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

[Laura Abbott]

On 02/14/2018 11:28 AM, Ard Biesheuvel wrote:
> 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?
> 

Page poisoning does exactly that. The argument I was trying to make
was that if nobody really uses page poisoning except for debugging
it might not be worth it to fix up the alias. Thinking a bit more,
this is a terrible argument for many reasons so yes I agree that
we can just fix up the alias if PAGE_POISONING (or other features)
are enabled.

Thanks,
Laura