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From:   Kees Cook <keescook@chromium.org>
Date:   Wed, 21 Feb 2018 14:22:10 -0800
Message-ID: <CAGXu5jKbuGcrdtVjEqp72e0+enzg26p1EGVPJ7goorbOnvcniA@mail.gmail.com>
Subject: Re: arm64 physmap (was Re: [kernel-hardening] [PATCH 4/6] Protectable Memory)
To:     Igor Stoppa <igor.stoppa@huawei.com>
Cc:     Laura Abbott <labbott@redhat.com>, Jann Horn <jannh@google.com>,
        Boris Lukashev <blukashev@sempervictus.com>,
        Christopher Lameter <cl@linux.com>,
        Matthew Wilcox <willy@infradead.org>,
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        Christoph Hellwig <hch@infradead.org>,
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On Tue, Feb 20, 2018 at 8:28 AM, Igor Stoppa <igor.stoppa@huawei.com> wrote:
>
>
> On 14/02/18 21:29, Kees Cook wrote:
>> On Wed, Feb 14, 2018 at 11:06 AM, Laura Abbott <labbott@redhat.com> wrote:
>
> [...]
>
>>> Kernel code should be fine, if it isn't that is a bug that should be
>>> fixed. Modules yes are not fully protected. The conclusion from past
>>
>> I think that's a pretty serious problem: we can't have aliases with
>> mismatched permissions; this degrades a deterministic protection
>> (read-only) to a probabilistic protection (knowing where the alias of
>> a target is mapped). Having an attack be "needs some info leaks"
>> instead of "need execution control to change perms" is a much lower
>> bar, IMO.
>
> Why "need execution control to change permission"?
> Or, iow, what does it mean exactly?
> ROP/JOP? Data-oriented control flow hijack?

Right, I mean, if an attacker has already gained execute control, they
can just call the needed functions to change memory permissions. But
that isn't needed if there is a mismatch between physmap and virtmap:
i.e. they can write to the physmap without needing to change perms
first.

> One can argue that this sort of R/W activity probably does require some
> form of execution control, but AFAIK, the only way to to prevent it, is
> to have CFI - btw, is there any standardization in that sense?

I meant that I don't want a difference in protection between physmap
and virtmap. I'd like to be able to reason the smae about the
exposures in either.

> So, from my (pessimistic?) perspective, the best that can be hoped for,
> is to make it much harder to figure out where the data is located.
>
> Virtual mapping has this side effect, compared to linear mapping.

Right, this is good, for sure. No complaints there at all. It's why I
think pmalloc and arm64 physmap perms are separate issues.

-Kees

-- 
Kees Cook
Pixel Security