netdev - Re: [PATCH net 000/117] net: avoid to remove module when its debugfs is being used

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Date:   Fri, 9 Oct 2020 19:15:14 +0900
From:   Taehee Yoo <ap420073@...il.com>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     Nicolai Stange <nstange@...e.de>,
        David Laight <David.Laight@...lab.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
        "wil6210@....qualcomm.com" <wil6210@....qualcomm.com>,
        "brcm80211-dev-list@...ress.com" <brcm80211-dev-list@...ress.com>,
        "b43-dev@...ts.infradead.org" <b43-dev@...ts.infradead.org>,
        "linux-bluetooth@...r.kernel.org" <linux-bluetooth@...r.kernel.org>
Subject: Re: [PATCH net 000/117] net: avoid to remove module when its debugfs
 is being used

On Fri, 9 Oct 2020 at 16:45, Johannes Berg <johannes@...solutions.net> wrote:
>

Hi Johannes,
Thank you for the review!

> On Fri, 2020-10-09 at 07:09 +0200, Nicolai Stange wrote:
> > Johannes Berg <johannes@...solutions.net> writes:
> >
> > > On Thu, 2020-10-08 at 15:59 +0000, David Laight wrote:
> > > > From: Taehee Yoo
> > > > > Sent: 08 October 2020 16:49
> > > > >
> > > > > When debugfs file is opened, its module should not be removed until
> > > > > it's closed.
> > > > > Because debugfs internally uses the module's data.
> > > > > So, it could access freed memory.
> > > > >
> > > > > In order to avoid panic, it just sets .owner to THIS_MODULE.
> > > > > So that all modules will be held when its debugfs file is opened.
> > > >
> > > > Can't you fix it in common code?
> >
> > Probably not: it's the call to ->release() that's faulting in the Oops
> > quoted in the cover letter and that one can't be protected by the
> > core debugfs code, unfortunately.
> >
> > There's a comment in full_proxy_release(), which reads as
> >
> >       /*
> >        * We must not protect this against removal races here: the
> >        * original releaser should be called unconditionally in order
> >        * not to leak any resources. Releasers must not assume that
> >        * ->i_private is still being meaningful here.
> >        */
>
> Yeah, found that too now :-)
>
> > > Yeah I was just wondering that too - weren't the proxy_fops even already
> > > intended to fix this?
> >
> > No, as far as file_operations are concerned, the proxy fops's intent was
> > only to ensure that the memory the file_operations' ->owner resides in
> > is still valid so that try_module_get() won't splat at file open
> > (c.f. [1]).
>
> Right.
>
> > You're right that the default "full" proxy fops do prevent all
> > file_operations but ->release() from getting invoked on removed files,
> > but the motivation had not been to protect the file_operations
> > themselves, but accesses to any stale data associated with removed files
> > ([2]).
>
> :)
>
> I actually got this to work in a crazy way, I'll send something out but
> I'm sure it's a better idea to add the .owner everywhere, but please
> let's do it in fewer than hundreds of patches :-)
>
Okay, as you mentioned earlier in 001/117 patch thread,
I will squash patches into per-driver/subsystem then send them as v2.

Thanks a lot!
Taehee