Re: [PATCH] spi: bcm2835: Fix buffer overflow with CS able to go beyond limit.

From: Mark Brown <broonie@kernel.org>
To: Joe Burmeister <joe.burmeister@devtank.co.uk>
Cc: Florian Fainelli <f.fainelli@gmail.com>,
	Ray Jui <rjui@broadcom.com>,
	Scott Branden <sbranden@broadcom.com>,
	bcm-kernel-feedback-list@broadcom.com, linux-spi@vger.kernel.org,
	linux-rpi-kernel@lists.infradead.org,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, nsaenz@kernel.org
Subject: Re: [PATCH] spi: bcm2835: Fix buffer overflow with CS able to go beyond limit.
Date: Fri, 23 Apr 2021 12:57:24 +0100	[thread overview]
Message-ID: <20210423115724.GB5507@sirena.org.uk> (raw)
In-Reply-To: <380624c4-82f3-0e6e-8cdb-8a9732636db8@devtank.co.uk>

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On Fri, Apr 23, 2021 at 11:03:22AM +0100, Joe Burmeister wrote:
> On 23/04/2021 00:49, Florian Fainelli wrote:

> > Right, which means that we should probably seek a solution within the
> > SPI core itself, even if you can only test with spi-bcm2835.c chances
> > are that the fix would be applicable for other controllers if done in
> > the core.

> I'm not sure it's possible to do in the core alone. The numb of the
> issue is the core changes ctlr->num_chipselect to what is in the device
> tree and some drivers are cool with that overs quietly stomp memory.

I wouldn't expect any controller to be OK with that?  Drivers can store
per-client data in spi_device->controller_data which doesn't need
scaling (but is also not so helpful if you need to look at clients other
than the one you're currently controlling).

> I've got a simple little patch to warn when the core expands
> ctlr->num_chipselect. This warning won't go off in bcm2835 with my patch
> because I am also extending ctlr->num_chipselect to the amount in the
> device tree before the core does that expansion. Hopefully that new
> warning would make people investigate and fix problem drivers.

> >> There is protection in spi_add_device, which will catch extra added
> >> later, but not ones in the device tree when the spi controller was
> >> registered.

> > Not sure I follow you, if we have the overlay before
> > spi_register_controller() is called, how can the check there not
> > trigger? And if we load the overlay later when the SPI controller is
> > already registered, why does not spi_add_device()'s check work?

> I think it might be a RPI thing. I think it is merging in the overlay
> and giving Linux one already merged.

If the overlay is handled by the bootloader then from the point of view
of Linux there is no overlay - sounds like there's an issue in the
overlay, it should be overriding something that it doesn't?

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