[PATCH 0/2] Introduce security_create_user_ns()

[Frederick Lawler]

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.

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/

Frederick Lawler (2):
  security, lsm: Introduce security_create_user_ns()
  bpf-lsm: Make bpf_lsm_create_user_ns() sleepable

 include/linux/lsm_hook_defs.h | 2 ++
 include/linux/lsm_hooks.h     | 5 +++++
 include/linux/security.h      | 8 ++++++++
 kernel/bpf/bpf_lsm.c          | 1 +
 kernel/user_namespace.c       | 5 +++++
 security/security.c           | 6 ++++++
 6 files changed, 27 insertions(+)

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2.30.2