[PATCH v4] aarch64: enforce >=64K guard size

[Florian Weimer]

* Szabolcs Nagy:

> There are several compiler implementations that allow large stack
> allocations to jump over the guard page at the end of the stack and
> corrupt memory beyond that. See CVE-2017-1000364.
>
> Compilers can emit code to probe the stack such that the guard page
> cannot be skipped, but on aarch64 the probe interval is 64K by default
> instead of the minimum supported page size (4K).
>
> This patch enforces at least 64K guard on aarch64 unless the guard
> is disabled by setting its size to 0.  For backward compatibility
> reasons the increased guard is not reported, so it is only observable
> by exhausting the address space or parsing /proc/self/maps on linux.

Is there anything special in the aarch64 probing sequence that
*requires* that the guard is 64 KiB, or is it just that it is not
effective if it is not?