Sandboxing

This document describes how sandboxing works in Fuchsia.

An empty process has nothing

On Fuchsia, a newly created process has nothing. A newly created process cannot access any kernel objects, cannot allocate memory, and cannot even execute code. Of course, such a process isn't very useful, which is why we typically create processes with some initial resources and capabilities.

Most commonly, a process starts executing some code with an initial stack, some command line arguments, environment variables, a set of initial handles. One of the most important initial handles is the PA_VMAR_ROOT, which the process can use to map additional memory into its address space.

Namespaces are the gateway to the world

Some of the initial handles given to a process are directories that the process mounts into its namespace. These handles let the process discover and communicate with other processes running on the system, including file systems and other servers. See Namespaces for more details.

The namespace given to a process strongly influences how much of the system the process can influence. Therefore, configuring the sandbox in which a process runs amounts to configuring the process's namespace.

Package namespace

A component run from a package is given access to /pkg, which is a read-only view of the package containing the component. To access these resources at runtime, a process can use the /pkg namespace. For example, the root_presenter can access cursor32.png using the absolute path /pkg/data/cursor32.png.

Services

Processes that are components receive an /svc directory in their namespace. The services available through /svc are a subset of the services provided by the component‘s environment. This subset is determined by the sandbox.services whitelist in the component’s manifest file.

A typical component will interact with a number of services from /svc in order to play some useful role in the system. For example, the service fuchsia.sys.Launcher is required if a component wishes to launch other components.

Processes that are not components may or may not have /svc. These processes receive whatever /svc their creator decided to provide to them.

NOTE: In the past, there existed no mechanism for service sandboxing and a component received all services in its environment. Pre-existing components have been grandfathered to receive all services with the deprecated-all-services feature, which will eventually be removed. Please do not use deprecated-all-services for new components.

Configuring additional namespaces

If a process requires access to additional resources (e.g., device drivers), the package can request access to additional names by including the sandbox property in its Component Manifest for the package. For example, the following meta/sandbox file requests direct access to the input driver:

{
    "dev": [ "class/input" ]
}

In the current implementation, the AppMgr grants all such requests, but that is likely to change as the system evolves.

Building a package

To build a package, use the package() macro in gn defined in //build/package.gni. See the documentation for the package() macro for details about including resources.

For examples, see [https://fuchsia.googlesource.com/garnet/+/master/packages/prod/fortune] and [https://fuchsia.googlesource.com/garnet/+/master/bin/fortune/BUILD.gn].