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

[Frederick Lawler]

On 7/8/22 7:10 AM, Christian Göttsche wrote:
> ,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?

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@xxxxxxxxxxxxxx/

> > 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