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Message-ID: <793a6ba5b534917018165d38bcb5e2c5704d82c7.camel@sipsolutions.net>
Subject: Re: [RFC] debugfs: protect against rmmod while files are open
From:   Johannes Berg <johannes@sipsolutions.net>
To:     David Laight <David.Laight@ACULAB.COM>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        "rafael@kernel.org" <rafael@kernel.org>
Date:   Fri, 09 Oct 2020 12:56:59 +0200
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On Fri, 2020-10-09 at 10:56 +0000, David Laight wrote:
> From: Johannes Berg
> > Sent: 09 October 2020 11:48
> > 
> > On Fri, 2020-10-09 at 12:41 +0200, Johannes Berg wrote:
> > 
> > > If the fops doesn't have a release method, we don't even need
> > > to keep a reference to the real_fops, we can just fops_put()
> > > them already in debugfs remove, and a later full_proxy_release()
> > > won't call anything anyway - this just crashed/UAFed because it
> > > used real_fops, not because there was actually a (now invalid)
> > > release() method.
> > 
> > I actually implemented something a bit better than what I described - we
> > never need a reference to the real_fops for the release method alone,
> > and that means if the release method is in the kernel image, rather than
> > a module, it can still be called.
> > 
> > That together should reduce the ~117 places you changed in the large
> > patchset to around a handful.
> 
> Is there an equivalent problem for normal cdev opens
> in any modules?

I guess so, but since there's no proxy_fops infrastructure and no
revoke(), you can't really do anything else other than adding .owner
properly, afaict.

johannes