Subject: Re: Network isolation with RLIMIT_NETWORK, cont'd.

From: Michael Stone <[email-masked]>
Date: Sun, 13 Dec 2009 08:44:25 -0500

Rémi,                                                                                                                                                                                                                                                               
> You explicitly mention the need to connect to the X server over local sockets.
> But won't that allow the sandboxed application to send synthetic events to any
> other X11 applications? 

X11 cookie authentication and socket ownership+permissions effectively control
access to the X server by local processes. Thus, as an isolation author, I may
easily grant my isolated process any of:

   a) full access to the main X server 
   b) some access to a nested X server (like a Xephyr) which I'm using to do
      some event filtering
   c) no access to any X server by witholding thec cookies or by changing the
      permissions on the X socket to be more restrictive

with existing techniques.

> Hence unless the whole X server has restricted network access, this seems a
> bit broken? 

Not broken for the reasons I mentioned above. However, using this rlimit to
disable fresh network access for the whole X server actually sounds like a
rather nice idea; thanks for suggesting it.

> D-Bus, which also uses local sockets, will exhibit similar issues, 

Absolutely. However, D-Bus, like X, already has strong authentication
mechanisms in place that permit me to use pre-existing Unix discretionary 
access control to limit what communication takes place. More specifically, I can 

   a) tell D-Bus to use a file-system socket and change the credentials on that
      socket

   b) use cookies to authenticate incoming connections

   c) explicitly tell D-Bus what users and groups may connect via configuration
      files

   d) explicitly tell D-Bus what users and groups may send and receive which
      messages via configuration files

> as will any unrestricted IPC mechanism in fact. I am not sure if restricting
> network access but not other file descriptors makes that much sense...? Then
> again, I'm not entirely clear what you are trying to solve.

Inadequately access-controlled IPC mechanisms are the specific problem that I
am trying to address. Fortunately, these mechanisms seem to be rare: the only
two that I know of are non-AF_UNIX sockets and ptrace(). All the other IPC
mechanisms that I have seen may be adequately restricted by changing file
permissions and ownership.

> If I had to sandbox something, I'd drop the process file limit to 0. 

That is a technique that is commonly used by many people in this space. It
works well for some limited use cases and, like SECCOMP, is too restrictive for
the kinds of general-purpose applications that I'm sandboxing.

If you're interested,

   http://cr.yp.to/unix/disablenetwork.html

lists several specific problems. To see more, just try dropping RLIMIT_NOFILE
to 0 before launching all your favorite apps. I'd be curious to hear how far
you get.

Regards,

Michael
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