[PATCH 0/2] arm64: permit KASLR in linear region even VArange == PArange

[Ard Biesheuvel]

(+ Marc)

On Thu, 16 Dec 2021 at 08:37, Kefeng Wang <wangkefeng.wang at huawei.com> wrote:
>
>
> On 2021/12/15 22:52, Ard Biesheuvel wrote:
> > Kefeng reports in [0] that using PArange to size the randomized linear
> > region offset leads to cases where randomization is no longer possible
> > even if the actual placement of DRAM in memory would otherwise have
> > permitted it.
> >
> > Instead of using CONFIG_MEMORY_HOTPLUG to choose at build time between
> > to different behaviors in this regard, let's try addressing this by
> > reducing the minimum relative aligment between VA and PA in the linear
> > region, and taking advantage of the space at the base of physical memory
> > below the first memblock to permit some randomization of the placement
> > of physical DRAM in the virtual address map.
> VArange == PArange is ok, but our case is Va=39/Pa=48, this is still not
> works :(
>
> Could we add a way(maybe cmdline) to set max parange, then we could make
>
> randomization works, or some other way?
>

We could, but it is not a very elegant way to recover this
randomization range. You would need to reduce the PArange to 36 bits
(which is the next valid option below 40) in order to ensure that a
39-bit VA kernel has some room for randomization, but this would not
work on many systems because they require 40-bit physical addressing,
due to the placement of DRAM in the PA space, not the DRAM size.

Android 5.10 is in the same boat (and needs CONFIG_MEMORY_HOTPLUG=y)
so I agree we need something better here.



>
> > Cc: Kefeng Wang <wangkefeng.wang at huawei.com>
> >
> > [0] https://lore.kernel.org/linux-arm-kernel/20211104062747.55206-1-wangkefeng.wang@huawei.com/
> >
> > Ard Biesheuvel (2):
> >    arm64: simplify rules for defining ARM64_MEMSTART_ALIGN
> >    arm64: kaslr: take free space at start of DRAM into account
> >
> >   arch/arm64/include/asm/kernel-pgtable.h | 27 +++-----------------
> >   arch/arm64/mm/init.c                    |  3 ++-
> >   2 files changed, 6 insertions(+), 24 deletions(-)
> >