Re: [PATCH v2 0/4] Introduce security_create_user_ns()

[Frederick Lawler <fred@cloudflare.com>]

On 7/14/22 9:27 AM, Serge E. Hallyn wrote:
> On Fri, Jul 08, 2022 at 09:11:15AM -0700, Casey Schaufler wrote:
>> On 7/8/2022 7:01 AM, Frederick Lawler wrote:
>>> 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,  const
>>>> struct 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?
>>
>> That would likely add confusion to the existing security module namespace
>> efforts. SELinux, Smack and AppArmor have all developed namespace models.
>> That, or it could replace the various independent efforts with a single,
> 
> I feel like you're attaching more meaning to this than there needs to be.
> I *think* he's just talking about a user_namespace->u_security void*.
> So that for instance while deciding whether to allow some transition,
> selinux could check whether the caller's user namespace was created by
> a task in an selinux context authorized to create user namespaces.
> 
> The "user namespaces are DAC and orthogonal to MAC" is of course true
> (where the LSM does not itself tie them together), except that we all
> know that a process running as root in a user namespace gains access to
> often-less-trustworthy code gated under CAP_SYS_ADMIN.
> 
>> unified security module namespace effort. There's more work to that than
>> adding a context to a namespace. Treating namespaces as objects is almost,
>> but not quite, solidifying containers as a kernel construct. We know we
>> can't do that.
> 
> What we "can't do" (imo) is to create a "full container" construct which
> ties together the various namespaces and other concepts in a restrictive
> way.
> 

Is this the direction we want to go with the SELinux implementation? If 
so, where can I find a similar implementation to make the userns_create 
work with this? If not, I have a v3 with the hook name change ready to post.

>>>> 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;
>>
>> While I'm not an expert on SELinux policy, I'd bet a refreshing beverage
>> that there's already a way to achieve this with existing constructs.
>> Smack, which is subject+object MAC couldn't care less about the user
>> namespace configuration. User namespaces are DAC constructs.
>>
>>>>
>>>
>>> Links:
>>> 1.
>>> https://lore.kernel.org/all/CAHC9VhSTkEMT90Tk+=iTyp3npWEm+3imrkFVX2qb=XsOPp9F=A@mail.gmail.com/
>>>
>>>>>
>>>>> 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/
>>>>>
>>>>> 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
>>>>>
>>>