# Linux `SUID` Sandbox

With [r20110](https://crrev.com/20110), Chromium on Linux can now sandbox its
renderers using a `SUID` helper binary. This is one of
[our layer-1 sandboxing solutions](linux_sandboxing.md).

## `SUID` helper executable

The `SUID` helper binary is called `chrome_sandbox` and you must build it
separately from the main 'chrome' target. To use this sandbox, you have to
specify its path in the `linux_sandbox_path` GYP variable. When spawning the
[zygote process](linux_zygote.md), if the `SUID` sandbox is enabled, Chromium
will check for the sandbox binary at the location specified by
`linux_sandbox_path`. For Google Chrome, this is set to
`/opt/google/chrome/chrome-sandbox`, and early version had this value hard coded
in `chrome/browser/zygote_host_linux.cc`.


In order for the sandbox to be used, the following conditions must be met:

*   The sandbox binary must be executable by the Chromium process.
*   It must be `SUID` and executable by other.

If these conditions are met then the sandbox binary is used to launch the zygote
process. Once the zygote has started, it asks a helper process to chroot it to a
temp directory.

## `CLONE_NEWPID` method

The sandbox does three things to restrict the authority of a sandboxed process.
The `SUID` helper is responsible for the first two:

*   The `SUID` helper chroots the process. This takes away access to the
    filesystem namespace.
*   The `SUID` helper puts the process in a PID namespace using the
    `CLONE_NEWPID` option to
    [clone()](http://www.kernel.org/doc/man-pages/online/pages/man2/clone.2.html).
    This stops the sandboxed process from being able to `ptrace()` or `kill()`
    unsandboxed processes.

In addition:

*   The [Linux Zygote](linux_zygote.md) startup code sets the process to be
    _undumpable_ using
    [prctl()](http://www.kernel.org/doc/man-pages/online/pages/man2/prctl.2.html).
    This stops sandboxed processes from being able to `ptrace()` each other.
    More specifically, it stops the sandboxed process from being `ptrace()`'d by
    any other process. This can be switched off with the
    `--allow-sandbox-debugging` option.

Limitations:

*   Not all kernel versions support `CLONE_NEWPID`. If the `SUID` helper is run
    on a kernel that does not support `CLONE_NEWPID`, it will ignore the problem
    without a warning, but the protection offered by the sandbox will be
    substantially reduced. See LinuxPidNamespaceSupport for how to test whether
    your system supports PID namespaces.
*   This does not restrict network access.
*   This does not prevent processes within a given sandbox from sending each
    other signals or killing each other.
*   Setting a process to be undumpable is not irreversible. A sandboxed process
    can make itself dumpable again, opening itself up to being taken over by
    another process (either unsandboxed or within the same sandbox).
    *   Breakpad (the crash reporting tool) makes use of this. If a process
        crashes, Breakpad makes it dumpable in order to use ptrace() to halt
        threads and capture the process's state at the time of the crash. This
        opens a small window of vulnerability.

## `setuid()` method

_This is an alternative to the `CLONE_NEWPID` method; it is not currently
implemented in the Chromium codebase._

Instead of using `CLONE_NEWPID`, the `SUID` helper can use `setuid()` to put the
process into a currently-unused UID, which is allocated out of a range of UIDs.
In order to ensure that the `UID` has not been allocated for another sandbox,
the `SUID` helper uses
[getrlimit()](http://www.kernel.org/doc/man-pages/online/pages/man2/getrlimit.2.html)
to set `RLIMIT_NPROC` temporarily to a soft limit of 1. (Note that the docs
specify that [setuid()](http://www.kernel.org/doc/man-pages/online/pages/man2/setuid.2.html)
returns `EAGAIN` if `RLIMIT_NPROC` is exceeded.)  We can reset `RLIMIT_NPROC`
afterwards in order to allow the sandboxed process to fork child processes.

As before, the `SUID` helper chroots the process.

As before, LinuxZygote can set itself to be undumpable to stop processes in the
sandbox from being able to `ptrace()` each other.

Limitations:

*   It is not possible for an unsandboxed process to `ptrace()` a sandboxed
    process because they run under different UIDs. This makes debugging harder.
    There is no equivalent of the `--allow-sandbox-debugging` other than turning
    the sandbox off with `--no-sandbox`.
*   The `SUID` helper can check that a `UID` is unused before it uses it (hence
    this is safe if the `SUID` helper is installed into multiple chroots), but
    it cannot prevent other root processes from putting processes into this
    `UID` after the sandbox has been started. This means we should make the
    `UID` range configurable, or distributions should reserve a `UID` range.

## `CLONE_NEWNET` method

The `SUID` helper uses
[CLONE_NEWNET](http://www.kernel.org/doc/man-pages/online/pages/man2/clone.2.html)
to restrict network access.

## Future work

We are splitting the `SUID` sandbox into a separate project which will support
both the `CLONE_NEWNS` and `setuid()` methods:
http://code.google.com/p/setuid-sandbox/

Having the `SUID` helper as a separate project should make it easier for
distributions to review and package.

## Possible extensions

## History

Older versions of the sandbox helper process will _only_ run
`/opt/google/chrome/chrome`. This string is hard coded
(`sandbox/linux/suid/sandbox.cc`). If your package is going to place the
Chromium binary somewhere else you need to modify this string.

## See also

*   [LinuxSUIDSandboxDevelopment](linux_suid_sandbox_development.md)
*   [LinuxSandboxing](linux_sandboxing.md)
*   General information on Chromium sandboxing:
    http://dev.chromium.org/developers/design-documents/sandbox