linux-kernel - Re: [PATCH 07/11] pidns: Wait in zap_pid_ns_processes until pid_ns->nr

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Date:	Fri, 21 Dec 2012 16:02:38 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Linux Containers <containers@...ts.linux-foundation.org>,
	linux-kernel@...r.kernel.org, Serge Hallyn <serge@...lyn.com>,
	Gao feng <gaofeng@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 07/11] pidns: Wait in zap_pid_ns_processes until
	pid_ns->nr_hashed == 1

On 12/21, Oleg Nesterov wrote:
>
> And in fact I think this is not strange, but simply wrong.
>
> Please consider the XXX case above. Suppose that free_pid(P) happens
> after ns->child_reaper exits and thus this pointer points to nowhere.
> Suppose also that there is another injected pid so nr_hashed == 2.
> In this case wake_up_process(ns->child_reaper) means use-after-free,
> no?

Hmm. And another minor problem, unless I missed something.

Once again, the parent namespace injects the task T after ns->reaper
sees nr_hashed == 1 and returns. Suppose that reaper's parent does
do_wait() and free_pidmap() clears the bit == 1.

Now, what if T doesn't exit but forks? We must not re-create the
task with pid_nr == 1 in the dead namespace. Normally this can't
happen, RESERVED_PIDS logic in alloc_pidmap() saves us. But it
seems that we need

	- .extra1 = &zero,
	+ .extra1 = &one,

in pid_ns_ctl_table.

Oleg.

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