# Linux `SUID` Sandbox *IMPORTANT NOTE: The Linux SUID sandbox is almost but not completely removed. See https://bugs.chromium.org/p/chromium/issues/detail?id=598454 This page is mostly out-of-date.* With [r20110](https://crrev.com/20110), Chromium on Linux can now sandbox its renderers using a `SUID` helper binary.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](sandboxing.md). ## `SUID` helper executable The `SUID` helper binary is called `chrome_sandbox` and you must build it separately from the main 'chrome' target. Chrome now just assumes it's next to the executable in the same directory. You can also control its path by CHROME_DEVEL_SANDBOX environment variable. 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](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](suid_sandbox_development.md) * [LinuxSandboxing](sandboxing.md) * General information on Chromium sandboxing: https://dev.chromium.org/developers/design-documents/sandbox