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Subject: Re: [PATCH] spi: bcm2835: Fix buffer overflow with CS able to go
 beyond limit.
To:     Joe Burmeister <joe.burmeister@devtank.co.uk>,
        Mark Brown <broonie@kernel.org>, 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
References: <20210420083402.6950-1-joe.burmeister@devtank.co.uk>
 <c087ba2c-7839-02d1-a522-b104d8ffb8d2@gmail.com>
 <7c9f9376-1a80-b624-7b9e-0f6d04437c02@devtank.co.uk>
From:   Florian Fainelli <f.fainelli@gmail.com>
Message-ID: <271ad212-a606-620e-3f0c-d6bff272be3c@gmail.com>
Date:   Thu, 22 Apr 2021 16:49:19 -0700
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On 4/22/2021 1:10 PM, Joe Burmeister wrote:
>> On 4/20/2021 1:34 AM, Joe Burmeister wrote:
>>> It was previoulsy possible to have a device tree with more chips than
>>> the driver supports and go off the end of CS arrays.
>> Do you mind walking me through the code how that could have happened? We
>> have spi_register_controller() call of_spi_get_gpio_numbers() which has
>> the following:
>>
>> ctlr->num_chipselect = max_t(int, nb, ctlr->num_chipselect);
>>
>> such that what the controller has is the maximum between the number of
>> 'cs-gpios' properties parsed and what was already populated in
>> ctrl->num_chipselect during bcm2835_spi_probe(), which for this driver
>> is BCM2835_SPI_NUM_CS (3).
> 
> If you make a initial device tree (or add overlay in the rpi's 
> config.txt) with more on the bus than BCM2835_SPI_NUM_CS (in my case 8
> devices), you get into this trampling memory state. As the devices are
> added, once the chip_select is equal to or greater than
> BCM2835_SPI_NUM_CS, it's writing off the end of the arrays.

OK.

> 
> There is no protection from this happening. By the looks of it, this
> isn't the only driver this could happen with, but it is the one I have
> hardware for to test. There are also drivers that look like they don't
> have a problem going well beyond the limit they gave.

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.

> 
> 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?

How would I go about reproducing this on a Pi4?

> 
>>> This patches inforces CS limit but sets that limit to the max of the
>>> default limit and what is in the device tree when driver is loaded.
>>>
>>> Signed-off-by: Joe Burmeister <joe.burmeister@devtank.co.uk>
>> You have changed many more things that just enforcing a limit on
>> BCM2835_SPI_NUM_CS you have now made all chip-select related data
>> structuresd dynamically allocated and you have changed a number of
>> prints to use the shorthand "dev" instead of &pdev->dev.
> The change to dynamic allocated arrays is just to support what is given
> in the device  tree rather than increase and enforce the CS limit just
> for my case.
> 
> The shorthand is of course not required. I'll drop it on resubmitting.
> 
> Regards,
> 
> Joe
> 
> 

-- 
Florian