linux-kernel - Re: [kernel-hardening] [PATCH 4/6] Protectable Memory

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Date:   Wed, 21 Feb 2018 14:37:11 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Igor Stoppa <igor.stoppa@...wei.com>
Cc:     Laura Abbott <labbott@...hat.com>,
        Boris Lukashev <blukashev@...pervictus.com>,
        Christopher Lameter <cl@...ux.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jann Horn <jannh@...gle.com>,
        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>
Subject: Re: [kernel-hardening] [PATCH 4/6] Protectable Memory

On Tue, Feb 20, 2018 at 9:16 AM, Igor Stoppa <igor.stoppa@...wei.com> wrote:
>
>
> On 13/02/18 20:10, Laura Abbott wrote:
>> On 02/13/2018 07:20 AM, Igor Stoppa wrote:
>>> Why alterations of page properties are not considered a risk and the physmap is?
>>> And how would it be easier (i suppose) to attack the latter?
>>
>> Alterations are certainly a risk but with the physmap the
>> mapping is already there. Find the address and you have
>> access vs. needing to actually modify the properties
>> then do the access. I could also be complete off base
>> on my threat model here so please correct me if I'm
>> wrong.
>
> It's difficult for me to comment on this without knowing *how* the
> attack would be performed, in your model.
>
> Ex: my expectation is that the attacked has R/W access to kernel data
> and has knowledge of the location of static variables.
>
> This is not just a guess, but a real-life scenario, found in attacks
> that, among other things, are capable of disabling SELinux, to proceed
> toward gaining full root capability.
>
> At that point, I think that variables which are allocated dynamically,
> in vmalloc address space, are harder to locate, because of the virtual
> mapping and the randomness of the address chosen (this I have not
> confirmed yet, but I suppose there is some randomness in picking the
> address to assign to a certain allocation request to vmalloc, otherwise,
> it could be added).

Machine-to-machine runtime variation certainly affects the mapping
location, but for early boot allocations, these become surprisingly
deterministic, especially across similar hardware/memory layouts (both
the virtmap and physmap locations). However, using
CONFIG_RANDOMIZE_MEMORY makes it MUCH more difficult. (Note that
RANDOMIZE_BASE on arm64 effectively includes RANDOMIZE_MEMORY, as it
uses the entropy for multiple base offsets, including the physmap,
IIRC.)

>> I think your other summaries are good points though
>> and should go in the cover letter.
>
> Ok, I'm just afraid it risks becoming a lengthy dissertation :-)

It's rare to have anyone say "your commit log is too long". :)

-Kees

-- 
Kees Cook
Pixel Security