On 7/8/22 7:10 AM, Christian Göttsche wrote:
,On Fri, 8 Jul 2022 at 00:32, Frederick Lawler fred@cloudflare.com 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?
The main issue with a generic hook is that different namespaces have different calling contexts. We decided in a previous discussion to opt-out of a generic hook for this reason. [1]
Also I think the naming scheme is <object>_<verb>.
That's a good call out. I was originally hoping to keep the security_*() match with the hook name matched with the caller function to keep things all aligned. If no one objects to renaming the hook, I can rename the hook for v3.
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, conststruct cred *obj, unsigned int flags)
IIUC, setns() will create a new namespace for the other namespaces except for user namespace. If we add a security hook for the other create_*_ns() functions, then we can catch setns() at that point.
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/CAHC9VhSTkEMT90Tk+=iTyp3npWEm+3imrkFVX2qb=XsOPp9...
Links:
- https://lore.kernel.org/all/20220608150942.776446-1-fred@cloudflare.com/
- https://lore.kernel.org/all/87y1xzyhub.fsf@email.froward.int.ebiederm.org/
- 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
-- 2.30.2