Port sandbox, sandbox faq, and startup design docs to markdown.

docs/design/sandbox_faq.md

+# Sandbox FAQ
+
+[TOC]
+
+### What is the sandbox?
+
+The sandbox is a C++ library that allows the creation of sandboxed processes —
+processes that execute within a very restrictive environment. The only resources
+sandboxed processes can freely use are CPU cycles and memory. For example,
+sandboxes processes cannot write to disk or display their own windows. What
+exactly they can do is controlled by an explicit policy. Chromium renderers are
+sandboxed processes.
+
+### What does and doesn't it protect against?
+
+The sandbox limits the severity of bugs in code running inside the sandbox. Such
+bugs cannot install persistent malware in the user's account (because writing to
+the filesystem is banned). Such bugs also cannot read and steal arbitrary files
+from the user's machine.
+
+(In Chromium, the renderer processes are sandboxed and have this
+protection. After the NPAPI removal, all remaining plugins are also
+sandboxed. Also note that Chromium renderer processes are isolated from the
+system, but not yet from the web. Therefore, domain-based data isolation is not
+yet provided.).
+
+The sandbox cannot provide any protection against bugs in system components such
+as the kernel it is running on.
+
+### Is the sandbox like what you get with the Java VM?
+
+Yeah, kind of... except that to take advantage of the Java sandbox, you must
+rewrite your code to use Java. With our sandbox you can add sandboxing to your
+existing C/C++ applications. Because the code is not executed inside a virtual
+machine, you get native speed and direct access to the Windows API.
+
+### Do I need to install a driver or kernel module? Does the user need to be Administrator?
+
+No and no. The sandbox is a pure user-mode library, and any user can run
+sandboxed processes.
+
+### How can you do this for C++ code if there is no virtual machine?
+
+We leverage the Windows security model. In Windows, code cannot perform any form
+of I/O (be it disk, keyboard, or screen) without making a system call. In most
+system calls, Windows performs some sort of security check. The sandbox sets
+things up so that these security checks fail for the kinds of actions that you
+don’t want the sandboxed process to perform. In Chromium, the sandbox is such
+that all access checks should fail.
+
+### So how can a sandboxed process such as a renderer accomplish anything?
+
+Certain communication channels are explicitly open for the sandboxed processes;
+the processes can write and read from these channels. A more privileged process
+can use these channels to do certain actions on behalf of the sandboxed
+process. In Chromium, the privileged process is usually the browser process.
+
+### Doesn't Vista have similar functionality? 
+
+Yes. It's called integrity levels (ILs). The sandbox detects Vista and uses
+integrity levels, as well. The main difference is that the sandbox also works
+well under Windows XP. The only application that we are aware of that uses ILs
+is Internet Explorer 7. In other words, leveraging the new Vista security
+features is one of the things that the sandbox library does for you.
+
+### This is very neat. Can I use the sandbox in my own programs?
+
+Yes. The sandbox does not have any hard dependencies on the Chromium browser and
+was designed to be used with other Internet-facing applications. The main hurdle
+is that you have to split your application into at least two interacting
+processes. One of the processes is privileged and does I/O and interacts with
+the user; the other is not privileged at all and does untrusted data processing.
+
+### Isn't that a lot of work?
+
+Possibly. But it's worth the trouble, especially if your application processes
+arbitrary untrusted data. Any buffer overflow or format decoding flaw that your
+code might have won't automatically result in malicious code compromising the
+whole computer. The sandbox is not a security silver bullet, but it is a strong
+last defense against nasty exploits.
+
+### Should I be aware of any gotchas?
+
+Well, the main thing to keep in mind is that you should only sandbox code that
+you fully control or that you fully understand. Sandboxing third-party code can
+be very difficult. For example, you might not be aware of third-party code's
+need to create temporary files or display warning dialogs; these operations will
+not succeed unless you explicitly allow them. Furthermore, third-party
+components could get updated on the end-user machine with new behaviors that you
+did not anticipate.
+
+### How about COM, Winsock, or DirectX — can I use them?
+
+For the most part, no. We recommend against using them before lock-down. Once a
+sandboxed process is locked down, use of Winsock, COM, or DirectX will either
+malfunction or outright fail.
+
+### What do you mean by before _lock-down_? Is the sandboxed process not locked down from the start?
+
+No, the sandboxed process does not start fully secured. The sandbox takes effect
+once the process makes a call to the sandbox method `LowerToken()`. This allows
+for a period during process startup when the sandboxed process can freely get
+hold of critical resources, load libraries, or read configuration files. The
+process should call `LowerToken()` as soon as feasible and certainly before it
+starts interacting with untrusted data.
+
+**Note:** Any OS handle that is left open after calling `LowerToken()` can be
+abused by malware if your process gets infected. That's why we discourage
+calling COM or other heavyweight APIs; they leave some open handles around for
+efficiency in later calls.
+
+### So what APIs can you call?
+
+There is no master list of safe APIs. In general, structure your code such that
+the sandboxed code reads and writes from pipes or shared memory and just does
+operations over this data. In the case of Chromium, the entire WebKit code runs
+this way, and the output is mostly a rendered bitmap of the web pages. You can
+use Chromium as inspiration for your own memory- or pipe-based IPC.
+
+### But can't malware just infect the process at the other end of the pipe or shared memory?
+
+Yes, it might, if there's a bug there. The key point is that it's easier to
+write and analyze a correct IPC mechanism than, say, a web browser
+engine. Strive to make the IPC code as simple as possible, and have it reviewed
+by others.