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Message-ID: <20181030175814.GB10491@bombadil.infradead.org>
Date: Tue, 30 Oct 2018 10:58:14 -0700
From: Matthew Wilcox <willy@...radead.org>
To: Andy Lutomirski <luto@...capital.net>
Cc: 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 10:06:51AM -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);
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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