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

[Frederick Lawler <fred@cloudflare.com>]

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?
> 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=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
>>