spi: bcm2835: Fix out-of-bounds access with more than 4 slaves

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: spi: bcm2835: Fix out-of-bounds access with more than 4 slaves Commit 571e31fa60b3 ("spi: bcm2835: Cache CS register value for ->prepare_message()") limited the number of slaves to 3 at compile-time. The limitation was necessitated by a statically-sized array prepare_cs[] in the driver private data which contains a per-slave register value. The commit sought to enforce the limitation at run-time by setting the controller's num_chipselect to 3: Slaves with a higher chipselect are rejected by spi_add_device(). However the commit neglected that num_chipselect only limits the number of *native* chipselects. If GPIO chipselects are specified in the device tree for more than 3 slaves, num_chipselect is silently raised by of_spi_get_gpio_numbers() and the result are out-of-bounds accesses to the statically-sized array prepare_cs[]. As a bandaid fix which is backportable to stable, raise the number of allowed slaves to 24 (which "ought to be enough for anybody"), enforce the limitation on slave ->setup and revert num_chipselect to 3 (which is the number of native chipselects supported by the controller). An upcoming for-next commit will allow an arbitrary number of slaves.

References

• https://git.kernel.org/stable/c/b5502580cf958b094f3b69dfe4eece90eae01fbc
• https://git.kernel.org/stable/c/82a8ffba54d31e97582051cb56ba1f988018681e
• https://git.kernel.org/stable/c/01415ff85a24308059e06ca3e97fd7bf75648690
• https://git.kernel.org/stable/c/13817d466eb8713a1ffd254f537402f091d48444