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

Avi Kivity
2011-05-26T06:08:55Z

On 05/26/2011 04:19 AM, James Morris wrote:
> On Wed, 25 May 2011, Linus Torvalds wrote:
>
> > And per-system-call permissions are very dubious. What system calls
> > don't you want to succeed? That ioctl? You just made it impossible to
> > do a modern graphical application. Yet the kind of thing where we
> > would _want_ to help users is in making it easier to sandbox something
> > like the adobe flash player. But without accelerated direct rendering,
> > that's not going to fly, is it?
>
> Going back to the initial idea proposed by Will, where seccomp is simply
> extended to filter all syscalls, there is potential benefit in being able
> to limit the attack surface of the syscall API.
>
> This is not security mediation in terms of interaction between things
> (e.g. "allow A to read B"). It's a _hardening_ feature which prevents a
> process from being able to invoke potentially hundreds of syscalls is has
> no need for. It would allow us to usefully restrict some well-established
> attack modes, e.g. triggering bugs in kernel code via unneeded syscalls.
>
> This is orthogonal to access control schemes (such as SELinux), which are
> about mediating security-relevant interactions between objects.
>
> One area of possible use is KVM/Qemu, where processes now contain entire
> operating systems, and the attack surface between them is now much broader
> e.g. a local unprivileged vulnerability is now effectively a 'remote' full
> system compromise.
>
> There has been some discussion of this within the KVM project. Using the
> existing seccomp facility is problematic in that it requires significant
> reworking of Qemu to a privsep model, which would also then incur a likely
> unacceptable context switching overhead. The generalized seccomp filter
> as proposed by Will would provide a significant reduction in exposed
> syscalls and thus guest->host attack surface.
>
> I've cc'd some KVM folk for more input on how this may or may not meet
> their requirements -- Avi/Gleb, there's a background writeup here:
> http://lwn.net/Articles/442569/ . We may need a proof of concept and/or
> commitment to use this feature for it to be accepted upstream.

Indeed are were looking at sandboxing as a means to mitigate the "guest
exploits qemu, proceeds to exploit host syscall interface" scenario, and
evolved seccomp looks like the best tradeoff in terms of security gains
vs effort needed.

Eric Paris (copied) prototyped this with his own version of enhanced
seccomp and achieved pretty good results, so a proof of concept will be
quite easy to provide.

Regarding dynamic filtering, the biggest question here is how this will
interact with hotplug, which requires new files to be opened in the
sandboxed process (or SCM_RIGHTed in). Any fd-based filtering will
defeat that, so we'll need some way for a privileged monitor to adjust
filters.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.