[PATCH bpf-next v13 0/7] Landlock LSM

Mickaël Salaün
2019-11-04T17:23:59Z

Hi,

Following the previous series [1], this thirteenth series mainly clean
up and add safeguards to the domain management. This series also
improves the documentation, add comments, and fixes some latent bugs.

This is the first step of the roadmap discussed at LPC [2]. While the
intended final goal is to allow unprivileged (or non-root) users to use
Landlock, this series allows only a process with global CAP_SYS_ADMIN to
load and enforce a rule. This may help to get feedback and avoid
unexpected behaviors.

This series can be applied on top of bpf-next, commit e93d99180abd
("selftests/bpf: Restore $(OUTPUT)/test_stub.o rule"). This can be
tested with CONFIG_BPF_SYSCALL, CONFIG_SECCOMP_FILTER and
CONFIG_SECURITY_LANDLOCK. This patch series can be found in a Git
repository here:
https://github.com/landlock-lsm/linux/commits/landlock-v13
I would really appreciate constructive comments on the design and the
code.

# Landlock LSM

Landlock is a stackable LSM [3] intended to be used as a low-level
framework to build custom access-control/audit systems or safe endpoint
security agents. There is currently one Landlock hook dedicated to
check ptrace(2). This hook accepts a dedicated eBPF program, called a
Landlock program, which can currently compare its position in the
hierarchy of similar programs tied to other processes. This enables to
enforce programmatic scoped ptrace restrictions.

The final goal of this new Linux Security Module (LSM) called Landlock
is to allow any process, including unprivileged ones, to create powerful
security sandboxes comparable to XNU Sandbox, FreeBSD Capsicum or
OpenBSD Pledge (which could be implemented with Landlock). This kind of
sandbox is expected to help mitigate the security impact of bugs or
unexpected/malicious behaviors in user-space applications.

The use of seccomp and Landlock is more suitable with the help of a
user-space library (e.g. libseccomp) that could help to specify a
high-level language to express a security policy instead of raw eBPF
programs. Moreover, thanks to the LLVM front-end, it is quite easy to
write an eBPF program with a subset of the C language.

The documentation patch contains some kernel documentation, explanations
on how to use Landlock and a FAQ. The compiled documentation and some
talks can be found here: https://landlock.io

# Frequently asked questions

## Why is seccomp-bpf not enough?

A seccomp filter can access only raw syscall arguments (i.e. the
register values) which means that it is not possible to filter according
to the value pointed to by an argument, such as a file pathname. As an
embryonic Landlock version demonstrated (i.e. seccomp-object), filtering
at the syscall level is complicated (e.g. need to take care of race
conditions). This is mainly because the access control checkpoints of
the kernel are not at this high-level but more underneath, at the
LSM-hook level. The LSM hooks are designed to handle this kind of
checks. Landlock abstracts this approach to leverage the ability of
unprivileged users to limit themselves.

Cf. section "What it isn't?" in
Documentation/userspace-api/seccomp_filter.rst

## Why use the seccomp(2) syscall?

Landlock use the same semantic as seccomp to apply access rule
restrictions. It add a new layer of security for the current process
which is inherited by its children. It makes sense to use an unique
access-restricting syscall (that should be allowed by seccomp filters)
which can only drop privileges. Moreover, a Landlock rule could come
from outside a process (e.g. passed through a UNIX socket). It is then
useful to differentiate the creation/load of Landlock eBPF programs via
bpf(2), from rule enforcement via seccomp(2).

## Why a new LSM? Are SELinux, AppArmor, Smack and Tomoyo not good
enough?

The current access control LSMs are fine for their purpose which is to
give the *root* the ability to enforce a security policy for the
*system*. What is missing is a way to enforce a security policy for any
application by its developer and *unprivileged user* as seccomp can do
for raw syscall filtering.

Differences from other (access control) LSMs:
* not only dedicated to administrators (i.e. no_new_priv);
* limited kernel attack surface (e.g. policy parsing);
* constrained policy rules (no DoS: deterministic execution time);
* do not leak more information than the loader process can legitimately
have access to (minimize metadata inference).

# Changes since v12

* make landlock_prepend_prog() more consistant by putting its caller in
charge of the lifetime of the Landlock domain being updated
* always copy a domain being updated, which garantee its immutability,
and behave as a copy-on-write thanks to commit_creds()
* fix preemption
* improve documentation and comments

# Changes since v11

* rework domain management
* minor fixes

# Changes since v10

* remove all the file system related features: program types, inode
map and expected_attach_triggers
* replace the static ptrace security policy with a new and simpler
ptrace program (attached) type and a task_landlock_ptrace_ancestor()
eBPF helper
* do not rely on seccomp internal structure but use stacked credentials
insdead
* extend ptrace tests
* add more documentation
* split and rename files/patches
* miscellaneous fixes

Previous changes can be found in a previous cover-letter [4].

[1] https://lore.kernel.org/lkml/20191031164445.29426-1-mic@digikod.net/
[2] https://lore.kernel.org/lkml/5828776A.1010104@digikod.net/
[3] https://lore.kernel.org/lkml/50db058a-7dde-441b-a7f9-f6837fe8b69f@schaufler-ca.com/
[4] https://lore.kernel.org/lkml/20190721213116.23476-1-mic@digikod.net/

Regards,

Mickaël Salaün (7):
bpf,landlock: Define an eBPF program type for Landlock hooks
landlock: Add the management of domains
landlock,seccomp: Apply Landlock programs to process hierarchy
landlock: Add ptrace LSM hooks
bpf,landlock: Add task_landlock_ptrace_ancestor() helper
bpf,landlock: Add tests for the Landlock ptrace program type
landlock: Add user and kernel documentation for Landlock

Documentation/security/index.rst | 1 +
Documentation/security/landlock/index.rst | 22 ++
Documentation/security/landlock/kernel.rst | 166 ++++++++++
Documentation/security/landlock/user.rst | 153 ++++++++++
MAINTAINERS | 9 +
include/linux/bpf.h | 3 +
include/linux/bpf_types.h | 3 +
include/linux/landlock.h | 25 ++
include/linux/lsm_hooks.h | 1 +
include/uapi/linux/bpf.h | 23 +-
include/uapi/linux/landlock.h | 39 +++
include/uapi/linux/seccomp.h | 1 +
kernel/bpf/syscall.c | 9 +
kernel/bpf/verifier.c | 11 +
kernel/seccomp.c | 4 +
scripts/bpf_helpers_doc.py | 1 +
security/Kconfig | 1 +
security/Makefile | 2 +
security/landlock/Kconfig | 19 ++
security/landlock/Makefile | 6 +
security/landlock/bpf_ptrace.c | 98 ++++++
security/landlock/bpf_ptrace.h | 17 ++
security/landlock/bpf_run.c | 65 ++++
security/landlock/bpf_run.h | 25 ++
security/landlock/bpf_verify.c | 87 ++++++
security/landlock/common.h | 84 +++++
security/landlock/domain_manage.c | 152 +++++++++
security/landlock/domain_manage.h | 22 ++
security/landlock/domain_syscall.c | 93 ++++++
security/landlock/hooks_cred.c | 47 +++
security/landlock/hooks_cred.h | 14 +
security/landlock/hooks_ptrace.c | 114 +++++++
security/landlock/hooks_ptrace.h | 19 ++
security/landlock/init.c | 32 ++
security/security.c | 15 +
tools/include/uapi/linux/bpf.h | 23 +-
tools/include/uapi/linux/landlock.h | 22 ++
tools/lib/bpf/libbpf_probes.c | 3 +
tools/testing/selftests/bpf/config | 3 +
tools/testing/selftests/bpf/test_verifier.c | 1 +
.../testing/selftests/bpf/verifier/landlock.c | 56 ++++
tools/testing/selftests/landlock/.gitignore | 5 +
tools/testing/selftests/landlock/Makefile | 27 ++
tools/testing/selftests/landlock/config | 5 +
tools/testing/selftests/landlock/test.h | 48 +++
tools/testing/selftests/landlock/test_base.c | 24 ++
.../testing/selftests/landlock/test_ptrace.c | 289 ++++++++++++++++++
47 files changed, 1887 insertions(+), 2 deletions(-)
create mode 100644 Documentation/security/landlock/index.rst
create mode 100644 Documentation/security/landlock/kernel.rst
create mode 100644 Documentation/security/landlock/user.rst
create mode 100644 include/linux/landlock.h
create mode 100644 include/uapi/linux/landlock.h
create mode 100644 security/landlock/Kconfig
create mode 100644 security/landlock/Makefile
create mode 100644 security/landlock/bpf_ptrace.c
create mode 100644 security/landlock/bpf_ptrace.h
create mode 100644 security/landlock/bpf_run.c
create mode 100644 security/landlock/bpf_run.h
create mode 100644 security/landlock/bpf_verify.c
create mode 100644 security/landlock/common.h
create mode 100644 security/landlock/domain_manage.c
create mode 100644 security/landlock/domain_manage.h
create mode 100644 security/landlock/domain_syscall.c
create mode 100644 security/landlock/hooks_cred.c
create mode 100644 security/landlock/hooks_cred.h
create mode 100644 security/landlock/hooks_ptrace.c
create mode 100644 security/landlock/hooks_ptrace.h
create mode 100644 security/landlock/init.c
create mode 100644 tools/include/uapi/linux/landlock.h
create mode 100644 tools/testing/selftests/bpf/verifier/landlock.c
create mode 100644 tools/testing/selftests/landlock/.gitignore
create mode 100644 tools/testing/selftests/landlock/Makefile
create mode 100644 tools/testing/selftests/landlock/config
create mode 100644 tools/testing/selftests/landlock/test.h
create mode 100644 tools/testing/selftests/landlock/test_base.c
create mode 100644 tools/testing/selftests/landlock/test_ptrace.c

--
2.23.0