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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: [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
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