From: "Christian A. Ehrhardt" <lk@c--e.de>
To: Tejun Heo <tj@kernel.org>
Cc: linux-kernel@vger.kernel.org,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH] kernfs: fix use-after-free in __kernfs_remove
Date: Fri, 9 Sep 2022 00:25:36 +0200 [thread overview]
Message-ID: <Yxpr4HSSxNw3SDrI@cae.in-ulm.de> (raw)
In-Reply-To: <YxojA7pDe8Ur7wpd@slm.duckdns.org>
Hello Tejun,
On Thu, Sep 08, 2022 at 07:14:43AM -1000, Tejun Heo wrote:
> 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().
Point taken. However, the syzkaller reproducer reliably triggers
the bug without the patch and the bug is gone with the patch.
> The caller must be responsible for ensuring that @kn
> remains allocated. Otherwise, it can't be made reliable.
In this case the caller of __kernfs_remove is not kernfs_remove but
kernfs_remove_by_name_ns and it fails to take a reference for the
node that it looks up and deletes. Thus a second call to
kernfs_remove_by_name_ns can remove the node while kernfs_drain
drops the semaphore.
I'll post an updated patch tomorrow.
regards Christian
next prev parent reply other threads:[~2022-09-08 22:25 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-07 20:08 [PATCH] kernfs: fix use-after-free in __kernfs_remove Christian A. Ehrhardt
2022-09-08 17:14 ` Tejun Heo
2022-09-08 22:25 ` Christian A. Ehrhardt [this message]
2022-09-12 21:24 ` Christian A. Ehrhardt
2022-09-12 21:39 ` Tejun Heo
2022-09-13 12:17 ` [PATCH v2] " Christian A. Ehrhardt
2022-09-19 17:35 ` Tejun Heo
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