Tuesday, 21 December 2010

A common misconception about the Chrome sandbox

A common misconception about the Chrome web browser is that its sandbox protects one web site from another.

For example, suppose you are logged into your e-mail account on mail.com in one tab, and have evil.com open in another tab. Suppose evil.com finds an exploit in the renderer process, such as a memory safety bug, that lets it run arbitrary code there. Can evil.com get hold of your HTTP cookies for mail.com, and thereby access your e-mail account?

Unfortunately, the answer is yes.

The reason is that mail.com and evil.com can be assigned to the same renderer process. The browser does not only do this to save memory. evil.com can cause this to happen by opening an iframe on mail.com. With mail.com's code running in the same exploited renderer process, evil.com can take it over and read the cookies for your mail.com account and use them for its own ends.

There are a couple of reasons why the browser puts a framed site in the same renderer process as the parent site. Firstly, if the sites were handled by separate processes, the browser would have to do costly compositing across renderer processes to make the child frame appear inside the parent frame. Secondly, in some cases the DOM allows Javascript objects in one frame to obtain references to DOM objects in other frames, even across origins, and it is easier for this to be managed within one renderer process.

I don't say this to pick on Chrome, of course. It is better to have the sandbox than not to have it.

Chrome has never claimed that the sandbox protects one site against another. In the tech report "The Security Architecture of the Chromium Browser" (Barth, Jackson, Reis and the Chrome Team; 2008), "Origin isolation" is specifically listed under "Out-of-scope goals". They state that "an attacker who compromises the rendering engine can act on behalf of any web site".

There are a couple of ways that web sites and users can mitigate this problem, which I'll discuss in another post. However, in the absence of those defences, what Chrome's multi-process architecture actually gives you is the following:

  • Robustness if a renderer crashes. Having multiple renderer processes means that a crash of one takes down only a limited number of tabs, and the browser and the other renderers will survive. It also helps memory management.

    But we can get this without sandboxing the renderers.

  • Protection of the rest of the user's system from vulnerabilities in the renderer process. For example, the sandboxed renderer cannot read any of the user's files, except for those the user has granted through a "File Upload" file chooser.

    But we can get this by sandboxing the whole browser (including any subprocesses), without needing to have the browser separated from the renderer.

    For example, since 2007 I have been running Firefox under Plash (a sandbox), on Linux.

    In principle, such a sandbox should be more effective at protecting applications and files outside the browser than the Chrome sandbox, because the sandbox covers all of the browser, including its network stack and the so-called browser "chrome" (this means the parts of the GUI outside of the DOM).

    In practice, Plash is not complete as a sandbox for GUI apps because it does not limit access to the X Window System, so apps can do things that X allows such as screen scraping other apps and sending them input.

The main reason Chrome was developed to sandbox its renderer processes but not the whole browser is that this is easier to implement with sandboxing technologies that are easily deployable today. Ideally, though, the whole browser would be sandboxed. One of the only components that would stay unsandboxed, and have access to all the user's files, would be the "File Open" dialog box for choosing files to upload.

Saturday, 18 December 2010

When printf debugging is a luxury

Inserting printf() calls is often considered to be a primitive fallback when other debugging tools are not available, such as stack backtraces with source line numbers.

But there are some situations in low-level programming where most libc calls don't work and so even printf() and assert() are unavailable luxuries. This can happen:

  • when libc is not properly initialised yet;
  • when we writing code that is called by libc and cannot re-enter libc code;
  • when we are in a signal handler;
  • when only limited stack space is available;
  • when we cannot allocate memory for some reason; or
  • when we are not even linked to libc.

Here's a fragment of code that has come in handy in these situations. It provides a simple assert() implementation:

#include <string.h>
#include <unistd.h>

static void debug(const char *msg) {
  write(2, msg, strlen(msg));

static void die(const char *msg) {

#define TO_STRING_1(x) #x
#define TO_STRING(x) TO_STRING_1(x)

#define assert(expr) {                                                        \
  if (!(expr)) die("assertion failed at " __FILE__ ":" TO_STRING(__LINE__)    \
                   ": " #expr "\n"); }

By using preprocessor trickery to construct the assertion failure string at compile time, it avoids having to format the string at runtime. So it does not need to allocate memory, and it doesn't need to do multiple write() calls (which can become interleaved with other output in the multi-threaded case).

Sometimes even libc's write() is a luxury. In some builds of GNU libc on Linux, glibc's syscall wrappers use the TLS register (%gs on i386) to fetch the address of a routine for making syscalls.

However, if %gs is not set up properly for some reason, this will fail. For example, for Native Client's i386 sandbox, %gs is set to a different value whenever sandboxed code is running, and %gs stays in this state if sandboxed code faults and triggers a signal handler. In Chromium's seccomp-sandbox, %gs is set to zero in the trusted thread.

In those situations we have to bypass libc and do the system calls ourselves. The following snippet comes from reference_trusted_thread.cc. The sys_*() functions are defined by linux_syscall_support.h, which provides wrappers for many Linux syscalls:

#include "linux_syscall_support.h"

void die(const char *msg) {
  sys_write(2, msg, strlen(msg));