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....
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