[PATCH v2 0/4] Introduce user namespace capabilities

[Jonathan Calmels <jcalmels@3xx0.net>]

This patch series introduces a new user namespace capability set, as
well as some plumbing around it (i.e. sysctl, secbit, lsm support).

First patch goes over the motivations for this as well as prior art.

In summary, while user namespaces are a great success today in that they
avoid running a lot of code as root, they also expand the attack surface
of the kernel substantially which is often abused by attackers. 
Methods exist to limit the creation of such namespaces [1], however,
application developers often need to assume that user namespaces are
available for various tasks such as sandboxing. Thus, instead of
restricting the creation of user namespaces, we offer ways for userspace
to limit the capabilities granted to them.

Why a new capability set and not something specific to the userns (e.g.
ioctl_ns)?

    1. We can't really expect userspace to patch every single callsite
    and opt-in this new security mechanism. 

    2. We don't necessarily want policies enforced at said callsites.
    For example a service like systemd-machined or a PAM session need to
    be able to place restrictions on any namespace spawned under it.

    3. We would need to come up with inheritance rules, querying
    capabilities, etc. At this point we're just reinventing capability
    sets.

    4. We can easily define interactions between capability sets, thus
    helping with adoption (patch 2 is an example of this)

Some examples of how this could be leveraged in userspace:

    - Prevent user from getting CAP_NET_ADMIN in user namespaces under SSH:
        echo "auth optional pam_cap.so" >> /etc/pam.d/sshd
        echo "!cap_net_admin $USER"     >> /etc/security/capability.conf
        capsh --secbits=$((1 << 8)) -- -c /usr/sbin/sshd

    - Prevent containers from ever getting CAP_DAC_OVERRIDE:
        systemd-run -p CapabilityBoundingSet=~CAP_DAC_OVERRIDE \
                    -p SecureBits=userns-strict-caps \
                    /usr/bin/dockerd
        systemd-run -p UserNSCapabilities=~CAP_DAC_OVERRIDE \
                    /usr/bin/incusd

    - Kernel could be vulnerable to CAP_SYS_RAWIO exploits, prevent it:
        sysctl -w cap_bound_userns_mask=0x1fffffdffff

    - Drop CAP_SYS_ADMIN for this shell and all the user namespaces below it:
        bwrap --unshare-user --cap-drop CAP_SYS_ADMIN /bin/sh

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7cd4c5c2101cb092db00f61f69d24380cf7a0ee8

---
Changes since v1:
- Add documentation
- Change commit wording
- Cleanup various aspects of the code based on feedback
- Add new CAP_SYS_CONTROL capability for sysctl check
- Add BPF-LSM support for modifying userns capabilities
---
Jonathan Calmels (4):
  capabilities: Add user namespace capabilities
  capabilities: Add securebit to restrict userns caps
  capabilities: Add sysctl to mask off userns caps
  bpf,lsm: Allow editing capabilities in BPF-LSM hooks

 Documentation/filesystems/proc.rst            |  1 +
 Documentation/security/credentials.rst        |  6 ++
 fs/proc/array.c                               |  9 +++
 include/linux/cred.h                          |  3 +
 include/linux/lsm_hook_defs.h                 |  2 +-
 include/linux/securebits.h                    |  1 +
 include/linux/security.h                      |  4 +-
 include/linux/user_namespace.h                |  7 ++
 include/uapi/linux/capability.h               |  6 +-
 include/uapi/linux/prctl.h                    |  7 ++
 include/uapi/linux/securebits.h               | 11 ++-
 kernel/bpf/bpf_lsm.c                          | 55 +++++++++++++
 kernel/cred.c                                 |  3 +
 kernel/sysctl.c                               | 10 +++
 kernel/umh.c                                  | 15 ++++
 kernel/user_namespace.c                       | 80 +++++++++++++++++--
 security/apparmor/lsm.c                       |  2 +-
 security/commoncap.c                          | 62 +++++++++++++-
 security/keys/process_keys.c                  |  3 +
 security/security.c                           |  6 +-
 security/selinux/hooks.c                      |  2 +-
 security/selinux/include/classmap.h           |  5 +-
 .../selftests/bpf/prog_tests/deny_namespace.c | 12 ++-
 .../selftests/bpf/progs/test_deny_namespace.c |  7 +-
 24 files changed, 291 insertions(+), 28 deletions(-)

-- 
2.45.2