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Date:   Mon, 11 Feb 2019 17:26:06 -0800
From:   Kees Cook <keescook@...omium.org>
To:     "igor.stoppa@...il.com" <igor.stoppa@...il.com>
Cc:     Igor Stoppa <igor.stoppa@...wei.com>,
        Ahmed Soliman <ahmedsoliman@...a.vt.edu>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v4 00/12] hardening: statically allocated protected memory

On Mon, Feb 11, 2019 at 5:08 PM igor.stoppa@...il.com
<igor.stoppa@...il.com> wrote:
>
>
>
> On Tue, 12 Feb 2019, 4.47 Kees Cook <keescook@...omium.org wrote:
>>
>> On Mon, Feb 11, 2019 at 4:37 PM Igor Stoppa <igor.stoppa@...il.com> wrote:
>> >
>> >
>> >
>> > On 12/02/2019 02:09, Kees Cook wrote:
>> > > On Mon, Feb 11, 2019 at 3:28 PM Igor Stoppa <igor.stoppa@...il.com> wrote:
>> > > It looked like only the memset() needed architecture support. Is there
>> > > a reason for not being able to implement memset() in terms of an
>> > > inefficient put_user() loop instead? That would eliminate the need for
>> > > per-arch support, yes?
>> >
>> > So far, yes, however from previous discussion about power arch, I
>> > understood this implementation would not be so easy to adapt.
>> > Lacking other examples where the extra mapping could be used, I did not
>> > want to add code without a use case.
>> >
>> > Probably both arm and x86 32 bit could do, but I would like to first get
>> > to the bitter end with memory protection (the other 2 thirds).
>> >
>> > Mostly, I hated having just one arch and I also really wanted to have arm64.
>>
>> Right, I meant, if you implemented the _memset() case with put_user()
>> in this version, you could drop the arch-specific _memset() and shrink
>> the patch series. Then you could also enable this across all the
>> architectures in one patch. (Would you even need the Kconfig patches,
>> i.e. won't this "Just Work" on everything with an MMU?)
>
>
> I had similar thoughts, but this answer [1] deflated my hopes (if I understood it correctly).
> It seems that each arch needs to be massaged in separately.

True, but I think x86_64, x86, arm64, and arm will all be "normal".
power may be that way too, but they always surprise me. :)

Anyway, series looks good, but since nothing uses _memset(), it might
make sense to leave it out and put all the arch-enabling into a single
patch to cover the 4 archs above, in an effort to make the series even
smaller.

-- 
Kees Cook

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