[PATCH v4 0/4] Introduce security_create_user_ns()

From: Frederick Lawler <fred@cloudflare.com>
To: kpsingh@kernel.org, revest@chromium.org, jackmanb@chromium.org,
	ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org,
	kafai@fb.com, songliubraving@fb.com, yhs@fb.com,
	john.fastabend@gmail.com, jmorris@namei.org, serge@hallyn.com,
	paul@paul-moore.com, stephen.smalley.work@gmail.com,
	eparis@parisplace.org, shuah@kernel.org, brauner@kernel.org,
	casey@schaufler-ca.com, ebiederm@xmission.com,
	bpf@vger.kernel.org, linux-security-module@vger.kernel.org,
	selinux@vger.kernel.org, linux-kselftest@vger.kernel.org
Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
	kernel-team@cloudflare.com, cgzones@googlemail.com,
	karl@bigbadwolfsecurity.com,
	Frederick Lawler <fred@cloudflare.com>
Subject: [PATCH v4 0/4] Introduce security_create_user_ns()
Date: Mon,  1 Aug 2022 13:01:42 -0500	[thread overview]
Message-ID: <20220801180146.1157914-1-fred@cloudflare.com> (raw)

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@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/

Past discussions:
V3: https://lore.kernel.org/all/20220721172808.585539-1-fred@cloudflare.com/
V2: https://lore.kernel.org/all/20220707223228.1940249-1-fred@cloudflare.com/
V1: https://lore.kernel.org/all/20220621233939.993579-1-fred@cloudflare.com/

Changes since v3:
- Explicitly set CAP_SYS_ADMIN to test namespace is created given
  permission
- Simplify BPF test to use sleepable hook only
- Prefer unshare() over clone() for tests
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 | 102 ++++++++++++++++++
 .../selftests/bpf/progs/test_deny_namespace.c |  33 ++++++
 10 files changed, 168 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