Some use-cases of bubblewrap want to ensure that the subprocess can't
further re-arrange the filesystem namespace, or do other more complex
namespace modification. For example, Flatpak wants to prevent sandboxed
processes from altering their /proc/$pid/root/.flatpak-info, so that
/.flatpak-info can safely be used as an indicator that a process is part
of a Flatpak app.
This approach was suggested by lukts30 on containers/bubblewrap#452.
The sysctl-controlled maximum numbers of namespaces are themselves
namespaced, so we can disable nested user namespaces by setting the
limit to 1 and then entering a new, nested user namespace. The resulting
process loses its privileges in the namespace where the limit was set
to 1, so it is unable to move the limit back up.
Co-authored-by: Alexander Larsson <alexl@redhat.com>
Signed-off-by: Simon McVittie <smcv@collabora.com>
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