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

["Daniel P. Berrange" <berrange@redhat.com>]

On Mon, Jun 18, 2012 at 09:31:03AM +0100, 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.

Or, of course, for a QEMU already confined by SELinux.

> 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.


Daniel
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