-                       kgid_t gid = make_kgid(ns->parent, id);
-                       if (gid_eq(gid, file->f_cred->fsgid))
-                               return true;
-               }
        }
 
        /* Allow anyone to set a mapping that doesn't require privilege */