Date: Wed, 31 Oct 2018 04:33:03 -0700 From: Matthew Wilcox <willy@...radead.org> To: Peter Zijlstra <peterz@...radead.org> Cc: Andy Lutomirski <luto@...capital.net>, Kees Cook <keescook@...omium.org>, Igor Stoppa <igor.stoppa@...il.com>, 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 Wed, Oct 31, 2018 at 10:36:59AM +0100, Peter Zijlstra wrote: > On Tue, Oct 30, 2018 at 10:58:14AM -0700, 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 <keescook@...omium.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); > > Why would you have rare_alloc() imply rare_modify() ? Would you have the > allocator meta data inside the rare section? Normally when I allocate some memory I need to initialise it before doing anything else with it ;-) I mean, you could do: ro = rare_alloc(..); rare = rare_modify(ro); rare->a = x; rare_protect(rare); but that's more typing.
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