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Date: Tue, 30 Oct 2018 12:28:41 -0600
From: Tycho Andersen <tycho@...ho.ws>
To: Matthew Wilcox <willy@...radead.org>
Cc: Andy Lutomirski <luto@...capital.net>,
Kees Cook <keescook@...omium.org>,
Peter Zijlstra <peterz@...radead.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 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;
Do you mean
p->a = y;
here? I assume the intent is that q isn't writable ever, but that's
the one we have in the structure at rest.
Tycho
> rare_protect(p);
>
> Under the covers, rare_modify() would switch to the rare_mm and return
> (void *)((unsigned long)q + ARCH_RARE_OFFSET). All of the rare data
> would then be modifiable, although you don't have any other pointers
> to it. rare_protect() would switch back to the previous mm and return
> (p - ARCH_RARE_OFFSET).
>
> Does this satisfy Igor's requirements? We wouldn't be able to
> copy_to/from_user() while rare_mm was active. I think that's a feature
> though! It certainly satisfies my interests (kernel code be able to
> mark things as dynamically-allocated-and-read-only-after-initialisation)
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