[RFC v4 00/18] Landlock LSM: Unprivileged sandboxing

From: MickaÃl SalaÃn
Date: Wed Oct 26 2016 - 02:58:46 EST

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Hi,

This fourth RFC brings some improvements over the previous one [1]. An important
new point is the abstraction from the raw types of LSM hook arguments. It is
now possible to call a Landlock function the same way for LSM hooks with
different internal argument types. Some parts of the code are revamped with RCU
to properly deal with concurrency. From a userland point of view, the only
remaining link with seccomp-bpf is the ability to use the seccomp(2) syscall to
load and enforce a Landlock rule. Seccomp filters cannot trigger Landlock rules
anymore. For now, it is no more possible for an unprivileged user to enforce a
Landlock rule on a cgroup through delegation.

As suggested, I plan to write documentation for userland and kernel developers
with some kind of guiding principles. A remaining question is how to enforce
limitations for the rule creation?

# Landlock LSM

The goal of this new stackable Linux Security Module (LSM) called Landlock is
to allow any process, including unprivileged ones, to create powerful security
sandboxes comparable to the Seatbelt/XNU Sandbox or the OpenBSD Pledge. This
kind of sandbox is expected to help mitigate the security impact of bugs or
unexpected/malicious behaviors in userland applications.

eBPF programs are used to create a security rule. They are very limited (i.e.
can only call a whitelist of functions) and cannot do a denial of service (i.e.
no loop). A new dedicated eBPF map allows to collect and compare Landlock
handles with system resources (e.g. files or network connections).

The approach taken is to add the minimum amount of code while still allowing
the userland to create quite complex access rules. A dedicated security policy
language as the one used by SELinux, AppArmor and other major LSMs involves a
lot of code and is usually dedicated to a trusted user (i.e. root).

# eBPF

To get an expressive language while still being safe and small, Landlock is
based on eBPF. Landlock should be usable by untrusted processes and must then
expose a minimal attack surface. The eBPF bytecode is minimal while powerful,
widely used and designed to be used by not so trusted application. Reusing this
code allows to not reproduce the same mistakes and minimize new code while
still taking a generic approach. Only a few additional features are added like
a new kind of arraymap and some dedicated eBPF functions.

An eBPF program has access to an eBPF context which contains the LSM hook
arguments (as does seccomp-bpf with syscall arguments). They can be used
directly or passed to helper functions according to their types. It is then
possible to do complex access checks without race conditions nor inconsistent
evaluation (i.e. incorrect mirroring of the OS code and state [2]).

There is one eBPF program subtype per LSM hook. This allows to statically check
which context access is performed by an eBPF program. This is needed to deny
kernel address leak and ensure the right use of LSM hook arguments with eBPF
functions. Moreover, this safe pointer handling removes the need for runtime
check or abstract data, which improves performances. Any user can add multiple
Landlock eBPF programs per LSM hook. They are stacked and evaluated one after
the other (cf. seccomp-bpf).

# LSM hooks

Unlike syscalls, LSM hooks are security checkpoints and are not architecture
dependent. They are designed to match a security need associated with a
security policy (e.g. access to a file). Exposing parts of some LSM hooks
instead of using the syscall API for sandboxing should help to avoid bugs and
hacks as encountered by the first RFC. Instead of redoing the work of the LSM
hooks through syscalls, we should use and expose them as does policies of
access control LSM.

Only a subset of the hooks are meaningful for an unprivileged sandbox mechanism
(e.g. file system or network access control). Landlock uses an abstraction of
raw LSM hooks, which allow to deal with possible future API changes of the LSM
hook API. Moreover, thanks to the ePBF program typing (per LSM hook) used by
Landlock, it should not be hard to make such evolutions backward compatible.

# Use case scenario

First, a process needs to create a new dedicated eBPF map containing handles.
This handles are references to system resources (e.g. file or directory) and
grouped in one or multiple maps to be efficiently managed and checked in
batches. This kind of map can be passed to Landlock eBPF functions to compare,
for example, with a file access request. The handles are only accessible from
the eBPF programs created by the same thread.

The loaded Landlock eBPF programs can be triggered by a seccomp filter
returning RET_LANDLOCK. In addition, a cookie (16-bit value) can be passed from
a seccomp filter to eBPF programs. This allow flexible security policies
between seccomp and Landlock.

Another way to enforce a Landlock security policy is to attach Landlock
programs to a dedicated cgroup. All the processes in this cgroup will then be
subject to this policy. For unprivileged processes, this can be done thanks to
cgroup delegation.

A triggered Landlock eBPF program can allow or deny an access, according to
its subtype (i.e. LSM hook), thanks to errno return values.

# Sandbox example with process hierarchy sandboxing (seccomp)

$ ls /home
user1
$ LANDLOCK_ALLOWED='/bin:/lib:/usr:/tmp:/proc/self/fd/0' \
./samples/landlock/sandbox /bin/sh -i
Launching a new sandboxed process.
$ ls /home
ls: cannot access '/home': No such file or directory

# Sandbox example with conditional access control depending on a cgroup

$ mkdir /sys/fs/cgroup/sandboxed
$ ls /home
user1
$ LANDLOCK_CGROUPS='/sys/fs/cgroup/sandboxed' \
LANDLOCK_ALLOWED='/bin:/lib:/usr:/tmp:/proc/self/fd/0' \
./samples/landlock/sandbox
Ready to sandbox with cgroups.
$ ls /home
user1
$ echo $$ > /sys/fs/cgroup/sandboxed/cgroup.procs
$ ls /home
ls: cannot access '/home': No such file or directory

# Current limitations and possible improvements

For now, eBPF programs can only return an errno code. It may be interesting to
be able to do other actions like seccomp-bpf does (e.g. kill process). Such
features can easily be implemented but the main advantage of the current
approach is to be able to only execute eBPF programs until one returns an errno
code instead of executing all programs like seccomp-bpf does.

It is quite easy to add new eBPF functions to extend Landlock. The main concern
should be about the possibility to leak information from current process to
another one (e.g. through maps) to not reproduce the same security sensitive
behavior as ptrace.

This design does not seem too intrusive but is flexible enough to allow a
powerful sandbox mechanism accessible by any process on Linux. The use of
seccomp and Landlock is more suitable with the help of a userland 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 LLVM, it is
possible to express an eBPF program with a subset of C.

# FAQ

## Why does seccomp-bpf is not enough?

A seccomp filter can access to raw syscall arguments which means that it is not
possible to filter according to pointed such as a file path. As the first
version of this patch series demonstrated, 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 LSM hooks level. The LSM hooks are designed to handle this
kind of checks. This series use this approach to leverage the ability of
unprivileged users to limit themselves.

Cf. "What it isn't?" in Documentation/prctl/seccomp_filter.txt

## Why using 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
childs. It makes sense to use an unique access-restricting syscall (that should
be allowed by seccomp-bpf rules) which can only drop privileges. Moreover, a
Landlock eBPF program 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 enforcing via seccomp(2).

## Why using cgroups?

cgroups are designed to handle groups of processes. One use case is to manage
containers. Sandboxing based on process hierarchy (seccomp) is design to handle
immutable security policies, which is a good security property but does not
match all use cases. A user can attach Landlock rules to a cgroup. Doing so,
all the processes in that cgroup will be subject to the security policy.
However, if the user is allowed to manage this cgroup, it could dynamically
move this group of processes to a cgroup with another security policy (or
none). Landlock rules can be applied either on a process hierarchy (e.g.
application with built-in sandboxing) or a group of processes (e.g. container
sandboxing). Both approaches can be combined for the same process.

## Does Landlock can limit network access or other resources?

Limiting network access is obviously in the scope of Landlock but it is not yet
implemented. The main goal now is to get feedback about the whole concept, the
API and the file access control part. More access control types could be
implemented in the future.

Sargun Dhillon sent a RFC (Checmate) [4] to deal with network manipulation.
This could be implemented on top of the Landlock framework.

## Why a new LSM? Are SELinux, AppArmor, Smack or 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 applications by its
developer and *unprivileged user* as seccomp can do for raw syscall filtering.
Moreover, Landlock handles stacked hook programs from different users. It must
then ensure there is no possible malicious interactions between these programs.

Differences with other (access control) LSMs:
* not only dedicated to administrators (i.e. no_new_priv);
* limited kernel attack surface (e.g. policy parsing);
* helpers to compare complex objects (path/FD), no access to internal kernel
data (do not leak addresses);
* constrained policy rules/programs (no DoS: deterministic execution time);
* do not leak more information than the loader process can legitimately have
access to (minimize metadata inference): must compare from an already allowed
file (through a handle).

## Why not use a policy language like used by SElinux or AppArmor?

This kind of LSMs are dedicated to administrators. They already manage the
system and are not a threat to the system security. However, seccomp, and
Landlock too, should be available to anyone, which potentially include
untrusted users and processes. To reduce the attack surface, Landlock should
expose the minimum amount of code, hence minimal complexity. Moreover, another
threat is to make accessible to a malicious code a new way to gain more
information. For example, Landlock features should not allow a program to get
the file owner if the directory containing this file is not readable. This data
could then be exfiltrated thanks to the access result. Thus, we should limit
the expressiveness of the available checks. The current approach is to do the
checks in such a way that only a comparison with an already accessed resource
(e.g. file descriptor) is possible. This allow to have a reference to compare
with, without exposing much information.

## As a developer, why do I need this feature?

Landlock's goal is to help userland to limit its attack surface.
Security-conscious developers would like to protect users from a security bug
in their applications and the third-party dependencies they are using. Such a
bug can compromise all the user data and help an attacker to perform a
privilege escalation. Using an *unprivileged sandbox* feature such as Landlock
empowers the developer with the ability to properly compartmentalize its
software and limit the impact of vulnerabilities.

## As a user, why do I need a this feature?

Any user can already use seccomp-bpf to whitelist a set of syscalls to
reduce the kernel attack surface for a predefined set of processes. However an
unprivileged user can't create a security policy like the root user can thanks to
SELinux and other access control LSMs. Landlock allows any unprivileged user to
protect their data from being accessed by any process they run but only an
identified subset. User tools can be created to help create such a high-level
access control policy. This policy may not be powerful enough to express the
same policies as the current access control LSMs, because of the threat an
unprivileged user can be to the system, but it should be enough for most
use-cases (e.g. blacklist or whitelist a set of file hierarchies).

# Changes since RFC v3

* use abstract LSM hook arguments with custom types (e.g. *_LANDLOCK_ARG_FS for
struct file, struct inode and struct path)
* add more LSM hooks to support full file system access control
* improve the sandbox example
* fix races and RCU issues:
* eBPF program execution and eBPF helpers
* revamp the arraymap of handles to cleanly deal with update/delete
* eBPF program subtype for Landlock:
* remove the "origin" field
* add an "option" field
* rebase onto Daniel Mack's patches v7 [3]
* remove merged commit 1955351da41c ("bpf: Set register type according to
is_valid_access()")
* fix spelling mistakes
* cleanup some type and variable names
* split patches
* for now, remove cgroup delegation handling for unprivileged user
* remove extra access check for cgroup_get_from_fd()
* remove unused example code dealing with skb
* remove seccomp-bpf link:
* no more seccomp cookie
* for now, it is no more possible to check the current syscall properties

# Changes since RFC v2

* revamp cgroup handling:
* use Daniel Mack's patches "Add eBPF hooks for cgroups" v5
* remove bpf_landlock_cmp_cgroup_beneath()
* make BPF_PROG_ATTACH usable with delegated cgroups
* add a new CGRP_NO_NEW_PRIVS flag for safe cgroups
* handle Landlock sandboxing for cgroups hierarchy
* allow unprivileged processes to attach Landlock eBPF program to cgroups
* add subtype to eBPF programs:
* replace Landlock hook identification by custom eBPF program types with a
dedicated subtype field
* manage fine-grained privileged Landlock programs
* register Landlock programs for dedicated trigger origins (e.g. syscall,
return from seccomp filter and/or interruption)
* performance and memory optimizations: use an array to access Landlock hooks
directly but do not duplicated it for each thread (seccomp-based)
* allow running Landlock programs without seccomp filter
* fix seccomp-related issues
* remove extra errno bounding check for Landlock programs
* add some examples for optional eBPF functions or context access (network
related) according to security checks to allow more features for privileged
programs (e.g. Checmate)

# Changes since RFC v1

* focus on the LSM hooks, not the syscalls:
* much more simple implementation
* does not need audit cache tricks to avoid race conditions
* more simple to use and more generic because using the LSM hook abstraction
directly
* more efficient because only checking in LSM hooks
* architecture agnostic
* switch from cBPF to eBPF:
* new eBPF program types dedicated to Landlock
* custom functions used by the eBPF program
* gain some new features (e.g. 10 registers, can load values of different
size, LLVM translator) but only a few functions allowed and a dedicated map
type
* new context: LSM hook ID, cookie and LSM hook arguments
* need to set the sysctl kernel.unprivileged_bpf_disable to 0 (default value)
to be able to load hook filters as unprivileged users
* smaller and simpler:
* no more checker groups but dedicated arraymap of handles
* simpler userland structs thanks to eBPF functions
* distinctive name: Landlock

This series can be applied on top of Daniel Mack's patches for BPF_PROG_ATTACH
v7 [3] on Linux v4.9-rc2. This can be tested with CONFIG_SECURITY_LANDLOCK,
CONFIG_SECCOMP_FILTER and CONFIG_CGROUP_BPF. I would really appreciate
constructive comments on the usability, architecture, code and userland API of
Landlock LSM.

[1] https://lkml.kernel.org/r/20160914072415.26021-1-mic@xxxxxxxxxxx
[2] https://crypto.stanford.edu/cs155/papers/traps.pdf
[3] https://lkml.kernel.org/r/1477390454-12553-1-git-send-email-daniel@xxxxxxxxxx
[4] https://lkml.kernel.org/r/20160829114542.GA20836@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Regards,

MickaÃl SalaÃn (18):
landlock: Add Kconfig
bpf: Move u64_to_ptr() to BPF headers and inline it
bpf,landlock: Add a new arraymap type to deal with (Landlock) handles
bpf,landlock: Add eBPF program subtype and is_valid_subtype() verifier
bpf,landlock: Define an eBPF program type for Landlock
fs: Constify path_is_under()'s arguments
landlock: Add LSM hooks
landlock: Handle file comparisons
landlock: Add manager functions
seccomp: Split put_seccomp_filter() with put_seccomp()
seccomp,landlock: Handle Landlock hooks per process hierarchy
bpf: Cosmetic change for bpf_prog_attach()
bpf/cgroup: Replace struct bpf_prog with struct bpf_object
bpf/cgroup: Make cgroup_bpf_update() return an error code
bpf/cgroup: Move capability check
bpf/cgroup,landlock: Handle Landlock hooks per cgroup
landlock: Add update and debug access flags
samples/landlock: Add sandbox example

fs/namespace.c | 2 +-
include/linux/bpf-cgroup.h | 19 +-
include/linux/bpf.h | 44 +++-
include/linux/cgroup-defs.h | 2 +
include/linux/filter.h | 1 +
include/linux/fs.h | 2 +-
include/linux/landlock.h | 95 +++++++++
include/linux/lsm_hooks.h | 5 +
include/linux/seccomp.h | 12 +-
include/uapi/linux/bpf.h | 105 ++++++++++
include/uapi/linux/seccomp.h | 1 +
kernel/bpf/arraymap.c | 270 +++++++++++++++++++++++++
kernel/bpf/cgroup.c | 139 ++++++++++---
kernel/bpf/syscall.c | 71 ++++---
kernel/bpf/verifier.c | 35 +++-
kernel/cgroup.c | 6 +-
kernel/fork.c | 15 +-
kernel/seccomp.c | 26 ++-
kernel/trace/bpf_trace.c | 12 +-
net/core/filter.c | 26 ++-
samples/Makefile | 2 +-
samples/bpf/bpf_helpers.h | 5 +
samples/landlock/.gitignore | 1 +
samples/landlock/Makefile | 16 ++
samples/landlock/sandbox.c | 405 +++++++++++++++++++++++++++++++++++++
security/Kconfig | 1 +
security/Makefile | 2 +
security/landlock/Kconfig | 23 +++
security/landlock/Makefile | 3 +
security/landlock/checker_fs.c | 152 ++++++++++++++
security/landlock/checker_fs.h | 20 ++
security/landlock/common.h | 58 ++++++
security/landlock/lsm.c | 449 +++++++++++++++++++++++++++++++++++++++++
security/landlock/manager.c | 379 ++++++++++++++++++++++++++++++++++
security/security.c | 1 +
35 files changed, 2309 insertions(+), 96 deletions(-)
create mode 100644 include/linux/landlock.h
create mode 100644 samples/landlock/.gitignore
create mode 100644 samples/landlock/Makefile
create mode 100644 samples/landlock/sandbox.c
create mode 100644 security/landlock/Kconfig
create mode 100644 security/landlock/Makefile
create mode 100644 security/landlock/checker_fs.c
create mode 100644 security/landlock/checker_fs.h
create mode 100644 security/landlock/common.h
create mode 100644 security/landlock/lsm.c
create mode 100644 security/landlock/manager.c

--
2.9.3

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