[PATCH v1] landlock: Document the threat model

[Jann Horn]

On Tue, Jul 7, 2026 at 11:03 PM Mickaël Salaün <mic at digikod.net> wrote:
> +Sandboxing is layered
> +---------------------
> +
> +Landlock is the access-control layer of a sandbox, not the whole sandbox.  A
> +robust sandbox also needs steps that are the program's responsibility: switching
> +to an unprivileged user, dropping capabilities, setting ``PR_SET_NO_NEW_PRIVS``,
> +and confining all threads of the process with the same domain.  A
> +single-threaded process gets the latter for free; a multithreaded one can
> +enforce a ruleset atomically on all its threads, or must otherwise synchronize
> +them before any untrusted work.  Landlock is typically applied last, to tighten
> +access and make the domain identifiable and auditable.
> +
> +Stronger isolation can come from combining Landlock with other mechanisms in a
> +defense-in-depth approach, notably seccomp-bpf (see
> +Documentation/userspace-api/seccomp_filter.rst) for what Landlock does not yet
> +cover.  A long-term goal of Landlock is to control access to any kind of kernel
> +resource in a way suited to sandboxing.

I think this part is something that should go near the top of the
document, and should have a title like "How to securely use landlock"
- most of the document is focused on telling kernel developers or
security researchers what the boundaries of the threat model are, but
I think it is also important (and maybe even more important) to
present this from the perspective of "if I want to design a sandbox
using landlock, what do I need to pay attention to".

I think it would also make sense to give more specific guidance on
which sets of syscalls, with what argument restrictions, can be
permitted in a seccomp policy because landlock covers any accesses
that these syscalls can perform to objects to which the process
doesn't already hold file descriptors or such.
I think it would be helpful to have instructions like "you can safely
allow socket operations if you ensure that socket creation (socket())
only works for AF_UNIX, and that the process has no existing socket
file descriptors for non-covered socket types, and you have fs_access
rules, and the kernel is sufficiently recent".