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Date: Wed, 31 Oct 2018 11:02:37 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Andy Lutomirski <luto@...capital.net>
Cc: Matthew Wilcox <willy@...radead.org>,
Igor Stoppa <igor.stoppa@...il.com>,
Tycho Andersen <tycho@...ho.ws>, Kees Cook <keescook@...omium.org>,
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>,
LSM List <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 09:41:13PM -0700, Andy Lutomirski wrote:
> To clarify some of this thread, I think that the fact that rare_write
> uses an mm_struct and alias mappings under the hood should be
> completely invisible to users of the API. No one should ever be
> handed a writable pointer to rare_write memory (except perhaps during
> bootup or when initializing a large complex data structure that will
> be rare_write but isn't yet, e.g. the policy db).
Being able to use pointers would make it far easier to do atomics and
other things though.
> For example, there could easily be architectures where having a
> writable alias is problematic.
Mostly we'd just have to be careful of cache aliases, alignment should
be able to sort that I think.
> If you have multiple pools and one mm_struct per pool, you'll need a
> way to find the mm_struct from a given allocation.
Or keep track of it externally. For example by context. If you modify
page-tables you pick the page-table pool, if you modify selinux state,
you pick the selinux pool.
> Regardless of how the mm_structs are set up, changing rare_write
> memory to normal memory or vice versa will require a global TLB flush
> (all ASIDs and global pages) on all CPUs, so having extra mm_structs
> doesn't seem to buy much.
The way I understand it, the point is that if you stick page-tables and
selinux state in different pools, a stray write in one will never affect
the other.
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