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seccomp-scopes

Goal: Make application level sandboxing easy and mitigate the consequences of an attacker taking control of a process.

BETA SOFTWARE: Do not use for production quality software yet, but give it a go and let me know if you run into issues.

TL;DR

Current implementation is like OpenBSD's pledge().

// Open a bunch of files.
if (pledge("stdio", NONE) == -1) {
    errx(1, "Could not pledge!");
}
// From now on only basic input and output on previously
// opened file descriptors are allowed, as well as acquiring
// anonymous virtual memory.

Any attempt to do system calls outside the previously "pledged" functionality will not be permitted and signal SIGSYS to the process.

For now, only a subset of OpenBSD's features are supported.

Motivation

Unsafe languages like C have a long history of memory corruption bugs, which can be exploited by attackers through specially crafted input data, giving them full control over the program.

Ideally, the world would abandon most software written in C, but

  • rewriting software is expensive
  • some software is purposefully written in C, such as system software and very performance-critical libraries

Moreover, there are entire software categories such as multimedia libraries, which are both regularly written in C, and which at the same time regularly handle data from untrusted sources.

This project wants to make it easy for Linux processes to lock themselves into a very narrow set of privileged operations (system calls), providing enough privileges for the task at hand, but restricting potential attackers who take over the process.

The project builds on Linux's seccomp(2) feature, which provides great value, but is also difficult to configure and use, requiring detailed understanding of Linux's system calls. Seccomp-scopes abstracts away these complexities with a simpler interface and more coarse-grained "permission scopes". The scopes provide sensible presets for common tasks and let application developers talk about operating system features in a more abstract way than by listing system call numbers.

Seccomp-scopes is strongly inspired by OpenBSD's pledge() feature.

In simple words

The system call is the fundamental interface between an application and the Linux kernel. -- syscalls(2)

On their own, Linux processes only have access to their own (virtual) memory, but not to system resources such as hardware, which is shared between processes and whose usage is coordinated by the operating system.

In order to do privileged operations such as reading data from disk, processes invoke a so-called system call to ask the operating system for help. The operating system does necessary permission checks and then performs the requested operation on behalf of the process.

For example, in order to open a file for reading, a process might invoke the open() system call, which returns a handle ("file descriptor") to the opened file:

int fd = open("hello.txt", O_RDONLY);

With the seccomp(2) Linux kernel feature, processes can drop their ability to do system calls of their own choosing. This ability can not be regained for that process, and the same restrictions also apply to any attacker who manages to take over the process.

Example: Multimedia processing

Multimedia libraries for parsing multimedia file formats

  • are often written in C for performance reasons,
  • regularly process untrusted data, and
  • are usually large enough to contain an unknown number of bugs.

However, after opening the required files for reading, and having a file descriptor for writing the results, a multimedia parser usually doesn't need many additional permissions. In fact, the only system calls still needed are now for:

  • allocating more memory (system calls that malloc() calls internally)
  • reading and writing on open file descriptors
  • exiting the process

The core of the multimedia processing is otherwise just making use of the CPU for processing and of the existing process virtual memory, which are both available without system calls.

Note that the time at which the parser is dropping permissions is before it started looking at any attacker-provided data. If an attacker gains control over the process through the multimedia data, he will be trapped in a sandboxed process where the only malevolence possible is to write a different result out over the output file descriptor.

The attacker

  • can not see or manipulate any files on the system
  • can not access the network
  • can not see or manipulate processes in the system
  • can not make use of other resources the process user has access to
  • etc.