Message-ID: <20181024145606.GA9019@cisco>
Date:   Wed, 24 Oct 2018 15:56:06 +0100
From:   Tycho Andersen <tycho@...ho.ws>
To:     Igor Stoppa <igor.stoppa@...il.com>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Kees Cook <keescook@...omium.org>,
        Matthew Wilcox <willy@...radead.org>,
        Dave Chinner <david@...morbit.com>,
        James Morris <jmorris@...ei.org>,
        Michal Hocko <mhocko@...nel.org>,
        kernel-hardening@...ts.openwall.com,
        linux-integrity@...r.kernel.org,
        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>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Josh Triplett <josh@...htriplett.org>,
        rostedt <rostedt@...dmis.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 14/17] prmem: llist, hlist, both plain and rcu

On Wed, Oct 24, 2018 at 05:03:01PM +0300, Igor Stoppa wrote:
> On 24/10/18 14:37, Mathieu Desnoyers wrote:
> > Also, is it the right approach to duplicate existing APIs, or should we
> > rather hook into page fault handlers and let the kernel do those "shadow"
> > mappings under the hood ?
> 
> This question is probably a good candidate for the small Q&A section I have
> in the 00/17.
> 
> 
> > Adding a new GFP flags for dynamic allocation, and a macro mapping to
> > a section attribute might suffice for allocation or definition of such
> > mostly-read-only/seldom-updated data.
> 
> I think what you are proposing makes sense from a pure hardening standpoint.
> From a more defensive one, I'd rather minimise the chances of giving a free
> pass to an attacker.
> 
> Maybe there is a better implementation of this, than what I have in mind.
> But, based on my current understanding of what you are describing, there
> would be few issues:
> 
> 1) where would the pool go? The pool is a way to manage multiple vmas and
> express common property they share. Even before a vma is associated to the
> pool.
> 
> 2) there would be more code that can seamlessly deal with both protected and
> regular data. Based on what? Some parameter, I suppose.
> That parameter would be the new target.
> If the code is "duplicated", as you say, the actual differences are baked in
> at compile time. The "duplication" would also allow to have always inlined
> functions for write-rare and leave more freedom to the compiler for their
> non-protected version.
> 
> Besides, I think the separate wr version also makes it very clear, to the
> user of the API, that there will be a price to pay, in terms of performance.
> The more seamlessly alternative might make this price less obvious.

What about something in the middle, where we move list to list_impl.h,
and add a few macros where you have list_set_prev() in prlist now, so
we could do,

// prlist.h

#define list_set_next(head, next) wr_ptr(&head->next, next)
#define list_set_prev(head, prev) wr_ptr(&head->prev, prev)

#include <linux/list_impl.h>

// list.h

#define list_set_next(next) (head->next = next)
#define list_set_next(prev) (head->prev = prev)

#include <linux/list_impl.h>

I wonder then if you can get rid of some of the type punning too? It's
not clear exactly why that's necessary from the series, but perhaps
I'm missing something obvious :)

I also wonder how much the actual differences being baked in at
compile time makes. Most (all?) of this code is inlined.

Cheers,

Tycho

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