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 <berra...@redhat.com> > 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 <berra...@redhat.com> > > > 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 -- |: 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 :|