Re: [Qemu-devel] [PATCH v3 3/3] seccomp: set the seccomp filter to all threads

[Daniel P . Berrangé]

On Wed, Aug 22, 2018 at 06:37:56PM +0200, Marc-André Lureau wrote:
> Hi
> 
> On Wed, Aug 22, 2018 at 6:08 PM Daniel P. Berrangé <address@hidden> wrote:
> >
> > On Wed, Aug 22, 2018 at 05:58:46PM +0200, Marc-André Lureau wrote:
> > > On Wed, Aug 22, 2018 at 5:46 PM, Daniel P. Berrangé <address@hidden> 
> > > wrote:
> > > > On Wed, Aug 22, 2018 at 04:29:56PM +0200, Marc-André Lureau wrote:
> > > >
> > > > At this point you might as well not bother using seccomp at all. The
> > > > thread that is confined merely needs to scribble something into the
> > > > stack of the unconfined thread and now it can do whatever it wants.
> > >
> > > Actually, that message is incorrect, it should rather be "not all
> > > threads will be filtered" (as described in commit message).
> > >
> > > > IMHO we need to find a way to get the policy to apply to those other
> > > > threads.
> > >
> > > That's what the patch is about ;)
> >
> > It only does it in some scenarios, leaving other unfixed. We need
> > a solution (or choice of multiple solutions) that works all the time
> >
> > >
> > > > The RCU thread is tricky as it is spawned from a __constructor__
> > > > function, which means it'll be active way before we setup seccomp.
> > > >
> > > > I think we need to figure out a way todo synchronization between
> > > > the RCU thread and the seccomp setup code. Could we have a global
> > > > variable 'int seccomp_initialized' that we check from the RCU
> > > > thread loop - when that toggles to non-zero, the RCU thread can
> > > > then call into the seccomp_start() method to activate policy in
> > > > its thread. We'd need a synchronous feedback mechansim back to
> > > > the main thread, as it must block startup until all the threads
> > > > have activated the seccomp filter.
> > >
> > > That's a bit like TSYNC, except we do it ourself with RCU thread. But
> > > what about other threads? For examples one that could be created by
> > > external libraries (like mesa)
> >
> > Does mesa create threads from library constructors too, or somewhere
> > else *before* we do -seccomp setup ?
> 
> That was an example, I don't think mesa creates threads before
> -seccomp. But what about the other 100 dependencies, or if we
> introduce other threads without the seccomp sync by mistake? I think
> we are better off using tsync.

Yeah we would have to actively check whether any unexpected threads
existed or not.

> > > > IMHO this should never exist, as setting "tsync" to anything other
> > > > than "yes", is akin to just running without any sandbox.
> > >
> > > Then we should just fail -sandbox on those systems.
> >
> > We would have to make libvirt probe for tsync support too, because it
> > now unconditionally uses -sandbox for new enough QEMU.
> 
> sigh :( that's where the -sandbox tsync option could have been helpful
> keeping the compatibility.

Probably if a distro knows they have a kernel which doesn't support
it, then should just biuld QEMU with seccomp disabled, at which point
the -sandbox arg stops being reported and libvirt "does the right thing"

IOW, most people probably won't hit the runtime check.

Regards,
Daniel
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