Subject: Re: [PATCH 3/5] v2 seccomp_filters: Enable ftrace-based system call filtering

From: Ingo Molnar <[email-masked]>
Date: Fri, 13 May 2011 14:10:34 +0200

* James Morris <[email-masked]> wrote:

> On Thu, 12 May 2011, Ingo Molnar wrote:
> > Funnily enough, back then you wrote this:
> > 
> >   " I'm concerned that we're seeing yet another security scheme being designed on 
> >     the fly, without a well-formed threat model, and without taking into account 
> >     lessons learned from the seemingly endless parade of similar, failed schemes. "
> > 
> > so when and how did your opinion of this scheme turn from it being an 
> > "endless parade of failed schemes" to it being a "well-defined and readily 
> > understandable feature"? :-)
> 
> When it was defined in a way which limited its purpose to reducing the attack 
> surface of the sycall interface.

Let me outline a simple example of a new filter expression based security 
feature that could be implemented outside the narrow system call boundary you 
find acceptable, and please tell what is bad about it.

Say i'm a user-space sandbox developer who wants to enforce that sandboxed code 
should only be allowed to open files in /home/sandbox/, /lib/ and /usr/lib/.

It is a simple and sensible security feature, agreed? It allows most code to 
run well and link to countless libraries - but no access to other files is 
allowed.

I would also like my sandbox app to be able to install this policy without 
having to be root. I do not want the sandbox app to have permission to create 
labels on /lib and /usr/lib and what not.

Firstly, using the filter code i deny the various link creation syscalls so 
that sandboxed code cannot escape for example by creating a symlink to outside 
the permitted VFS namespace. (Note: we opt-in to syscalls, that way new 
syscalls added by new kernels are denied by defalt. The current symlink 
creation syscalls are not opted in to.)

But the next step, actually checking filenames, poses a big hurdle: i cannot 
implement the filename checking at the sys_open() syscall level in a secure 
way: because the pathname is passed to sys_open() by pointer, and if i check it 
at the generic sys_open() syscall level, another thread in the sandbox might 
modify the underlying filename *after* i've checked it.

But if i had a VFS event at the fs/namei.c::getname() level, i would have 
access to a central point where the VFS string becomes stable to the kernel and 
can be checked (and denied if necessary).

A sidenote, and not surprisingly, the audit subsystem already has an event 
callback there:

        audit_getname(result);

Unfortunately this audit callback cannot be used for my purposes, because the 
event is single-purpose for auditd and because it allows no feedback (no 
deny/accept discretion for the security policy).

But if had this simple event there:

	err = event_vfs_getname(result);

I could implement this new filename based sandboxing policy, using a filter 
like this installed on the vfs::getname event and inherited by all sandboxed 
tasks (which cannot uninstall the filter, obviously):

  "
	if (strstr(name, ".."))
		return -EACCESS;

	if (!strncmp(name, "/home/sandbox/", 14) &&
	    !strncmp(name, "/lib/", 5) &&
	    !strncmp(name, "/usr/lib/", 9))
		return -EACCESS;

  "

  #
  # Note1: Obviously the filter engine would be extended to allow such simple string
  #        match functions. )
  #
  # Note2: ".." is disallowed so that sandboxed code cannot escape the restrictions
  #         using "/..".
  #

This kind of flexible and dynamic sandboxing would allow a wide range of file 
ops within the sandbox, while still isolating it from files not included in the 
specified VFS namespace.

( Note that there are tons of other examples as well, for useful security features
  that are best done using events outside the syscall boundary. )

The security event filters code tied to seccomp and syscalls at the moment is 
useful, but limited in its future potential.

So i argue that it should go slightly further and should become:

 - unprivileged:  application-definable, allowing the embedding of security 
                  policy in *apps* as well, not just the system

 - flexible:      can be added/removed runtime unprivileged, and cheaply so

 - transparent:   does not impact executing code that meets the policy

 - nestable:      it is inherited by child tasks and is fundamentally stackable,
                  multiple policies will have the combined effect and they
                  are transparent to each other. So if a child task within a
                  sandbox adds *more* checks then those add to the already
                  existing set of checks. We only narrow permissions, never
                  extend them.

 - generic:       allowing observation and (safe) control of security relevant
                  parameters not just at the system call boundary but at other
                  relevant places of kernel execution as well: which 
                  points/callbacks could also be used for other types of event 
                  extraction such as perf. It could even be shared with audit ...

I argue that this is the LSM and audit subsystems designed right: in the long 
run it could allow everything that LSM does at the moment - and so much more 
...

And you argue that allowing this would be bad, if it was extended like that 
then you'd consider it a failed scheme? Why?

Thanks,

	Ingo