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Subject: Re: [PATCH 10/17] prmem: documentation
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <28C8CD2A-BDC0-49A5-854E-1E18968528B8@amacapital.net>
Date:   Tue, 30 Oct 2018 16:18:39 -0700
Cc:     Kees Cook <keescook@chromium.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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To:     Andy Lutomirski <luto@amacapital.net>,
        Matthew Wilcox <willy@infradead.org>,
        Peter Zijlstra <peterz@infradead.org>
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From: Andy Lutomirski
Sent: October 30, 2018 at 6:51:17 PM GMT
> To: Matthew Wilcox <willy@infradead.org>, nadav.amit@gmail.com
> 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>
> Subject: Re: [PATCH 10/17] prmem: documentation
>=20
>=20
>=20
>=20
>> 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);
>=20
> How about:
>=20
> rare_write(&q->a, y);
>=20
> Or, for big writes:
>=20
> rare_write_copy(&q, local_q);
>=20
> This avoids a whole ton of issues. In practice, actually running with =
a
> special mm requires preemption disabled as well as some other stuff, =
which
> Nadav carefully dealt with.
>=20
> Also, can we maybe focus on getting something merged for statically
> allocated data first?
>=20
> Finally, one issue: rare_alloc() is going to utterly suck =
performance-wise
> due to the global IPI when the region gets zapped out of the direct =
map or
> otherwise 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 need to make sure it=E2=80=99s not called except during rare =
events like
> device enumeration.
>=20
> Nadav, want to resubmit your series? IIRC the only thing wrong with it =
was
> that 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. =
:)

I guess since it was based on your ideas=E2=80=A6

Anyhow, the only open issue that I have with v2 is Peter=E2=80=99s wish =
that I would
make kgdb use of poke_text() less disgusting. I still don=E2=80=99t know =
exactly
how to deal with it.

Perhaps it (fixing kgdb) can be postponed? In that case I can just =
resend
v2.