linux-kernel - Re: [PATCH 10/17] prmem: documentation

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <2E29C35B-3F98-473D-A874-0D9B9B15B399@amacapital.net>
Date:   Wed, 31 Oct 2018 15:57:21 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Igor Stoppa <igor.stoppa@...il.com>,
        Tycho Andersen <tycho@...ho.ws>,
        Kees Cook <keescook@...omium.org>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Dave Chinner <david@...morbit.com>,
        James Morris <jmorris@...ei.org>,
        Michal Hocko <mhocko@...nel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        Igor Stoppa <igor.stoppa@...wei.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Laura Abbott <labbott@...hat.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 10/17] prmem: documentation



> On Oct 31, 2018, at 2:00 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
>> On Wed, Oct 31, 2018 at 01:36:48PM -0700, Andy Lutomirski wrote:
>> 
>>>> On Oct 31, 2018, at 3:02 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>>>> 
>>>> On Tue, Oct 30, 2018 at 09:41:13PM -0700, Andy Lutomirski wrote:
>>>> To clarify some of this thread, I think that the fact that rare_write
>>>> uses an mm_struct and alias mappings under the hood should be
>>>> completely invisible to users of the API.  No one should ever be
>>>> handed a writable pointer to rare_write memory (except perhaps during
>>>> bootup or when initializing a large complex data structure that will
>>>> be rare_write but isn't yet, e.g. the policy db).
>>> 
>>> Being able to use pointers would make it far easier to do atomics and
>>> other things though.
>> 
>> This stuff is called *rare* write for a reason. Do we really want to
>> allow atomics beyond just store-release?  Taking a big lock and then
>> writing in the right order should cover everything, no?
> 
> Ah, so no. That naming is very misleading.
> 
> We modify page-tables a _lot_. The point is that only a few sanctioned
> sites are allowed writing to it, not everybody.
> 
> I _think_ the use-case for atomics is updating the reference counts of
> objects that are in this write-rare domain. But I'm not entirely clear
> on that myself either. I just really want to avoid duplicating that
> stuff.

Sounds nuts. Doing a rare-write is many hundreds of cycles at best. Using that for a reference count sounds wacky.

Can we see a *real* use case before we over complicate the API?