arm64: kernel: Fix incorrect brk randomization

arm64: kernel: Fix incorrect brk randomization

This fixes two issues with the arm64 brk randomziation. First, the
STACK_RND_MASK was being used incorrectly. The original code was:

	unsigned long range_end = base + (STACK_RND_MASK << PAGE_SHIFT) + 1;

STACK_RND_MASK is 0x7ff (32-bit) or 0x3ffff (64-bit), with 4K pages where
PAGE_SHIFT is 12:

	#define STACK_RND_MASK	(test_thread_flag(TIF_32BIT) ? \
						0x7ff >> (PAGE_SHIFT - 12) : \
						0x3ffff >> (PAGE_SHIFT - 12))

This means the resulting offset from base would be 0x7ff0001 or 0x3ffff0001,
which is wrong since it creates an unaligned end address. It was likely
intended to be:

	unsigned long range_end = base + ((STACK_RND_MASK + 1) << PAGE_SHIFT)

Which would result in offsets of 0x800000 (32-bit) and 0x40000000 (64-bit).

However, even this corrected 32-bit compat offset (0x00800000) is much
smaller than native ARM's brk randomization value (0x02000000):

	unsigned long arch_randomize_brk(struct mm_struct *mm)
	{
	        unsigned long range_end = mm->brk + 0x02000000;
	        return randomize_range(mm->brk, range_end, 0) ? : mm->brk;
	}

So, instead of basing arm64's brk randomization on mistaken STACK_RND_MASK
calculations, just use specific corrected values for compat (0x2000000)
and native arm64 (0x40000000).

Reviewed-by: Jon Medhurst <tixy@linaro.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
[will: use is_compat_task() as suggested by tixy]
Signed-off-by: Will Deacon <will.deacon@arm.com>