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.