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From:   Kees Cook <keescook@chromium.org>
Date:   Tue, 30 Oct 2018 12:14:50 -0700
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Subject: Re: [PATCH 10/17] prmem: documentation
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Nadav Amit <nadav.amit@gmail.com>,
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On Tue, Oct 30, 2018 at 11:51 AM, Andy Lutomirski <luto@amacapital.net> wrote:
>
>
>> On Oct 30, 2018, at 10:58 AM, Matthew Wilcox <willy@infradead.org> wrote:
>>
>> 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.
>>
>> 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.
>>
>> So the API might look something like this:
>>
>>    void *p = rare_alloc(...);    /* writable pointer */
>>    p->a = x;
>>    q = rare_protect(p);        /* read-only pointer */
>>
>> To subsequently modify q,
>>
>>    p = rare_modify(q);
>>    q->a = 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 special mm requires preemption disabled as well as some other stuff, which Nadav carefully dealt with.

This is what I had before, yes:
https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=kspp/write-rarely&id=9ab0cb2618ebbc51f830ceaa06b7d2182fe1a52d

It just needs the switch_mm() backend.

-- 
Kees Cook