linux-kernel - Re: [PATCH] configfs: fix a race in configfs

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Message-ID: <20210823074636.GA23822@lst.de>
Date:   Mon, 23 Aug 2021 09:46:36 +0200
From:   Christoph Hellwig <hch@....de>
To:     sishuaigong <sishuai@...due.edu>
Cc:     jlbec@...lplan.org, hch@....de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] configfs: fix a race in configfs_lookup()

On Fri, Aug 20, 2021 at 05:44:58PM -0400, sishuaigong wrote:
> When configfs_lookup() is executing list_for_each_entry(),
> it is possible that configfs_dir_lseek() is calling list_del().
> Some unfortunate interleavings of them can cause a kernel NULL
> pointer dereference error
> 
> Thread 1                  Thread 2
> //configfs_dir_lseek()    //configfs_lookup()
> list_del(&cursor->s_sibling);
>                           list_for_each_entry(sd, ...)
> 
> Fix this bug by using list_for_each_entry_safe() instead.

I don't see how list_for_each_entry_safe would save you there.
You need a lock to sychronize the two, list_for_each_entry_safe
only ensures the next entry is looked up before iterating over
the current one.