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

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Date:   Wed, 14 Feb 2018 12:13:36 -0800
From:   Laura Abbott <labbott@...hat.com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Kees Cook <keescook@...omium.org>, Jann Horn <jannh@...gle.com>,
        Igor Stoppa <igor.stoppa@...wei.com>,
        Boris Lukashev <blukashev@...pervictus.com>,
        Christopher Lameter <cl@...ux.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jerome Glisse <jglisse@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: arm64 physmap (was Re: [kernel-hardening] [PATCH 4/6] Protectable
 Memory)

On 02/14/2018 11:28 AM, Ard Biesheuvel wrote:
> On 14 February 2018 at 19:06, Laura Abbott <labbott@...hat.com> wrote:
>> On 02/13/2018 01:43 PM, Kees Cook wrote:
>>>
>>> On Tue, Feb 13, 2018 at 8:09 AM, Laura Abbott <labbott@...hat.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