linux-kernel - Re: [PATCH] mtd: spi-nor: intel-spi: Do not try to make the SPI flash chip writable

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Message-ID: <20200824082227.GU1375436@lahna.fi.intel.com>
Date:   Mon, 24 Aug 2020 11:22:27 +0300
From:   Mika Westerberg <mika.westerberg@...ux.intel.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Daniel Gutson <daniel@...ypsium.com>,
        Tudor Ambarus <tudor.ambarus@...rochip.com>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Richard Weinberger <richard@....at>,
        Vignesh Raghavendra <vigneshr@...com>,
        Boris Brezillon <bbrezillon@...nel.org>,
        linux-mtd <linux-mtd@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Alex Bazhaniuk <alex@...ypsium.com>,
        Richard Hughes <hughsient@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] mtd: spi-nor: intel-spi: Do not try to make the SPI
 flash chip writable

On Sat, Aug 22, 2020 at 06:06:03PM +0200, Arnd Bergmann wrote:
> On Wed, Aug 19, 2020 at 11:11 AM Mika Westerberg
> <mika.westerberg@...ux.intel.com> wrote:
> > On Wed, Aug 19, 2020 at 10:38:24AM +0200, Arnd Bergmann wrote:
> > > On Wed, Aug 19, 2020 at 8:57 AM Mika Westerberg <mika.westerberg@...ux.intel.com> wrote:
> >
> > > > Actually thinking about this bit more, to make PCI and the platform
> > > > parts consistent we can make the "writeable" control this for the PCI
> > > > part as well. So what if we add a callback to struct intel_spi_boardinfo
> > > > that the PCI driver populates and then the "core" driver uses to enable
> > > > writing when "writeable" is set to 1.
> > >
> > > If you are really worried about the write protection being bypassed by
> > > a different driver or code injection, the best way would seem to be to
> > > only enable writing in the mtd write callback and disable it immediately
> > > after the write is complete. I still don't see why this hardware would
> > > be more susceptible to this kind of attack than other drivers though,
> > > as it already has the safeguard against writing through the MTD layer
> > > without the module parameter.
> >
> > Hmm, is there already a mechanism at the MTD level to prevent writes? If
> > that's the case then sure we can use that instead.
> 
> The mtd core just checks both the permissions on the device node (which
> default to 0600 without any special udev rules) and the MTD_WRITEABLE
> on the underlying device that is controlled by the module parameter
> in case of intel-spi{,-platform,-pci}.c.

OK, thanks.

Since we cannot really get rid of the module parameter (AFAIK there are
users for it), I still think we should just make the "writeable" to
apply to the PCI part as well. That should at least make it consistent,
and it also solves Daniel's case.