[PATCH 0/2] ARM: support THREAD_INFO_IN_TASK (v7 only) (v2)

Keith Packard
2021-09-04T06:09:22Z

Placing thread_info in the kernel stack leaves it vulnerable to stack
overflow attacks. This short series addresses that by using the
existing THREAD_INFO_IN_TASK infrastructure.

This is the second version of this series, in this version the changes
are restricted to v7 hardware which offers a way to identify each cpu
in the system without reference to the stack it is using.

The series is broken into three pieces:

1) Change the secondary_start_kernel API to pass the cpu number to
this function. This is required for the following patch because the
raw_smp_processor_id() macro will use the per_cpu_offset value which
needs to have the cpu number to get the right value.

2) Enable THREAD_INFO_IN_TASK by creating a new per-cpu variable,
current_task, just like the x86 architecture. The largest changes
are in the assembly code where fetching the current_task value
requires a temporary register. Fortunately, each location in the
code performing this had a reasonably obvious register to use.

3) Optimize access to the cpu number using another new per-cpu
variable. This is not functionally necessary, but avoids
de-referencing through two pointers at modest memory cost.

Signed-off-by: Keith Packard <keithpac@amazon.com>