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

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Date:	Tue, 27 Sep 2011 09:28:33 -0500
From:	Serge Hallyn <serge.hallyn@...onical.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	lkml <linux-kernel@...r.kernel.org>, richard@....at,
	Andrew Morton <akpm@...gle.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Tejun Heo <tj@...nel.org>, serge@...lyn.com
Subject: Re: [PATCH] user namespace: make signal.c respect user namespaces

Quoting Oleg Nesterov (oleg@...hat.com):
> On 09/25, Serge E. Hallyn wrote:
> >
> > Quoting Oleg Nesterov (oleg@...hat.com):
> > > On 09/23, Serge E. Hallyn wrote:
> > > >
> > > > Quoting Oleg Nesterov (oleg@...hat.com):
> > > > > On 09/23, Serge E. Hallyn wrote:
> > > > > >
> > > > > > It looks like I can fix all the
> > > > > > cases
> > > > >
> > > > > except ptrace_signal(). Although we can simply ignore this case, imho.
> > > >
> > > > ptrace_signal() calls send_signal() though.
> > >
> > > Confused... I meant the "if (signr != info->si_signo)" case. This is
> > > simple, and I only meant that this case is not that important.
> >
> > Yes, that's the case I was talking about.  That then proceeds through
> > send_signal().
> 
> It doesn't?

No, I was saying it *does*.

> I am even more confuused. Anyway, your patch adds map_cred_ns()
> into ptrace_signal().

Yes.  Which is fine, because target task is current, so the code I add
in send_signal() which tries to map the uid if necessary will check
that current == current, and not re-map the uid.

> > The whole new patch (so far only compile-tested) is below.
> 
> Perhaps I missed something, but it looks overcomplicated.

That may be :)

> I was thinking
> about the (uncompiled/untested) simple patch below (it ignores ptrace_signal
> for clarity).
> 
> And note that this way we do not need to modify do_notify_parent*()
> or ipc/mqueue.c:__do_notify() (your patch doesn't cover the latter).
> Unless I missed something of course.
> 
> And we do not need to handle the SEND_SIG_NOINFO case separately.
> 
> 
> However, we still have the problems with sigqueueinfo, 
> 
> > > > > > by checking whether si_fromuser(info)
> > > > >
> > > > > I am not sure... sys_rt_queueinfo() is nasty. Plus we have to handle
> > > > > the "fromkernel" case too. May be we can ignore this too.
> > > >
> > > > sys_rt_tgsigqueueinfo() still seems to go through send_signal().
> > >
> > > Yes. But how can you fix si_uid? We do not even know if it exists.
> > > Please look at siginfo/_uid, there is a union. We can't know what
> > > the caller of sys_rt_sigqueueinfo() puts in this location.
> >
> > But it's a union alongside the pid.
> 
> Again, I do not understand... Yes, we have the same problem with
> 
> 	if (from_ancestor_ns)
> 		q->info.si_pid = 0;
> 
> This was discussed, we do not know what we can do.

I see.

> My point was, this
> change is not sigqueueinfo-friendly too.

Yup.

> Oleg.
> 
> --- x/kernel/signal.c
> +++ x/kernel/signal.c
> @@ -1019,6 +1019,27 @@ static inline int legacy_queue(struct si
>  	return (sig < SIGRTMIN) && sigismember(&signals->signal, sig);
>  }
>  
> +static inline fixup_uid(struct siginfo *info, struct task_struct *t)
> +{
> +#ifdef CONFIG_USER_NS
> +	if (current_user_ns() == task_cred_xxx(t, user_ns)))
> +#endif
> +		return;
> +
> +	if (SI_FROMKERNEL(info))
> +		switch (info->si_code & __SI_MASK) {
> +			default:
> +				return;
> +
> +			case __SI_CHLD:
> +			case __SI_MESGQ:
> +				break;
> +		}
> +
> +	info->si_uid = user_ns_map_uid(task_cred_xxx(t, user_ns),
> +					current_cred(), info->si_uid);
> +}
> +
>  static int __send_signal(int sig, struct siginfo *info, struct task_struct *t,
>  			int group, int from_ancestor_ns)
>  {
> @@ -1088,6 +1109,9 @@ static int __send_signal(int sig, struct
>  				q->info.si_pid = 0;
>  			break;
>  		}
> +
> +		fixup_uid(info, t);
> +
>  	} else if (!is_si_special(info)) {
>  		if (sig >= SIGRTMIN && info->si_code != SI_USER) {
>  			/*

It certainly is much simpler.  I'll take some time to walk through all
of send_signal again and make sure I understand what it does in all
the cases.

thanks,
-serge
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