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Message-ID: <z2bgjrzeq7crqx24chdbxnaanuhczbjnq6da3xw6al6omjj5xz@mqbzzzfva5sw>
Date: Tue, 11 Jun 2024 01:09:18 -0700
From: Jonathan Calmels <jcalmels@...0.net>
To: Paul Moore <paul@...l-moore.com>
Cc: brauner@...nel.org, ebiederm@...ssion.com,
 	Jonathan Corbet <corbet@....net>, James Morris <jmorris@...ei.org>,
 	"Serge E. Hallyn" <serge@...lyn.com>, KP Singh <kpsingh@...nel.org>,
 	Matt Bobrowski <mattbobrowski@...gle.com>,
 Alexei Starovoitov <ast@...nel.org>,
 	Daniel Borkmann <daniel@...earbox.net>,
 Andrii Nakryiko <andrii@...nel.org>,
 	Martin KaFai Lau <martin.lau@...ux.dev>,
 Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
 	Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>,
 	Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, 	Luis Chamberlain <mcgrof@...nel.org>,
 Kees Cook <kees@...nel.org>, 	Joel Granados <j.granados@...sung.com>,
 John Johansen <john.johansen@...onical.com>,
 	David Howells <dhowells@...hat.com>,
 Jarkko Sakkinen <jarkko@...nel.org>,
 	Stephen Smalley <stephen.smalley.work@...il.com>,
 Ondrej Mosnacek <omosnace@...hat.com>, 	Mykola Lysenko <mykolal@...com>,
 Shuah Khan <shuah@...nel.org>, containers@...ts.linux.dev,
 	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-doc@...r.kernel.org, 	linux-security-module@...r.kernel.org,
 bpf@...r.kernel.org, apparmor@...ts.ubuntu.com,
 	keyrings@...r.kernel.org, selinux@...r.kernel.org,
 linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 4/4] bpf,lsm: Allow editing capabilities in BPF-LSM
 hooks

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@...0.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@...0.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

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