linux-kernel - Re: [PATCH 16/17] prmem: pratomic-long

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Message-ID: <CAGXu5jKmhoSsOTqDJmr3ijwDAh57Bfq3KVE+AjrBdLXjPz83+g@mail.gmail.com>
Date:   Wed, 31 Oct 2018 20:28:33 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Will Deacon <will.deacon@....com>,
        Igor Stoppa <igor.stoppa@...il.com>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Matthew Wilcox <willy@...radead.org>,
        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>,
        linux-security-module <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>,
        Boqun Feng <boqun.feng@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        linux-arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 16/17] prmem: pratomic-long

On Wed, Oct 31, 2018 at 2:10 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, Oct 30, 2018 at 04:28:16PM +0000, Will Deacon wrote:
>> On Tue, Oct 30, 2018 at 04:58:41PM +0100, Peter Zijlstra wrote:
>> > Like mentioned elsewhere; if you do write_enable() + write_disable()
>> > thingies, it all becomes:
>> >
>> >     write_enable();
>> >     atomic_foo(&bar);
>> >     write_disable();
>> >
>> > No magic gunk infested duplication at all. Of course, ideally you'd then
>> > teach objtool about this (or a GCC plugin I suppose) to ensure any
>> > enable reached a disable.
>>
>> Isn't the issue here that we don't want to change the page tables for the
>> mapping of &bar, but instead want to create a temporary writable alias
>> at a random virtual address?
>>
>> So you'd want:
>>
>>       wbar = write_enable(&bar);
>>       atomic_foo(wbar);
>>       write_disable(wbar);
>>
>> which is probably better expressed as a map/unmap API. I suspect this
>> would also be the only way to do things for cmpxchg() loops, where you
>> want to create the mapping outside of the loop to minimise your time in
>> the critical section.
>
> Ah, so I was thikning that the altnerative mm would have stuff in the
> same location, just RW instead of RO.

I was hoping for the same location too. That allows use to use a gcc
plugin to mark, say, function pointer tables, as read-only, and
annotate their rare updates with write_rare() without any
recalculation.

-Kees

-- 
Kees Cook