arm64: acpi: Harden get_cpu_for_acpi_id() against missing CPU entry

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: arm64: acpi: Harden get_cpu_for_acpi_id() against missing CPU entry In a review discussion of the changes to support vCPU hotplug where a check was added on the GICC being enabled if was online, it was noted that there is need to map back to the cpu and use that to index into a cpumask. As such, a valid ID is needed. If an MPIDR check fails in acpi_map_gic_cpu_interface() it is possible for the entry in cpu_madt_gicc[cpu] == NULL. This function would then cause a NULL pointer dereference. Whilst a path to trigger this has not been established, harden this caller against the possibility.

References

• https://git.kernel.org/stable/c/f57769ff6fa7f97f1296965f20e8a2bb3ee9fd0f
• https://git.kernel.org/stable/c/4c3b21204abb4fa3ab310fbbb5cf7f0e85f3a1bc
• https://git.kernel.org/stable/c/945be49f4e832a9184c313fdf8917475438a795b
• https://git.kernel.org/stable/c/40cae0df42e5e7f7a1c0f32deed9c4027c1ba94e
• https://git.kernel.org/stable/c/62ca6d3a905b4c40cd942f3cc645a6718f8bc7e7
• https://git.kernel.org/stable/c/bc7fbb37e3d2df59336eadbd6a56be632e3c7df7
• https://git.kernel.org/stable/c/2488444274c70038eb6b686cba5f1ce48ebb9cdd