Openwall GNU/*/Linux - a small security-enhanced Linux distro for servers
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Date: Thu, 19 Oct 2017 03:03:48 +0200
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: "Tobin C. Harding" <me@...in.cc>
Cc: kernel-hardening@...ts.openwall.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>, Steven Rostedt <rostedt@...dmis.org>, 
	Chris Fries <cfries@...gle.com>, Dave Weinstein <olorin@...gle.com>, 
	Daniel Micay <danielmicay@...il.com>, Djalal Harouni <tixxdz@...il.com>, 
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5] printk: hash addresses printed with %p

On Wed, Oct 18, 2017 at 11:30 PM, Tobin C. Harding <me@...in.cc> wrote:
> +static siphash_key_t ptr_secret __read_mostly;
> +static atomic_t have_key = ATOMIC_INIT(0);
> +
> +static void initialize_ptr_secret(void)
> +{
> +       if (atomic_read(&have_key) == 1)
> +               return;
> +
> +       get_random_bytes(&ptr_secret, sizeof(ptr_secret));
> +       atomic_set(&have_key, 1);
> +}

> +               case -EALREADY:
> +                       initialize_ptr_secret();
> +                       break;

Unfortunately the above is racy, and the spinlock you had before was
actually correct (though using an atomic inside a spinlock wasn't
strictly necessary). The race is that two callers might hit
initialize_ptr_secret at the same time, and have_key will be zero at
the beginning for both. Then they'll both scribble over ptr_secret,
and might wind up using a different value after if one finishes before
the other. I see two ways of correcting this:

1) Go back to the spinlock yourself.
2) Use get_random_bytes_once(&ptr_secret, sizeof(ptr_secret)). I don't
know lib/once.c especially well, but from cursory look, it appears to
be taking a spinlock too, which means you're probably good.


+       if (atomic_read(&have_key) == 0) {
+               random_ready.owner = NULL;
+               random_ready.func = schedule_async_key_init;

You can probably take care of this part in the initialization:

static struct random_ready_callback random_ready = {
        .func = schedule_async_key_init
};

Alternatively, you could put the actual call to
add_random_ready_callback in an init function, but maybe how you have
it is easier.


Jason

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