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

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Date:   Wed, 19 Aug 2020 12:11:23 +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 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:
> >
> > On Tue, Aug 18, 2020 at 12:55:59PM -0300, Daniel Gutson wrote:
> > > > If you care about other (malicious) code writing to the driver, please explain
> > > > what the specific attack scenario is that you are worried about, and
> > > > why you think
> > > > this is not sufficient. What code would be able to write to the device
> > > > if not the
> > > > device driver itself?
> > >
> > > Maybe Mika can answer this better, but what I'm trying to do is to
> > > limit the possibility of
> > > damage, as explained in the Kconfig:
> > > "Intel PCH/PCU SPI flash PCI driver (DANGEROUS)"
> > > "Say N here unless you know what you are doing. Overwriting the
> > >   SPI flash may render the system unbootable."
> >
> > Right, the PCI part of the driver unconditionally (and wrongly) tried to
> > set the chip writeable.
> >
> > What this whole thing tries to protect is that the user does not
> > accidentally write to the flash chip. It contains BIOS and other
> > important firmware so touching it (if it is not locked in the BIOS side)
> > may potentially brick the system. That's why we also require that
> > command line parameter so the user who knows what he or she is doing can
> > enable it for writing.
> 
> The same thing can happen with the platform driver if you load it
> once with 'writeable=1' and then unload, leaving the chip in writeable
> state. If you load it a second time without the module parameter, it
> will be in the same state as the PCI driver: the hardware bit allows
> writing, but the MTD layer prevents writes from being issued to the
> device.

Right.

> > 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.