Introduce security_create_user_ns()

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 userns_create LSM hook, then marks the hook as sleepable in BPF.

Links:
1. https://lore.kernel.org/all/20220608150942.776446-1-fred@...
2. https://lore.kernel.org/all/87y1xzyhub.fsf@email.froward....
3. https://lore.kernel.org/all/9fe9cd9f-1ded-a179-8ded-5fde8...

Past discussions:
V2: https://lore.kernel.org/all/20220707223228.1940249-1-fred...
V1: https://lore.kernel.org/all/20220621233939.993579-1-fred@...

Changes since v2:
- Rename create_user_ns hook to userns_create
- Use user_namespace as an object opposed to a generic namespace object
- s/domB_t/domA_t in commit message
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_userns_create() sleepable
  selftests/bpf: Add tests verifying bpf lsm userns_create hook
  selinux: Implement userns_create 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

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2.30.2