linux-kernel - Re: [PATCH] vfs: check i_count under lock in evict

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Message-ID: <CAK1sfubpLBv3P5ttpuq1z+EyNGHFiiMEVLSCv5HoQ51f_pFHig@mail.gmail.com>
Date:	Mon, 11 Jul 2016 18:31:57 -0700
From:	David Chen <david.chen@...exus.com>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	lkml <linux-kernel@...r.kernel.org>, linux-fsdevel@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH] vfs: check i_count under lock in evict_inodes

Hi Al,

I'm not sure about the in-tree fs, but in zfsonlinux, it would offload
iput to a thread, so this would happen there. And it would wait for
the thread in put_super(), so that part is not a problem...

Thanks

2016-07-11 17:46 GMT-07:00 Al Viro <viro@...iv.linux.org.uk>:
> On Mon, Jul 11, 2016 at 05:15:04PM -0700, Chunwei Chen wrote:
>> We need to check i_count again with i_lock held, because iput might re-add
>> i_count when lazytime is on. Without this check, we could end up with
>> double-free or use-after-free.
>
> Details, please.  Ideally - with a reproducer.  Who is calling that iput()
> at that point of generic_shutdown_super() (has to be another thread) and
> just what will happen if the same iput() is delayed until *after*
> evict_inodes(), all the way into ->put_super().  At which point there's
> no promise whatsoever that the data structures used by ->evict_inode()
> hadn't been already freed...
>