linux-kernel - Re: [PATCH] kernfs: fix use-after-free in __kernfs

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Message-ID: <YxojA7pDe8Ur7wpd@slm.duckdns.org>
Date:   Thu, 8 Sep 2022 07:14:43 -1000
From:   Tejun Heo <tj@...nel.org>
To:     "Christian A. Ehrhardt" <lk@...e.de>
Cc:     linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] kernfs: fix use-after-free in __kernfs_remove

Hello, Christian.

On Wed, Sep 07, 2022 at 10:08:11PM +0200, Christian A. Ehrhardt wrote:
> Concurrent calls to __kernfs_remove can race on the removal
> of the root node: The race occurs if the root node(kn) is freed
> during kernfs_drain. The child node(pos) is explicitly protected
> with an additional ref count. Do the same for the root node.

I don't think this is right. We don't support parallel invocations of
__kernfs_remove() this way. If @kn can be freed during kernfs_drain(), it
also means that it can be freed before kernfs_rwsem is grabbed in
kernfs_remove(). The caller must be responsible for ensuring that @kn
remains allocated. Otherwise, it can't be made reliable.

Thanks.

-- 
tejun