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

[Joe Burmeister <joe.burmeister@devtank.co.uk>]

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On 23/04/2021 00:49, Florian Fainelli wrote:
> 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.

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.

If we stop the core changing ctlr->num_chipselect then sod's law says
that we'd break existing devices which exceed the drivers num_chipselect
without a problem.

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.


> How would I go about reproducing this on a Pi4?

Attached is a device tree overlay. If you compile that up and stick it
in /boot/overlays and add dtoverlay=rpi-bug to your config.txt, you can
get into this state.

If you do dtoverlay, you don't see anything, but if you do:

ls /dev/spi*

You can see all the spidev added by this are added. 2 of which go beyond
the drivers CS arrays.


>>>> 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
>>
>>

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