CVE-2021-47282

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.