Re: [RFC v5 1/1] ns: add binfmt_misc to the user namespace

[Jann Horn <jannh@google.com>]

On Tue, Oct 9, 2018 at 12:38 PM Laurent Vivier <laurent@vivier.eu> wrote:
> This patch allows to have a different binfmt_misc configuration
> for each new user namespace. By default, the binfmt_misc configuration
> is the one of the previous level, but if the binfmt_misc filesystem is
> mounted in the new namespace a new empty binfmt instance is created and
> used in this namespace.
>
> For instance, using "unshare" we can start a chroot of an another
> architecture and configure the binfmt_misc interpreter without being root
> to run the binaries in this chroot.
[...]
> @@ -823,12 +847,34 @@ static const struct super_operations s_ops = {
>  static int bm_fill_super(struct super_block *sb, void *data, int silent)
>  {
>         int err;
> +       struct user_namespace *ns = sb->s_user_ns;
>         static const struct tree_descr bm_files[] = {
>                 [2] = {"status", &bm_status_operations, S_IWUSR|S_IRUGO},
>                 [3] = {"register", &bm_register_operations, S_IWUSR},
>                 /* last one */ {""}
>         };
>
> +       /* create a new binfmt namespace
> +        * if we are not in the first user namespace
> +        * but the binfmt namespace is the first one
> +        */
> +       if (READ_ONCE(ns->binfmt_ns) == NULL) {
> +               struct binfmt_namespace *new_ns;
> +
> +               new_ns = kmalloc(sizeof(struct binfmt_namespace),
> +                                GFP_KERNEL);
> +               if (new_ns == NULL)
> +                       return -ENOMEM;
> +               INIT_LIST_HEAD(&new_ns->entries);
> +               new_ns->enabled = 1;
> +               rwlock_init(&new_ns->entries_lock);
> +               new_ns->bm_mnt = NULL;
> +               new_ns->entry_count = 0;
> +               /* ensure new_ns is completely initialized before sharing it */
> +               smp_wmb();
> +               WRITE_ONCE(ns->binfmt_ns, new_ns);
> +       }

You're still not preventing a concurrent race of two mount() calls,
right? What prevents two instances of this code block from running
concurrently in two different namespaces? I think you want to take
some sort of global lock around this.