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

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Message-ID: <20160712004623.GA14480@ZenIV.linux.org.uk>
Date:	Tue, 12 Jul 2016 01:46:23 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Chunwei Chen <david.chen@...exus.com>
Cc:	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

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...