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Date: Tue, 30 Oct 2018 21:41:13 -0700
From: Andy Lutomirski <luto@...capital.net>
To: Matthew Wilcox <willy@...radead.org>
Cc: Igor Stoppa <igor.stoppa@...il.com>, Tycho Andersen <tycho@...ho.ws>,
Kees Cook <keescook@...omium.org>, Peter Zijlstra <peterz@...radead.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 2:36 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Tue, Oct 30, 2018 at 10:43:14PM +0200, Igor Stoppa wrote:
> > On 30/10/2018 21:20, Matthew Wilcox wrote:
> > > > > So the API might look something like this:
> > > > >
> > > > > void *p = rare_alloc(...); /* writable pointer */
> > > > > p->a = x;
> > > > > q = rare_protect(p); /* read-only pointer */
> >
> > With pools and memory allocated from vmap_areas, I was able to say
> >
> > protect(pool)
> >
> > and that would do a swipe on all the pages currently in use.
> > In the SELinux policyDB, for example, one doesn't really want to
> > individually protect each allocation.
> >
> > The loading phase happens usually at boot, when the system can be assumed to
> > be sane (one might even preload a bare-bone set of rules from initramfs and
> > then replace it later on, with the full blown set).
> >
> > There is no need to process each of these tens of thousands allocations and
> > initialization as write-rare.
> >
> > Would it be possible to do the same here?
>
> What Andy is proposing effectively puts all rare allocations into
> one pool. Although I suppose it could be generalised to multiple pools
> ... one mm_struct per pool. Andy, what do you think to doing that?
Hmm. Let's see.
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).
For example, there could easily be architectures where having a
writable alias is problematic. On such architectures, an entirely
different mechanism might work better. And, if a tool like KNOX ever
becomes a *part* of the Linux kernel (hint hint!)
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. 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.
(It's just possible that changing rare_write back to normal might be
able to avoid the flush if the spurious faults can be handled
reliably.)
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