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....
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