Re: [RFC] Null Namespaces

On Mon, Jul 6, 2026 at 9:55 AM Jann Horn <[email protected]> wrote:
>

> I mostly agree, though we might want to gate this on no_new_privs just
> to be sure - you could theoretically have a setuid root program that
> gives the caller more privileges if it can't find its config file.
> That's kind of a far-fetched scenario, and in reality it would
> probably fail because the dynamic linker can't be found, but there is
> precedent for other sandboxing stuff also requiring no_new_privs, so
> we might as well require that here, too...
>

My ancient patch did this gating.

FWIW, if we are contemplating letting unprivileged tasks chroot to an
*empty* mountns and they don't have privileges to bind anything there,
then the mnt_may_suid() will prevent them from using execveat to run a
setuid program.

> (I think it actually might be fine to just make chroot entirely
> unprivileged as long as no_new_privs is set, but I don't think we
> should actually do that, that would just be unnecessarily playing with
> fire and would probably confuse some security monitoring tools or
> such.)
>
> We should probably also reject if current_chrooted() is true, for the
> same reason we reject userns creation when that's true.
>

These empty-tree proposals might expose a little issue in
current_chrooted().  We don't want to check whether our root is the
namespace root -- I think we may want to check whether our root has a
parent.  Otherwise once you chroot to an empty tree once, you can't
chroot again, which is silly.  Maybe if we add this empty tree thing,
the definition of current_chrooted should change.

(This is already an issue with open_tree and probably even with
detached mounts before that, but I don't think it's as easily
observable.)