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Message-ID: <20181031113303.GJ10491@bombadil.infradead.org>
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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