Re: [PATCH v1 2/2] signal: add procfd_signal() syscall

Aleksa Sarai
2018-11-19T21:18:26Z

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On 2018-11-19, Christian Brauner <christian@brauner.io> wrote:
> On Tue, Nov 20, 2018 at 07:28:57AM +1100, Aleksa Sarai wrote:
> > On 2018-11-19, Christian Brauner <christian@brauner.io> wrote:
> > > + if (info) {
> > > + ret = __copy_siginfo_from_user(sig, &kinfo, info);
> > > + if (unlikely(ret))
> > > + goto err;
> > > + /*
> > > + * Not even root can pretend to send signals from the kernel.
> > > + * Nor can they impersonate a kill()/tgkill(), which adds
> > > + * source info.
> > > + */
> > > + ret = -EPERM;
> > > + if ((kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL) &&
> > > + (task_pid(current) != pid))
> > > + goto err;
> > > + } else {
> > > + prepare_kill_siginfo(sig, &kinfo);
> > > + }
> >
> > I wonder whether we should also have a pidns restriction here, since
> > currently it isn't possible for a container process using a pidns to
> > signal processes outside its pidns. AFAICS, this isn't done through an
> > explicit check -- it's a side-effect of processes in a pidns not being
> > able to address non-descendant-pidns processes.
> >
> > But maybe it's reasonable to allow sending a procfd to a different pidns
> > and the same operations working on it? If we extend the procfd API to
>
> No, I don't think so. I really don't want any fancy semantics in here.
> Fancy doesn't get merged and fancy is hard to maintain. So we should do
> something like:
>
> if (proc_pid_ns() != current_pid_ns)
> return EINVAL

This isn't quite sufficient. The key thing is that you have to be in an
*ancestor* (or same) pidns, not the *same* pidns. Ideally you can re-use
the check already in pidns_get_parent, and expose it. It would be
something as trivial as:

bool pidns_is_descendant(struct pid_namespace *ns,
struct pid_namespace *ancestor)
{
for (;;) {
if (!ns)
return false;
if (ns == ancestor)
break;
ns = ns->parent;
}
return true;
}

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.)

--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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