[PATCH v2 4/4] bpf,lsm: Allow editing capabilities in BPF-LSM hooks

[Jonathan Calmels]

On Sun, Jun 09, 2024 at 08:18:48PM GMT, Paul Moore wrote:
> On Sun, Jun 9, 2024 at 6:40 AM Jonathan Calmels <jcalmels at 3xx0.net> wrote:
> >
> > This patch allows modifying the various capabilities of the struct cred
> > in BPF-LSM hooks. More specifically, the userns_create hook called
> > prior to creating a new user namespace.
> >
> > With the introduction of userns capabilities, this effectively provides
> > a simple way for LSMs to control the capabilities granted to a user
> > namespace and all its descendants.
> >
> > Update the selftests accordingly by dropping CAP_SYS_ADMIN in
> > namespaces and checking the resulting task's bounding set.
> >
> > Signed-off-by: Jonathan Calmels <jcalmels at 3xx0.net>
> > ---
> >  include/linux/lsm_hook_defs.h                 |  2 +-
> >  include/linux/security.h                      |  4 +-
> >  kernel/bpf/bpf_lsm.c                          | 55 +++++++++++++++++++
> >  security/apparmor/lsm.c                       |  2 +-
> >  security/security.c                           |  6 +-
> >  security/selinux/hooks.c                      |  2 +-
> >  .../selftests/bpf/prog_tests/deny_namespace.c | 12 ++--
> >  .../selftests/bpf/progs/test_deny_namespace.c |  7 ++-
> >  8 files changed, 76 insertions(+), 14 deletions(-)
> 
> I'm not sure we want to go down the path of a LSM modifying the POSIX
> capabilities of a task, other than the capabilities/commoncap LSM.  It
> sets a bad precedent and could further complicate issues around LSM
> ordering.

Well unless I'm misunderstanding, this does allow modifying the
capabilities/commoncap LSM through BTF. The reason for allowing
`userns_create` to be modified is that it is functionally very similar
to `cred_prepare` in that it operates with new creds (but specific to
user namespaces because of reasons detailed in [1]). 

There were some concerns in previous threads that the userns caps by
themselves wouldn't be granular enough, hence the LSM integration.
Ubuntu for example, currently has to resort to a hardcoded profile
transition to achieve this [2].

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7cd4c5c2101cb092db00f61f69d24380cf7a0ee8
[2] https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/noble/commit/?id=43a6c29532f517179fea8c94949d657d71f4fc13