seccomp-pledge

Introduction

seccomp-pledge is a systems hardening tool for Linux that marries seccomp-bpf to pledge and unveil.

For reference, seccomp-bpf is a feature in the Linux kernel that allows specifying filters for system calls spawned by processes in the form of Berkeley Packet Filter (BPF) programs. A configurable set of policies (Allow, Errno, Trap, etc.) determines the kind of filtering that will be applied to system calls intercepted by seccomp. This minimizes the attack surface of the kernel that is exposed to userland applications.

pledge is a sandboxing mechanism implemented in OpenBSD that restricts the operational capabilities of processes by defining a set of promises that determine the system calls which will be made unavailable to the respective process. It has been ported to Linux as a standalone binary by Justine Tunney. Find more information about pledge here.

unveil is another OpenBSD sandboxing mechanism that is used to provide path permissions to processes. By default, a pledge sandbox will restrict access to the entire filesystem. It is possible to allow a process access to some path using unveil if required. The type of permissions granted (read-only, read-write, etc.) can also be specified. Justine Tunney's pledge port incorporates support for unveil.

This tool upholds the principle of the least privilege (PoLP) and limits processes to exactly what they are designed for, disallowng any non-essential operations as desired. It will cause core dumps if the user blocks a syscall that is fundamental to the execution of the process, which implies that the seccomp filters need to be constructed with care and pledge promises must be chosen appropriately.

A detailed list of syscalls with an explanation of the parameters can be found here.

Supported platforms

Since seccomp is Linux-specific, syscall filtering using this feature is supported only on Linux systems. Non-Linux systems will have to proceed without seccomp-bpf filters.

Features

• Accept the process to be executed (with optional flags) as an argument
• Perform optional dependency checking
• Display the list of syscalls (name and arguments) spawned by the process using lurk
• Use seccompiler as a high-level interface for defining seccomp-bpf filters
• Serialize the syscall list and filter list into JSON objects using serde and save at current path for later reference
• Install user-defined filters as BPF programs for current and child processes
• Support for running in three modes: interactively, non-interactively by directly supplying all necessary arguments during execution and via a Unix IPC socket as an API layer for communication
• Optionally fetch Justine Tunney's Linux port of pledge from upstream or use the local version to wrap around command invocations
• Provide intuitive prompts to simplify the process of constructing seccomp filters and selecting pledge promises and unveil path permissions

Dependencies

seccomp-pledge has the following dependencies:

• seccompiler - Provides easy-to-use Linux seccomp-bpf jailing
• lurk - A pretty (simple) alternative to strace
• pledge - Linux port of OpenBSD's pledge(2)
• serde - Framework for (de)serializing data structures in Rust
• wget - Retrieve files from the web using HTTP(S)
• optional-field - Provides a Rust type for values that can be missing/null

Installation

To run seccomp-pledge, ensure cargo is installed on your system and follow these steps:

git clone https://github.com/DeviousCilantro/seccomp-pledge.git
cd seccomp-pledge
cargo build --release
./target/release/seccomp-pledge [FLAGS] [COMMAND]

Usage

Here is a quick overview of seccomp-pledge:

USAGE:
    <path to seccomp-pledge binary> [--check | --no-check] [--local | --remote] [--api | --no-api] [COMMAND]...
    
ARGS:
    <command>... Sandbox command

FLAGS:
    --check
        Proceed with dependency checking
    --no-check
        Continue without dependency checking
    --local
        Use the local pledge binary supplied
    --remote
        Fetch remote pledge binary from upstream using wget
    --api
        Communicate via Unix socket API
    --no-api
        Disable Unix socket API
    -v [PERMS]:PATH
        Unveil path. Defaults to read-only operations
    -p  PROMISES
        Specify the pledge promises to be granted to the process. Defaults to 'stdio rpath'

When the -v and -p flags are supplied, seccomp-pledge will run in non-interactive mode, assuming all the necessary arguments have been provided during execution. This skips seccomp filtering.

When the --api flag is supplied, seccomp-pledge will run in API mode, creating a Unix socket at /tmp/seccomp-pledge.sock which can then be used for communication using, for instance, the Linux port of OpenBSD's netcat available in most package manager repositories. After installing netcat, run the following command to use the socket: nc -U /tmp/seccomp-pledge.sock

Otherwise, seccomp-pledge runs in standard interactive mode, guiding the user through the different stages of creating the seccomp filtering policy and defining the pledge sandbox.

Demonstrations

This is a series of demonstrations on the three modes of using seccomp-pledge for sandboxing, for instance, ls. The pledge promises supplied are stdio, rpath, and tty and the current directory is unveiled with the default read-only permissions. seccomp filtering is disabled for the non-interactive mode.

Non-interactive mode:

Interactive mode:

API mode: