linux-kernel - Re: [RESEND PATCH] pid_ns: Fix proc_flush_task() accessing freed proc

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Message-ID: <20100625212758.GA30474@redhat.com>
Date:	Fri, 25 Jun 2010 23:27:59 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Containers <containers@...ts.osdl.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH] pid_ns: Fix proc_flush_task() accessing freed
	proc_mnt

On 06/25, Sukadev Bhattiprolu wrote:
>
> Oleg Nesterov [oleg@...hat.com] wrote:
> | On 06/25, Sukadev Bhattiprolu wrote:
> | >
> | > Louis Rilling [Louis.Rilling@...labs.com] wrote:
> | > | - proc_pid_readdir():
> | > |   Needs similar check and protection to proc_pid_lookup(), but there is another
> | > |   issue: next_tgid() can find a dying task:
> | >
> | > Hmm, I thought proc_pid_readdir() would be a problem too but convinced myself
> | > that it would not - since a process running proc_pid_readdir() would have
> | > a reference to the pid namespace,
> |
> | Where does this reference comes from ?
>
> Caller of proc_pid_readdir() would be living in the same pid namespace right ?

Afaics, in general not.

Suppose that we do something like

	if (!clone(CLONE_NEWPID)) {
		mount("none", "/SUB_PROC", "proc", 0, NULL);
		exit();
	}

After that /SUB_PROC/ still exists, one can do "ls /SUB_PROC/".

This particular case is fine, ns->child_reaper was already cleared.
But, as Louis pointed out, ls can race with the exiting init.

> | But I won't be surprised if I am wrong again ;)

Yes ;)

Oleg.

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