Linux sandboxing with LSMSB (07 Jun 2009)

Chrome Linux got a dev channel release and I'm very happy with it. It's now my primary browser.

However, one of the big selling points for Chrome on Windows is that the renderers (which deal with decoding the HTML, CSS, image files etc) are sandboxed. We've had exploitable issues in the renderers which which have been stopped by the sandbox. It's a Good Thing.

However, we don't have a sandbox on Linux! The Mac team have been talking about how nice their sandbox is (and I expect we'll get some official documentation about it after WWDC this week). We have to hack around with SUID binaries, chrooting, seccomp and one-size-fits all SELinux solutions.

(I don't wish to discount the good work that the SELinux folks have done: we'll probably use something like that sandbox on Fedora, but Chromium was very carefully written to be sandboxed and we should aim higher.)

So, as part of the exploration of what we could do with sandboxing on Linux, longer term, I have a prototype implementation of LSMSB. It's another literate program of mine, so you can usefully read the source too. The README is included below:

This is LSMSB, a sandboxing scheme for Linux based on the ideas of the OS X
sandbox (which, in turn, was inspired by TrustedBSD and FreeBSD).

Imagine that you're working on a university computer and you get a binary which
promises to do some fiendishly complex calculation, reading from a file ./input
and writing to a file ./output. It also talks to a specific server to access a
pre-computed lookup table. You want to run it, but you don't want to have to
trust that it won't do anything malicious (save giving the wrong answer).

This code is incomplete, but currently you can take a sandbox specification
like this:

filter dentry-open {
  constants {
    var etc-prefix bytestring = "/etc/";

  ldc r2,etc-prefix;
  isprefixof r2,r2,r0;
  jc r2,#fail;
  ldi r0,1;
  ret r0;
  ldi r0,0;
  ret r0;

... and use it to remove access to /etc.

*** This code functions, but is incomplete ***

It's written in a literate programming style, but the derived sources are
included so that you don't have to bother with that in order to build. You'll
need a recent (> 2.6.30-rc1) kernel in order to apply the included patch. Once
you've applied the patch, drop lsmsb.c into security/lsmsb and rebuild.

You can assemble a sandbox file with:
  ./lsmsb-as > sandbox
And then run a shell in the sandbox with:
  ./lsmsb-install sandbox

To read the code, see