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

Ingo Molnar
2011-05-26T09:31:24Z

* Avi Kivity <avi@redhat.com> wrote:

> > Note that tools/kvm/ would probably like to implement its own
> > object manager model as well in addition to access method
> > restrictions: by being virtual hardware it deals with many
> > resources and object hierarchies that are simply not known to the
> > host OS's LSM.
> >
> > Unlike Qemu tools/kvm/ has a design that is very fit for MAC
> > concepts: it uses separate helper threads for separate resources
> > (this could in many cases even be changed to be separate
> > processes which only share access to the guest RAM image) - while
> > Qemu is in most parts a state machine, so in tools/kvm/ we can
> > realistically have a good object manager and keep an exploit in a
> > networking interface driver from being able to access disk driver
> > state.
>
> 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.

You are missing the geniality of the tools/kvm/ thread pool! :-)

It could be switched to a worker *process* model rather easily. Guest
RAM and (a limited amount of) global resources would be shared via
mmap(SHARED), but otherwise each worker process would have its own
stack, its own subsystem-specific state, etc.

Exploiting other device domains via the shared guest RAM image is not
possible, we treat guest RAM as untrusted data already.

Devices, like real hardware devices, are functionally pretty
independent from each other, so this security model is rather natural
and makes a lot of sense.

> A multi process model works better but it has significant memory
> and performance overhead.

Not in Linux :-) We context-switch between processes almost as
quickly as we do between threads. With modern tagged TLB hardware
it's even faster.

> (well the memory overhead is much smaller when using transparent
> huge pages, but these only work for anonymous memory).

The biggest amount of RAM is the guest RAM image - but if that is
mmap(SHARED) and mapped using hugepages then the pte overhead from a
process model is largely mitigated.

Once we have a process model then isolation and MAC between devices
becomes a very real possibility: exploit via one network interface
cannot break into a disk interface.

Maybe even the isolation and per device access control of
*same-class* devices from each other is possible: with careful
implementation of the subsystem shared data structures. (which isnt
much really)

Thanks,

Ingo