,On Fri, 8 Jul 2022 at 00:32, Frederick Lawler <fred@xxxxxxxxxxxxxx> wrote:
>
> 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.
Some thoughts:
I.
Why not make the hook more generic, e.g. support all other existing
and potential future namespaces?
Also I think the naming scheme is <object>_<verb>.
LSM_HOOK(int, 0, namespace_create, const struct cred *cred,
unsigned int flags)
where flags is a bitmap of CLONE flags from include/uapi/linux/sched.h
(like CLONE_NEWUSER).
II.
While adding policing for namespaces maybe also add a new hook for setns(2)
LSM_HOOK(int, 0, namespace_join, const struct cred *subj, const
struct cred *obj, unsigned int flags)
III.
Maybe even attach a security context to namespaces so they can be
further governed?
SELinux example:
type domainA_userns_t;
type_transition domainA_t domainA_t : namespace domainA_userns_t "user";
allow domainA_t domainA_userns_t:namespace create;
# domainB calling setns(2) with domainA as target
allow domainB_t domainA_userns_t:namespace join;
>
> Links:
> 1. https://lore.kernel.org/all/20220608150942.776446-1-fred@xxxxxxxxxxxxxx/
> 2. https://lore.kernel.org/all/87y1xzyhub.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> 3. https://lore.kernel.org/all/9fe9cd9f-1ded-a179-8ded-5fde8960a586@xxxxxxxxxxxxxx/
>
> 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
>
> --
> 2.30.2
>
