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

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Date:   Wed, 31 Oct 2018 13:36:48 -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 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?

> 
>> For example, there could easily be architectures where having a
>> writable alias is problematic.
> 
> Mostly we'd just have to be careful of cache aliases, alignment should
> be able to sort that I think.
> 
>> If you have multiple pools and one mm_struct per pool, you'll need a
>> way to find the mm_struct from a given allocation.
> 
> Or keep track of it externally. For example by context. If you modify
> page-tables you pick the page-table pool, if you modify selinux state,
> you pick the selinux pool.
> 
>> Regardless of how the mm_structs are set up, changing rare_write
>> memory to normal memory or vice versa will require a global TLB flush
>> (all ASIDs and global pages) on all CPUs, so having extra mm_structs
>> doesn't seem to buy much.
> 
> The way I understand it, the point is that if you stick page-tables and
> selinux state in different pools, a stray write in one will never affect
> the other.
> 

Hmm. That’s not totally crazy, but the API would need to be carefully designed. And some argument would have to be made as to why it’s better to use a different address space as opposed to checking in software along the lines of the uaccess checking.