linux-kernel - Re: [PATCH v3] i2c: dev: don't allow user-space to deadlock the kernel

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Date:   Mon, 6 Feb 2023 19:39:30 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Bartosz Golaszewski <brgl@...ev.pl>
Cc:     Wolfram Sang <wsa@...nel.org>, linux-i2c@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH v3] i2c: dev: don't allow user-space to deadlock the
 kernel

Hello Bartosz,

On Mon, Feb 06, 2023 at 07:01:11PM +0100, Bartosz Golaszewski wrote:
> On Mon, Feb 6, 2023 at 4:51 PM Uwe Kleine-König
> <u.kleine-koenig@...gutronix.de> wrote:
> >
> > Hello,
> >
> > ah, this is the mail I missed before.
> >
> > On Wed, Jan 25, 2023 at 11:11:59PM +0100, Wolfram Sang wrote:
> > >
> > > > So, this code handled all my stress-testing well so far. I'll try to
> > > > think of some more ideas until this evening, but likely I will apply it
> > > > later. Nonetheless, more review eyes are still welcome!
> > >
> > > Ah yes, I now recalled why I had the gut feeling that this solution is
> > > not complete. See this mail thread from 2015:
> > >
> > > https://lkml.iu.edu/hypermail/linux/kernel/1501.2/01700.html
> > >
> > > There are still drivers using i2c_del_adapter()+kfree(), so removing the
> > > completion could cause use-after-free there, or?
> >
> > There is also a strange construct in spi that I understand at one point
> > in time, but I failed to swap it in quickly. It's about commit
> > 794aaf01444d4e765e2b067cba01cc69c1c68ed9. I think there should be a
> > nicer solution than to track if the controller was allocated using devm,
> > but I don't remember the details. But before addressing the i2c problem
> > it might be worth to invest some time into that spi issue to not make
> > the same mistake for i2c.
> 
> Yeah, I've seen these constructs before elsewhere... Sadly, we have
> workarounds upon workarounds within workarounds chased by other
> workarounds due to this issue.

Another pointer: Some time ago I did to the counter framework what would
be needed here for i2c. See
https://lore.kernel.org/all/20211230150300.72196-1-u.kleine-koenig@pengutronix.de/.
This was applied in commit f2ee4759fb700b32a1bd830960fe86bf6bdfd0ab and
its parents.

Maybe it can serve as a template for i2c.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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