arm64: mm: account for hotplug memory when randomizing the linear region

arm64: mm: account for hotplug memory when randomizing the linear region

As a hardening measure, we currently randomize the placement of
physical memory inside the linear region when KASLR is in effect.
Since the random offset at which to place the available physical
memory inside the linear region is chosen early at boot, it is
based on the memblock description of memory, which does not cover
hotplug memory. The consequence of this is that the randomization
offset may be chosen such that any hotplugged memory located above
memblock_end_of_DRAM() that appears later is pushed off the end of
the linear region, where it cannot be accessed.

So let's limit this randomization of the linear region to ensure
that this can no longer happen, by using the CPU's addressable PA
range instead. As it is guaranteed that no hotpluggable memory will
appear that falls outside of that range, we can safely put this PA
range sized window anywhere in the linear region.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Steven Price <steven.price@arm.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/20201014081857.3288-1-ardb@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>