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

Avi Kivity
2011-05-26T10:47:06Z

On 05/26/2011 01:38 PM, Ingo Molnar wrote:
> * Gleb Natapov<gleb@redhat.com> wrote:
>
> > On Thu, May 26, 2011 at 11:57:51AM +0300, Pekka Enberg wrote:
> > > Hi Avi,
> > >
> > > On Thu, May 26, 2011 at 11:49 AM, Avi Kivity<avi@redhat.com> wrote:
> > >
> > > > You mean each thread will have a different security context? I
> > > > don't see the point. All threads share all of memory so it
> > > > would be trivial for one thread to exploit another and gain all
> > > > of its privileges.
> > >
> > > So how would that happen? I'm assuming that once the security
> > > context has been set up for a thread, you're not able to change
> > > it after that. You'd be able to exploit other threads through
> > > shared memory but how would you gain privileges?
> >
> > By tricking other threads to execute code for you. Just replace
> > return address on the other's thread stack.
>
> That kind of exploit is not possible if the worker pool consists of
> processes - which would be rather easy to achieve with tools/kvm/.
>
> In that model each process has its own stack, not accessible to other
> worker processes. They'd only share the guest RAM image and some
> (minimal) global state.
>
> This way the individual devices are (optionally) isolated from each
> other. In a way this is a microkernel done right ;-)

It's really hard to achieve, since devices have global interactions.
For example a PCI device can change the memory layout when a BAR is
programmed. So you would have a lot of message passing going on (not at
runtime, so no huge impact on performance). The programming model is
very different.

Note that message passing is in fact quite a good way to model hardware,
since what different devices actually do is pass messages to each
other. I expect if done this way, the device model would be better than
what we have today. But it's not an easy step away from threads.

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