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Subject: Re: [PATCH 10/17] prmem: documentation
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <34204E6C-53C2-427D-A3B2-3D2E091D3E4B@amacapital.net>
Date:   Wed, 31 Oct 2018 13:53:11 -0700
Cc:     Igor Stoppa <igor.stoppa@gmail.com>,
        Matthew Wilcox <willy@infradead.org>,
        Tycho Andersen <tycho@tycho.ws>,
        Kees Cook <keescook@chromium.org>,
        Mimi Zohar <zohar@linux.vnet.ibm.com>,
        Dave Chinner <david@fromorbit.com>,
        James Morris <jmorris@namei.org>,
        Michal Hocko <mhocko@kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-integrity <linux-integrity@vger.kernel.org>,
        linux-security-module <linux-security-module@vger.kernel.org>,
        Igor Stoppa <igor.stoppa@huawei.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Laura Abbott <labbott@redhat.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
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To:     Peter Zijlstra <peterz@infradead.org>
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> On Oct 31, 2018, at 1:38 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>=20
>=20
>=20
>>> On Oct 31, 2018, at 3:11 AM, Peter Zijlstra <peterz@infradead.org> wrote=
:
>>>=20
>>> On Wed, Oct 31, 2018 at 12:15:46AM +0200, Igor Stoppa wrote:
>>> On 30/10/2018 23:02, Andy Lutomirski wrote:
>>=20
>>>> But I dislike allowing regular writes in the protected region. We
>>>> really only need four write primitives:
>>>>=20
>>>> 1. Just write one value.  Call at any time (except NMI).
>>>>=20
>>>> 2. Just copy some bytes. Same as (1) but any number of bytes.
>>>>=20
>>>> 3,4: Same as 1 and 2 but must be called inside a special rare write
>>>> region. This is purely an optimization.
>>>=20
>>> Atomic? RCU?
>>=20
>> RCU can be done, that's not really a problem. Atomics otoh are a
>> problem. Having pointers makes them just work.
>>=20
>> Andy; I understand your reason for not wanting them, but I really don't
>> want to duplicate everything. Is there something we can do with static
>> analysis to make you more comfortable with the pointer thing?
>=20
> I=E2=80=99m sure we could do something with static analysis, but I think s=
eeing a real use case where all this fanciness makes sense would be good.
>=20
> And I don=E2=80=99t know if s390 *can* have an efficient implementation th=
at uses pointers. OTOH they have all kinds of magic stuff, so who knows?

Also, if we=E2=80=99re using a hypervisor, then there are a couple ways it c=
ould be done:

1. VMFUNC.  Pointers work fine.  This is stronger than any amount of CR3 tri=
ckery because it can=E2=80=99t be defeated by page table attacks.

2. A hypercall to do the write. No pointers.

Basically, I think that if we can get away without writable pointers, we get=
 more flexibility and less need for fancy static analysis. If we do need poi=
nters, then so be it.=