linux-kernel - Re: [PATCH] [RFC] fix potential access after free: return value of blk_check

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Date:	Wed, 06 Apr 2016 13:10:57 +1000
From:	NeilBrown <neilb@...e.de>
To:	Shaohua Li <shli@...nel.org>,
	Lars Ellenberg <lars.ellenberg@...bit.com>
Cc:	Jens Axboe <axboe@...nel.dk>, Chris Mason <clm@...com>,
	Josef Bacik <jbacik@...com>, David Sterba <dsterba@...e.com>,
	linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-btrfs@...r.kernel.org
Subject: Re: [PATCH] [RFC] fix potential access after free: return value of blk_check_plugged() must be used schedule() safe

On Wed, Apr 06 2016, Shaohua Li wrote:

> On Tue, Apr 05, 2016 at 03:36:57PM +0200, Lars Ellenberg wrote:
>> blk_check_plugged() will return a pointer
>> to an object linked on current->plug->cb_list.
>> 
>> That list may "at any time" be implicitly cleared by
>> blk_flush_plug_list()
>>  flush_plug_callbacks()
>> either as a result of blk_finish_plug(),
>> or implicitly by schedule() [and maybe other implicit mechanisms?]
>> 
>> If there is no protection against an implicit unplug
>> between the call to blk_check_plug() and using its return value,
>> that implicit unplug may have already happened,
>> even before the plug is actually initialized or populated,
>> and we may be using a pointer to already free()d data.
>
> This isn't correct. flush plug is never called in preemption, which is designed
> only called when the task is going to sleep. See sched_submit_work. Am I
> missing anything?

Ahh yes, thanks.

Only two places call blk_schedule_flush_plug().
One is io_schedule_timeout() which must be called explicitly.
There other is, as you say, sched_submit_work().  It starts:

static inline void sched_submit_work(struct task_struct *tsk)
{
        if (!tsk->state || tsk_is_pi_blocked(tsk))
                return;

so if the task is runnable, then as
include/linux/sched.h:#define TASK_RUNNING              0

it will never call blk_schedule_flush_plug().

So I don't think you are missing anything, we were.

Lars:  have your concerns been relieved or do you still have reason to
think there is a problem?

Thanks,
NeilBrown

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