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

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Date:   Fri, 26 Jan 2018 11:36:30 -0500
From:   Boris Lukashev <blukashev@...pervictus.com>
To:     Igor Stoppa <igor.stoppa@...wei.com>
Cc:     Jerome Glisse <jglisse@...hat.com>, Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Michal Hocko <mhocko@...nel.org>,
        Laura Abbott <labbott@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Christoph Lameter <cl@...ux.com>,
        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 Fri, Jan 26, 2018 at 7:28 AM, Igor Stoppa <igor.stoppa@...wei.com> wrote:
> On 25/01/18 17:38, Jerome Glisse wrote:
>> On Thu, Jan 25, 2018 at 10:14:28AM -0500, Boris Lukashev wrote:
>>> On Thu, Jan 25, 2018 at 6:59 AM, Igor Stoppa <igor.stoppa@...wei.com> wrote:
>>
>> [...]
>>
>>> DMA/physmap access coupled with a knowledge of which virtual mappings
>>> are in the physical space should be enough for an attacker to bypass
>>> the gating mechanism this work imposes. Not trivial, but not
>>> impossible. Since there's no way to prevent that sort of access in
>>> current hardware (especially something like a NIC or GPU working
>>> independently of the CPU altogether)
>
> [...]
>
>> I am not saying that this can not happen but that we are trying our best
>> to avoid it.
>
> How about an opt-in verification, similar to what proposed by Boris
> Lukashev?
>
> When reading back the data, one could access the pointer directly and
> bypass the verification, or could use a function that explicitly checks
> the integrity of the data.
>
> Starting from an unprotected kmalloc allocation, even just turning the
> data into R/O is an improvement, but if one can afford the overhead of
> performing the verification, why not?
>

I like the idea of making the verification call optional for consumers
allowing for fast/slow+hard paths depending on their needs.
Cant see any additional vectors for abuse (other than the original
ones effecting out-of-band modification) introduced by having
verify/normal callers, but i've not had enough coffee yet. Any access
races or things like that come to mind for anyone? Shouldn't happen
with a write-once allocation, but again, lacking coffee.

> It would still be better if the service was provided by the library,
> instead than implemented by individual users, I think.
>
> --
> igor

-Boris