arm64 physmap (was Re: [kernel-hardening] [PATCH 4/6] Protectable Memory)

[Ard Biesheuvel]

On 14 February 2018 at 19:06, Laura Abbott <labbott at redhat.com> wrote:
> On 02/13/2018 01:43 PM, Kees Cook wrote:
>>
>> On Tue, Feb 13, 2018 at 8:09 AM, Laura Abbott <labbott at redhat.com> wrote:
>>>
>>> No, arm64 doesn't fixup the aliases, mostly because arm64 uses larger
>>> page sizes which can't be broken down at runtime. CONFIG_PAGE_POISONING
>>> does use 4K pages which could be adjusted at runtime. So yes, you are
>>> right we would have physmap exposure on arm64 as well.
>>
>>
>> Errr, so that means even modules and kernel code are writable via the
>> arm64 physmap? That seems extraordinarily bad. :(
>>
>> -Kees
>>
>
> (adding linux-arm-kernel and changing the subject)
>
> Kernel code should be fine, if it isn't that is a bug that should be
> fixed.

We take care to ensure that the linear alias of the core kernel's
.text and .rodata segments are mapped read-only. When we first moved
the kernel out of the linear region, we did not map it there at all
anymore, but that broke hibernation so we had to put something back.

> Modules yes are not fully protected. The conclusion from past
> experience has been that we cannot safely break down larger page sizes
> at runtime like x86 does. We could theoretically
> add support for fixing up the alias if PAGE_POISONING is enabled but
> I don't know who would actually use that in production. Performance
> is very poor at that point.
>

As long as the linear alias of the module is mapped down to pages, we
should be able to tweak the permissions. I take it that PAGE_POISONING
does more than just that?