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

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Date:	Tue, 25 Nov 2014 19:59:19 +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 11/25, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@...hat.com> writes:
>
> > On 11/25, Eric W. Biederman wrote:
> >>
> >> My concern is exposing a half initialized struct pid to the world via an
> >> rcu data structure.  In particular could one of the rcu users get into
> >> trouble because we haven't called get_pid_ns yet?  That is unclear to me.
> >
> > They can't. This pid was fully initialized, in particular
> > pid->numbers[pid->level].ns == ns has a reference.
> >
> > Just it is not ready for put_pid() which will be called by the "owner" of
> > this pid, the caller or the new child. So in this sense it doesn't matter
> > when we call get_pid_ns(), just we need to do this before return.
>
> Or by someone calling find_get_pid() ... put_pid().
>
> Now the reference count should not hit zero in that case

Yes. this get_pid_ns() connects to atomic_set(&pid->count, 1) set by us.

> but I hate to
> think of that case separately.

OK.

> At this point, and especially since we need to Cc stable and get this
> fix backported to who knows how many kernel releases having something
> that is trivial to validate is correct is important.

OK. I'll send V2 with -stable cc'ed.

Oleg.

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