[PATCH] signal: Avoid corrupting si_pid and si_uid in do_notify_parent

[Christian Brauner]

On Tue, Apr 21, 2020 at 11:28:47AM +0200, Oleg Nesterov wrote:
> On 04/21, Christian Brauner wrote:
> >
> > > -		__group_send_sig_info(sig, &info, tsk->parent);
> > > +		__send_signal(sig, &info, tsk->parent, PIDTYPE_TGID, false);
> >
> > So below you switch to __send_signal() but set the "force" argument to
> > to "false".
> 
> it must be false, the signal is generated from the parent's namespace or
> its descendant
> 
> > Before that, if the signal was generated from another pid
> > namespace and we fixed up si_pid and si_uid the "force" argument was set
> > to "true",
> 
> before that the "force" argument could be falsely true by the same reason,
> task_pid_nr_ns(tsk, tsk->parent) can return 0 because "tsk" no longer have
> pids after __unhash_process().

As I said in my mail, looking at the codepath it seems safe. My question
was whether it is _always_ incorrectly false which I do think it is
because child subreapers can't come from outside the pid namespace. If
they could you could create a scenario where the signal is generated
from a sibling pid namespace in which case it would be correctly set to
true.

Christian