Re: [Qemu-devel] [RFC] [PATCHv2 2/2] Adding basic calls to libseccomp in vl.c

[Corey Bryant <coreyb@linux.vnet.ibm.com>]

On 06/18/2012 04:31 AM, Daniel P. Berrange wrote:
> On Fri, Jun 15, 2012 at 05:02:19PM -0400, Paul Moore wrote:
>> On Friday, June 15, 2012 07:06:10 PM Blue Swirl wrote:
>>> I think allowing execve() would render seccomp pretty much useless.
>>
>> Not necessarily.
>>
>> I'll agree that it does seem a bit odd to allow execve(), but there is still
>> value in enabling seccomp to disable potentially buggy/exploitable syscalls.
>> Let's not forget that we have over 300 syscalls on x86_64, not including the
>> 32 bit versions, and even if we add all of the new syscalls suggested in this
>> thread we are still talking about a small subset of syscalls.  As far as
>> security goes, the old adage of "less is more" applies.
>
> I can sort of see this argument, but *only* if the QEMU process is being
> run under a dedicated, fully unprivileged (from a DAC pov) user, completely
> separate from anything else on the system.

This might be a good point to plug Marcelo Cerri's DAC isolation patches 
that are on the libvirt mailing list. :)

http://www.redhat.com/archives/libvir-list/2012-May/msg01005.html
http://www.redhat.com/archives/libvir-list/2012-June/msg00020.html

-- 
Regards,
Corey

>
> If QEMU were being run as root, then even with seccomp, it could trivially
> just overwrite some binary in /bin, update /proc/core-pattern to point to
> this binary, and then crash itself. Now that core handling binary will
> execute without any of the seccomp filters applied.
>
> Similarly if QEMU is being run in the user's desktop session, I'm sure there
> is some kind of similar attack possible by changing a config setting for the
> user's GNOME/KDE session, and then waiting for GNOME/KDE to execute the script
> that QEMU just wrote out, once again bypassing seccomp.
>
> Regards,
> Daniel
>