[PATCH 10/17] prmem: documentation

[Nadav Amit]

From: Andy Lutomirski
Sent: October 30, 2018 at 6:51:17 PM GMT
> To: Matthew Wilcox <willy at infradead.org>, nadav.amit at gmail.com
> Cc: Kees Cook <keescook at chromium.org>, Peter Zijlstra <peterz at infradead.org>, Igor Stoppa <igor.stoppa at gmail.com>, Mimi Zohar <zohar at linux.vnet.ibm.com>, Dave Chinner <david at fromorbit.com>, James Morris <jmorris at namei.org>, Michal Hocko <mhocko at kernel.org>, Kernel Hardening <kernel-hardening at lists.openwall.com>, linux-integrity <linux-integrity at vger.kernel.org>, linux-security-module <linux-security-module at vger.kernel.org>, Igor Stoppa <igor.stoppa at huawei.com>, Dave Hansen <dave.hansen at linux.intel.com>, Jonathan Corbet <corbet at lwn.net>, Laura Abbott <labbott at redhat.com>, Randy Dunlap <rdunlap at infradead.org>, Mike Rapoport <rppt at linux.vnet.ibm.com>, open list:DOCUMENTATION <linux-doc at vger.kernel.org>, LKML <linux-kernel at vger.kernel.org>, Thomas Gleixner <tglx at linutronix.de>
> Subject: Re: [PATCH 10/17] prmem: documentation
> 
> 
> 
> 
>> On Oct 30, 2018, at 10:58 AM, Matthew Wilcox <willy at 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 at 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.
> 
> 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
v2.