Date: Wed, 31 Oct 2018 13:53:11 -0700 From: Andy Lutomirski <luto@...capital.net> To: Peter Zijlstra <peterz@...radead.org> Cc: Igor Stoppa <igor.stoppa@...il.com>, Matthew Wilcox <willy@...radead.org>, Tycho Andersen <tycho@...ho.ws>, Kees Cook <keescook@...omium.org>, Mimi Zohar <zohar@...ux.vnet.ibm.com>, 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>, Randy Dunlap <rdunlap@...radead.org>, Mike Rapoport <rppt@...ux.vnet.ibm.com>, "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de> Subject: Re: [PATCH 10/17] prmem: documentation > On Oct 31, 2018, at 1:38 PM, Andy Lutomirski <luto@...capital.net> wrote: > > > >>> On Oct 31, 2018, at 3:11 AM, Peter Zijlstra <peterz@...radead.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.
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