[PATCH v2 0/4] Introduce security_create_user_ns()

[Christian Göttsche]

,On Fri, 8 Jul 2022 at 00:32, Frederick Lawler <fred at cloudflare.com> wrote:
>
> While creating a LSM BPF MAC policy to block user namespace creation, we
> used the LSM cred_prepare hook because that is the closest hook to prevent
> a call to create_user_ns().
>
> The calls look something like this:
>
>     cred = prepare_creds()
>         security_prepare_creds()
>             call_int_hook(cred_prepare, ...
>     if (cred)
>         create_user_ns(cred)
>
> We noticed that error codes were not propagated from this hook and
> introduced a patch [1] to propagate those errors.
>
> The discussion notes that security_prepare_creds()
> is not appropriate for MAC policies, and instead the hook is
> meant for LSM authors to prepare credentials for mutation. [2]
>
> Ultimately, we concluded that a better course of action is to introduce
> a new security hook for LSM authors. [3]
>
> This patch set first introduces a new security_create_user_ns() function
> and create_user_ns LSM hook, then marks the hook as sleepable in BPF.

Some thoughts:

I.

Why not make the hook more generic, e.g. support all other existing
and potential future namespaces?
Also I think the naming scheme is <object>_<verb>.

    LSM_HOOK(int, 0, namespace_create, const struct cred *cred,
unsigned int flags)

where flags is a bitmap of CLONE flags from include/uapi/linux/sched.h
(like CLONE_NEWUSER).

II.

While adding policing for namespaces maybe also add a new hook for setns(2)

    LSM_HOOK(int, 0, namespace_join, const struct cred *subj,  const
struct cred *obj, unsigned int flags)

III.

Maybe even attach a security context to namespaces so they can be
further governed?
SELinux example:

    type domainA_userns_t;
    type_transition domainA_t domainA_t : namespace domainA_userns_t "user";
    allow domainA_t domainA_userns_t:namespace create;

    # domainB calling setns(2) with domainA as target
    allow domainB_t domainA_userns_t:namespace join;

>
> Links:
> 1. https://lore.kernel.org/all/20220608150942.776446-1-fred@cloudflare.com/
> 2. https://lore.kernel.org/all/87y1xzyhub.fsf@email.froward.int.ebiederm.org/
> 3. https://lore.kernel.org/all/9fe9cd9f-1ded-a179-8ded-5fde8960a586@cloudflare.com/
>
> Changes since v1:
> - Add selftests/bpf: Add tests verifying bpf lsm create_user_ns hook patch
> - Add selinux: Implement create_user_ns hook patch
> - Change function signature of security_create_user_ns() to only take
>   struct cred
> - Move security_create_user_ns() call after id mapping check in
>   create_user_ns()
> - Update documentation to reflect changes
>
> Frederick Lawler (4):
>   security, lsm: Introduce security_create_user_ns()
>   bpf-lsm: Make bpf_lsm_create_user_ns() sleepable
>   selftests/bpf: Add tests verifying bpf lsm create_user_ns hook
>   selinux: Implement create_user_ns hook
>
>  include/linux/lsm_hook_defs.h                 |  1 +
>  include/linux/lsm_hooks.h                     |  4 +
>  include/linux/security.h                      |  6 ++
>  kernel/bpf/bpf_lsm.c                          |  1 +
>  kernel/user_namespace.c                       |  5 ++
>  security/security.c                           |  5 ++
>  security/selinux/hooks.c                      |  9 ++
>  security/selinux/include/classmap.h           |  2 +
>  .../selftests/bpf/prog_tests/deny_namespace.c | 88 +++++++++++++++++++
>  .../selftests/bpf/progs/test_deny_namespace.c | 39 ++++++++
>  10 files changed, 160 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/deny_namespace.c
>  create mode 100644 tools/testing/selftests/bpf/progs/test_deny_namespace.c
>
> --
> 2.30.2
>