linux-kernel - Re: [PATCH v2 1/2] tty: add missing rcu_read_lock for task

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Message-ID: <5590492F.9010507@hurleysoftware.com>
Date:	Sun, 28 Jun 2015 15:21:19 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Patrick Donnelly <batrick@...bytes.com>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] tty: add missing rcu_read_lock for task_pgrp

On 06/28/2015 01:20 PM, Patrick Donnelly wrote:
> On Sun, Jun 28, 2015 at 11:23 AM, Peter Hurley <peter@...leysoftware.com> wrote:
>> On 06/27/2015 08:51 PM, Patrick Donnelly wrote:
>>> task_pgrp requires an rcu or tasklist lock to be obtained if the returned pid
>>> is to be dereferenced, which kill_pgrp does. Obtain an RCU lock for the
>>> duration of use.
>>
>> kill_pgrp() obtains tasklist_lock, so I don't see an unsafe deref.
> 
> I see a race between looking up the pgrp via task_pgrp and passing it
> to kill_pgrp. The pgrp struct pid may be freed via setpgid/setsid, as
> mentioned in the comment for task_pgrp:
> 
>  * Without tasklist or rcu lock it is not safe to dereference
>  * the result of task_pgrp/task_session even if task == current,
>  * we can race with another thread doing sys_setsid/sys_setpgid.
> 
> Getting the lock after the lookup is getting the lock too late. I
> could be wrong though as I'm no expert on locking in Linux.

I suppose it can't hurt; please add similar logic to job_control() in
drivers/tty/n_tty.c which handles the corresponding SIGTTIN signal conditions.

Regards,
Peter Hurley
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