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

[Marc-André Lureau]

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.

>
> > >> 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.

sigh :( that's where the -sandbox tsync option could have been helpful
keeping the compatibility.


-- 
Marc-André Lureau