linux-kernel - Re: [PATCH 2/2] exit: pidns: alloc_pid() leaks pid_namespace if child

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Date:	Tue, 25 Nov 2014 18:07:18 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Aaron Tomlin <atomlin@...hat.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Sterling Alexander <stalexan@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] exit: pidns: alloc_pid() leaks pid_namespace if
	child_reaper is exiting

On 11/24, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@...hat.com> writes:
>
> > --- a/kernel/pid.c
> > +++ b/kernel/pid.c
> > @@ -320,7 +320,6 @@ struct pid *alloc_pid(struct pid_namespace *ns)
> >  			goto out_free;
> >  	}
> >
> > -	get_pid_ns(ns);
> >  	atomic_set(&pid->count, 1);
> >  	for (type = 0; type < PIDTYPE_MAX; ++type)
> >  		INIT_HLIST_HEAD(&pid->tasks[type]);
> > @@ -336,7 +335,7 @@ struct pid *alloc_pid(struct pid_namespace *ns)
> >  	}
> >  	spin_unlock_irq(&pidmap_lock);
> >
> > -out:
> > +	get_pid_ns(ns);
>
> Moving the label and changing the goto out logic is gratuitous confusing
> and I think it probably even generates worse code.
>
> Furthermore multiple exits make adding debugging code more difficult.

Oh, I strongly disagree but I am not going to argue ;) cleanups are
always subjective, and I do believe in "maintainer is always right"
mantra. I can make v2 without this change.

> Moving get_pid_ns down does close a leak in the error handling path.

OK, good.

> However at the moment my I can't figure out if it is safe to move
> get_pid_ns elow hlist_add_head_rcu.  Because once we are on the rcu list
> the pid is findable, and being publicly visible with a bad refcount could cause
> problems.

The caller has a reference, this ns can't go away. Obviously, otherwise
get_pid_ns(ns) is not safe.

We need this get_pid_ns() to balance put_pid()->put_pid_ns() which obviously
won't be called until we return this pid, otherwise everything is wrong.

So I think this should be safe?

Oleg.

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