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Subject: Re: [PATCH 10/17] prmem: documentation
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20181030175814.GB10491@bombadil.infradead.org>
Date:   Tue, 30 Oct 2018 11:51:17 -0700
Cc:     Kees Cook <keescook@chromium.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Igor Stoppa <igor.stoppa@gmail.com>,
        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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References: <20181023213504.28905-1-igor.stoppa@huawei.com> <20181023213504.28905-11-igor.stoppa@huawei.com> <20181026092609.GB3159@worktop.c.hoisthospitality.com> <CAGXu5jKCs01jvtvpEFS7R8qCR-5iRqK11kJxLJY99NicGWc4aQ@mail.gmail.com> <20181028183126.GB744@hirez.programming.kicks-ass.net> <40cd77ce-f234-3213-f3cb-0c3137c5e201@gmail.com> <20181030152641.GE8177@hirez.programming.kicks-ass.net> <CAGXu5jJftJ+Hq+jrZGHX5CTDLWOog7kGsd_Ne=yMvrRs__skSg@mail.gmail.com> <0A7AFB50-9ADE-4E12-B541-EC7839223B65@amacapital.net> <20181030175814.GB10491@bombadil.infradead.org>
To:     Matthew Wilcox <willy@infradead.org>, nadav.amit@gmail.com
Sender: linux-kernel-owner@vger.kernel.org
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> On Oct 30, 2018, at 10:58 AM, Matthew Wilcox <willy@infradead.org> wrote:
>=20
> On Tue, Oct 30, 2018 at 10:06:51AM -0700, Andy Lutomirski wrote:
>>> On Oct 30, 2018, at 9:37 AM, Kees Cook <keescook@chromium.org> wrote:
>> I support the addition of a rare-write mechanism to the upstream kernel.
>> And I think that there is only one sane way to implement it: using an
>> mm_struct. That mm_struct, just like any sane mm_struct, should only
>> differ from init_mm in that it has extra mappings in the *user* region.
>=20
> I'd like to understand this approach a little better.  In a syscall path,
> we run with the user task's mm.  What you're proposing is that when we
> want to modify rare data, we switch to rare_mm which contains a
> writable mapping to all the kernel data which is rare-write.
>=20
> So the API might look something like this:
>=20
>    void *p =3D rare_alloc(...);    /* writable pointer */
>    p->a =3D x;
>    q =3D rare_protect(p);        /* read-only pointer */
>=20
> To subsequently modify q,
>=20
>    p =3D rare_modify(q);
>    q->a =3D y;
>    rare_protect(p);

How about:

rare_write(&q->a, y);

Or, for big writes:

rare_write_copy(&q, local_q);

This avoids a whole ton of issues.  In practice, actually running with a spe=
cial mm requires preemption disabled as well as some other stuff, which Nada=
v carefully dealt with.

Also, can we maybe focus on getting something merged for statically allocate=
d data first?

Finally, one issue: rare_alloc() is going to utterly suck performance-wise d=
ue to the global IPI when the region gets zapped out of the direct map or ot=
herwise made RO.  This is the same issue that makes all existing XPO efforts=
 so painful. We need to either optimize the crap out of it somehow or we nee=
d to make sure it=E2=80=99s not called except during rare events like device=
 enumeration.

Nadav, want to resubmit your series?  IIRC the only thing wrong with it was t=
hat it was a big change and we wanted a simpler fix to backport. But that=E2=
=80=99s all done now, and I, at least, rather liked your code. :)