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

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Message-ID: <20160406004956.GA102852@kernel.org>
Date:	Tue, 5 Apr 2016 17:49:56 -0700
From:	Shaohua Li <shli@...nel.org>
To:	Lars Ellenberg <lars.ellenberg@...bit.com>
Cc:	Neil Brown <neilb@...e.de>, 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 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?

Thanks,
Shaohua