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

[Daniel P . Berrangé]

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 ?

> >> diff --git a/qemu-options.hx b/qemu-options.hx
> >> index 5515dfaba5..dafacb60c6 100644
> >> --- a/qemu-options.hx
> >> +++ b/qemu-options.hx
> >> @@ -3864,6 +3864,8 @@ Disable set*uid|gid system calls
> >>  Disable *fork and execve
> >>  @item address@hidden
> >>  Disable process affinity and schedular priority
> >> address@hidden address@hidden
> >> +Apply seccomp filter to all threads (default is auto, and will warn if 
> >> fail)
> >
> > 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.

Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|