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Date:   Tue, 30 Oct 2018 10:58:14 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Kees Cook <keescook@chromium.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Igor Stoppa <igor.stoppa@gmail.com>,
        Mimi Zohar <zohar@linux.vnet.ibm.com>,
        Dave Chinner <david@fromorbit.com>,
        James Morris <jmorris@namei.org>,
        Michal Hocko <mhocko@kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-integrity <linux-integrity@vger.kernel.org>,
        linux-security-module <linux-security-module@vger.kernel.org>,
        Igor Stoppa <igor.stoppa@huawei.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Laura Abbott <labbott@redhat.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 10/17] prmem: documentation
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On Tue, Oct 30, 2018 at 10:06:51AM -0700, Andy Lutomirski wrote:
> > On Oct 30, 2018, at 9:37 AM, Kees Cook <keescook@chromium.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)