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Date: Tue, 31 Oct 2017 09:41:02 +1100
From: "Tobin C. Harding" <me@...in.cc>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: kernel-hardening@...ts.openwall.com,
	"Jason A. Donenfeld" <Jason@...c4.com>,
	Theodore Ts'o <tytso@....edu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Tycho Andersen <tycho@...ker.com>,
	"Roberts, William C" <william.c.roberts@...el.com>,
	Tejun Heo <tj@...nel.org>,
	Jordan Glover <Golden_Miller83@...tonmail.ch>,
	Greg KH <gregkh@...uxfoundation.org>,
	Petr Mladek <pmladek@...e.com>, Joe Perches <joe@...ches.com>,
	Ian Campbell <ijc@...lion.org.uk>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <wilal.deacon@....com>, Chris Fries <cfries@...gle.com>,
	Dave Weinstein <olorin@...gle.com>,
	Daniel Micay <danielmicay@...il.com>,
	Djalal Harouni <tixxdz@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V8 2/2] printk: hash addresses printed with %p

On Mon, Oct 30, 2017 at 05:33:22PM -0400, Steven Rostedt wrote:
> On Thu, 26 Oct 2017 13:58:38 +1100
> "Tobin C. Harding" <me@...in.cc> wrote:
> 
> > > +static bool have_filled_random_ptr_key;
> > > +static siphash_key_t ptr_key __read_mostly;
> > > +
> > > +static void fill_random_ptr_key(struct random_ready_callback *unused)
> > > +{
> > > +	get_random_bytes(&ptr_key, sizeof(ptr_key));
> > > +	WRITE_ONCE(have_filled_random_ptr_key, true);  
> > 
> > This usage of WRITE_ONCE was suggested by Jason A. Donenfeld. I read
> > include/linux/compiler.h but was not able to grok it. Is this enough to
> > stop the compiler re-ordering these two statements? 
> > 
> > Or do I need to read Documentation/memory-barriers.txt [again]?
> 
> No, the WRITE_ONCE does not stop the compiler from reordering those
> statements. If you need that, then you need to do:
> 
> 	get_random_bytes(&ptr_key, sizeof(ptr_key));
> 	barrier();
> 	WRITE_ONCE(have_filled_random_ptr_key, true);
> 
> and that only works against interrupts. If you need synchronization
> across CPUs, then you need smp_mb().

Cool. So I think we need

 	get_random_bytes(&ptr_key, sizeof(ptr_key));
	smp_mb();
 	WRITE_ONCE(have_filled_random_ptr_key, true);

V10 to include this unless I have it wrong.

thanks,
Tobin.

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