linux-kernel - Re: Use after free in bcm2835_spi

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Date:   Wed, 14 Oct 2020 21:25:05 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Lukas Wunner <lukas@...ner.de>,
        Florian Fainelli <f.fainelli@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-spi <linux-spi@...r.kernel.org>
Subject: Re: Use after free in bcm2835_spi_remove()

On Wed, Oct 14, 2020 at 10:40:35PM +0300, Vladimir Oltean wrote:
> On Wed, Oct 14, 2020 at 04:09:12PM +0200, Lukas Wunner wrote:

> > Apparently the problem is that spi_unregister_controller() drops the
> > last ref on the controller, causing it to be freed, and afterwards we
> > access the controller's private data, which is part of the same
> > allocation as struct spi_controller:

> > bcm2835_spi_remove()
> >   spi_unregister_controller()
> >     device_unregister()
> >       put_device()
> >         spi_controller_release()  #  spi_master_class.dev_release()
> > 	  kfree(ctlr)
> >   bcm2835_dma_release(ctlr, bs)

> Also see these threads:
> https://lore.kernel.org/linux-spi/20200922112241.GO4792@sirena.org.uk/T/#t
> https://lore.kernel.org/linux-spi/270b94fd1e546d0c17a735c1f55500e58522da04.camel@suse.de/T/#u

Right, the proposed patch is yet another way to fix the issue - it all
comes back to the fact that you shouldn't be using the driver data after
unregistering if it was allocated as part of allocating the controller.
This framework feature is unfortunately quite error prone.

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