Re: [PATCH 10/17] prmem: documentation

From: Andy Lutomirski <luto@amacapital.net>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Igor Stoppa <igor.stoppa@gmail.com>,
	Matthew Wilcox <willy@infradead.org>,
	Tycho Andersen <tycho@tycho.ws>,
	Kees Cook <keescook@chromium.org>,
	Mimi Zohar <zohar@linux.vnet.ibm.com>,
	Dave Chinner <david@fromorbit.com>,
	James Morris <jmorris@namei.org>,
	Michal Hocko <mhocko@kernel.org>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	linux-integrity <linux-integrity@vger.kernel.org>,
	linux-security-module <linux-security-module@vger.kernel.org>,
	Igor Stoppa <igor.stoppa@huawei.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Laura Abbott <labbott@redhat.com>,
	Randy Dunlap <rdunlap@infradead.org>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 10/17] prmem: documentation
Date: Wed, 31 Oct 2018 13:53:11 -0700	[thread overview]
Message-ID: <83ED6142-A370-42FD-BBA4-95E5C3F998B3@amacapital.net> (raw)
In-Reply-To: <34204E6C-53C2-427D-A3B2-3D2E091D3E4B@amacapital.net>

> On Oct 31, 2018, at 1:38 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> 
> 
> 
>>> On Oct 31, 2018, at 3:11 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>>> 
>>> On Wed, Oct 31, 2018 at 12:15:46AM +0200, Igor Stoppa wrote:
>>> On 30/10/2018 23:02, Andy Lutomirski wrote:
>> 
>>>> But I dislike allowing regular writes in the protected region. We
>>>> really only need four write primitives:
>>>> 
>>>> 1. Just write one value.  Call at any time (except NMI).
>>>> 
>>>> 2. Just copy some bytes. Same as (1) but any number of bytes.
>>>> 
>>>> 3,4: Same as 1 and 2 but must be called inside a special rare write
>>>> region. This is purely an optimization.
>>> 
>>> Atomic? RCU?
>> 
>> RCU can be done, that's not really a problem. Atomics otoh are a
>> problem. Having pointers makes them just work.
>> 
>> Andy; I understand your reason for not wanting them, but I really don't
>> want to duplicate everything. Is there something we can do with static
>> analysis to make you more comfortable with the pointer thing?
> 
> I’m sure we could do something with static analysis, but I think seeing a real use case where all this fanciness makes sense would be good.
> 
> And I don’t know if s390 *can* have an efficient implementation that uses pointers. OTOH they have all kinds of magic stuff, so who knows?

Also, if we’re using a hypervisor, then there are a couple ways it could be done:

1. VMFUNC.  Pointers work fine.  This is stronger than any amount of CR3 trickery because it can’t be defeated by page table attacks.

2. A hypercall to do the write. No pointers.

Basically, I think that if we can get away without writable pointers, we get more flexibility and less need for fancy static analysis. If we do need pointers, then so be it.