On Tue, Nov 20, 2018 at 11:31:13AM +0100, Christian Brauner wrote: > On Mon, Nov 19, 2018 at 10:59:12PM -0600, Eric W. Biederman wrote: > > Daniel Colascione <dan...@google.com> writes: > > > > > On Mon, Nov 19, 2018 at 1:37 PM Christian Brauner <christ...@brauner.io> > > > wrote: > > >> > > >> On Mon, Nov 19, 2018 at 01:26:22PM -0800, Daniel Colascione wrote: > > >> > On Mon, Nov 19, 2018 at 1:21 PM, Christian Brauner > > >> > <christ...@brauner.io> wrote: > > >> > > That can be done without a loop by comparing the level counter for > > >> > > the > > >> > > two pid namespaces. > > >> > > > > >> > >> > > >> > >> And you can rewrite pidns_get_parent to use it. So you would > > >> > >> instead be > > >> > >> doing: > > >> > >> > > >> > >> if (pidns_is_descendant(proc_pid_ns, > > >> > >> task_active_pid_ns(current))) > > >> > >> return -EPERM; > > >> > >> > > >> > >> (Or you can just copy the 5-line loop into procfd_signal -- though I > > >> > >> imagine we'll need this for all of the procfd_* APIs.) > > >> > > > >> > Why is any of this even necessary? Why does the child namespace we're > > >> > considering even have a file descriptor to its ancestor's procfs? If > > >> > > >> Because you can send file descriptors between processes and container > > >> runtimes tend to do that. > > > > > > Right. But why *would* a container runtime send one of these procfs > > > FDs to a container? > > > > > >> > it has one of these FDs, it can already *read* all sorts of > > >> > information it really shouldn't be able to acquire, so the additional > > >> > ability to send a signal (subject to the usual permission checks) > > >> > feels like sticking a finger in a dike that's already well-perforated. > > >> > IMHO, we shouldn't bother with this check. The patch would be simpler > > >> > without it. > > >> > > >> We will definitely not allow signaling processes in an ancestor pid > > >> namespace! That is a security issue! I can imagine container runtimes > > >> killing their monitoring process etc. pp. Not happening, unless someone > > >> with deep expertise in signals can convince me otherwise. > > > > > > If parent namespace procfs FDs or mounts really can leak into child > > > namespaces as easily as Aleksa says, then I don't mind adding the > > > check. I was under the impression that if you find yourself in this > > > situation, you already have a big problem. > > > > There is one big reason to have the check, and I have not seen it > > mentioned yet in this thread. > > > > When SI_USER is set we report the pid of the sender of the signal in > > si_pid. When the signal comes from the kernel si_pid == 0. When signal > > is sent from an ancestor pid namespace si_pid also equals 0 (which is > > reasonable). > > > > A signal out to a process in a parent pid namespace such as SIGCHLD is > > reasonable as we can map the pid. I really don't see the point of > > forbidding that. From the perspective of the process in the parent pid > > namespace it is just another process in it's pid namespace. So it > > should pose no problem from the perspective of the receiving process. > > > > A signal to a process in a pid namespace that is neither a parent nor a > > descendent pid namespace would be a problem, as there is no well defined > > notion of what si_pid should be set to. So for that case perhaps we > > should have something like a noprocess pid that we can set. Perhaps we > > could set si_pid to 0xffffffff. That would take a small extension to > > pid_nr_ns. > > > > File descriptors are not namespaced. It is completely legitimate to use > > file descriptors to get around limitations of namespaces. > > Frankly, I don't see a good argument for why we would allow that even if > safe. I have not heard a legitimate use-case or need for this. > At this point I care about very simple semantics. Being able to signal > into ancestor pid namespaces and cousin namespaces is interesting but > makes the syscall more brittle and harder to understand.
Yeah, I'm with you on that. We can always open that door later if a good use case comes up, but I prefer simple at first. > Changing pid_nr_ns() might be the solution but this function is called > all over the place in the kernel and I'm not going to risk breaking > something by changing it for a feature that no one so far has ever > asked for. > If you are ok with this then we should hold off on this. We can always > add this feature later by removing the check when someone has a use-case > for it. > I'll send a v2 of the patch that keeps the restriction for now. If you > insist on it being removed we can make the change in a follow-up > iteration. > > Christian > > > > > Adding limitations to a file descriptor based api because someone else > > can't set up their processes in such a way as to get the restrictions > > they are looking for seems very sad. > > > > Frankly I think it is one of the better features of namespaces that we > > have to carefully handle and define these cases so that when the > > inevitable leaks happen you are not immediately in a world of hurt. All > > of the other permission checks etc continue to do their job. Plus you > > are prepared for the case when someone wants their containers to have an > > interesting communication primitive. > > > > Eric > > > > > > > >
