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SubjectRe: [PATCH 10/17] prmem: documentation
From: Andy Lutomirski
Sent: October 30, 2018 at 6:51:17 PM GMT
> To: Matthew Wilcox <>,
> Cc: Kees Cook <>, Peter Zijlstra <>, Igor Stoppa <>, Mimi Zohar <>, Dave Chinner <>, James Morris <>, Michal Hocko <>, Kernel Hardening <>, linux-integrity <>, linux-security-module <>, Igor Stoppa <>, Dave Hansen <>, Jonathan Corbet <>, Laura Abbott <>, Randy Dunlap <>, Mike Rapoport <>, open list:DOCUMENTATION <>, LKML <>, Thomas Gleixner <>
> Subject: Re: [PATCH 10/17] prmem: documentation
>> On Oct 30, 2018, at 10:58 AM, Matthew Wilcox <> wrote:
>> On Tue, Oct 30, 2018 at 10:06:51AM -0700, Andy Lutomirski wrote:
>>>> On Oct 30, 2018, at 9:37 AM, Kees Cook <> 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.
> Also, can we maybe focus on getting something merged for statically
> allocated data first?
> 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’s not called except during rare events like
> device enumeration.
> 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’s all done now, and I, at least, rather liked your code. :)

I guess since it was based on your ideas…

Anyhow, the only open issue that I have with v2 is Peter’s wish that I would
make kgdb use of poke_text() less disgusting. I still don’t know exactly
how to deal with it.

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

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