userns: Allow setting gid_maps without privilege when setgroups is disabled

Now that setgroups can be disabled and not reenabled, setting gid_map
without privielge can now be enabled when setgroups is disabled.

This restores most of the functionality that was lost when unprivileged
setting of gid_map was removed.  Applications that use this functionality
will need to check to see if they use setgroups or init_groups, and if they
don't they can be fixed by simply disabling setgroups before writing to
gid_map.

Cc: stable@vger.kernel.org
Reviewed-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 6e80f4c..a2e37c5 100644 (file)
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -826,6 +826,11 @@ static bool new_idmap_permitted(const struct file *file,
                        kuid_t uid = make_kuid(ns->parent, id);
                        if (uid_eq(uid, cred->euid))
                                return true;
+               } else if (cap_setid == CAP_SETGID) {
+                       kgid_t gid = make_kgid(ns->parent, id);
+                       if (!(ns->flags & USERNS_SETGROUPS_ALLOWED) &&
+                           gid_eq(gid, cred->egid))
+                               return true;
                }
        }