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From:   Boris Lukashev <blukashev@sempervictus.com>
Date:   Fri, 26 Jan 2018 11:36:30 -0500
Message-ID: <CAFUG7CeAfymvCC5jpBSM88X=8nSu-ktE0h81Ws1dAO0KrZk=9w@mail.gmail.com>
Subject: Re: [kernel-hardening] [PATCH 4/6] Protectable Memory
To:     Igor Stoppa <igor.stoppa@huawei.com>
Cc:     Jerome Glisse <jglisse@redhat.com>, Jann Horn <jannh@google.com>,
        Kees Cook <keescook@chromium.org>,
        Michal Hocko <mhocko@kernel.org>,
        Laura Abbott <labbott@redhat.com>,
        Christoph Hellwig <hch@infradead.org>,
        Matthew Wilcox <willy@infradead.org>,
        Christoph Lameter <cl@linux.com>,
        linux-security-module <linux-security-module@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>
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On Fri, Jan 26, 2018 at 7:28 AM, Igor Stoppa <igor.stoppa@huawei.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@huawei.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