Re: [PATCH] user namespace: make signal.c respect user namespaces

[Serge Hallyn <serge.hallyn@canonical.com>]

Quoting Oleg Nesterov (oleg@redhat.com):
> On 09/19, Serge E. Hallyn wrote:
> >
> > __send_signal: convert the uid being sent in SI_USER to the target task's
> > user namespace.
> >
> > do_notify_parent and do_notify_parent_cldstop: map task's uid to parent's
> >  user namespace
> >
> > ptrace_signal: map parent's uid into current's user namespace before
> > including in signal to current.
> 
> And all of them follow the same pattern,
> 
> > @@ -1073,7 +1074,8 @@ static int __send_signal(int sig, struct siginfo *info, struct task_struct *t,
> >  			q->info.si_code = SI_USER;
> >  			q->info.si_pid = task_tgid_nr_ns(current,
> >  							task_active_pid_ns(t));
> > -			q->info.si_uid = current_uid();
> > +			q->info.si_uid = user_ns_map_uid(task_cred_xxx(t, user_ns),
> > +					current_cred(), current_uid());
> 
> Up to you, but may be we can add a helper? Something like
> 
> 	static inline uid_t good_name(struct task_struct *from, struct task_struct *to)
> 	{
> 		// the caller does rcu_read_lock() if needed
> 		const struct cred *from_cred = __task_cred(from);
> 		return user_ns_map_uid(task_cred_xxx(to, user_ns),
> 					from_cred, from_cred->uid);
> 	}
> 
> As for __send_signal() in particular, I guess we could do
> 
> 		q->info.si_uid = from_ancestor_ns ? 0 : current_uid();

But wait - can't __send_signal end up being triggered through fcntl
F_SETSIG+F_SETOWN to another but none-ancestor namespace?  So we
do actually need to call the full user_ns_map_uid() there.  However,
the uid sent in ptrace_signal() can (if i'm thinking right) be set
using

	info->si_uid = (current_user_ns() == parent_user_ns()) ?
			0 : current_uid();

Not much of a win performance-wise, but perhaps better by making it
clear that those are the only possibilities (and overflowuid is not
possible).

thanks,
-serge