netdev - Re: [patch 1/1] net: convert %p usage to %pK

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Date:	Wed, 25 May 2011 21:50:40 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	kees.cook@...onical.com
Cc:	eric.dumazet@...il.com, joe@...ches.com, mingo@...e.hu,
	akpm@...ux-foundation.org, netdev@...r.kernel.org,
	drosenberg@...curity.com, a.p.zijlstra@...llo.nl,
	eparis@...isplace.org, eugeneteo@...nel.org, jmorris@...ei.org,
	tgraf@...radead.org
Subject: Re: [patch 1/1] net: convert %p usage to %pK

From: Kees Cook <kees.cook@...onical.com>
Date: Wed, 25 May 2011 16:29:21 -0700

> Hi David,
> 
> On Tue, May 24, 2011 at 03:58:01AM -0400, David Miller wrote:
>> From: Eric Dumazet <eric.dumazet@...il.com>
>> Date: Tue, 24 May 2011 09:45:01 +0200
>> 
>> > Le mardi 24 mai 2011 à 00:35 -0700, Joe Perches a écrit :
>> > 
>> >> I think it's be better without the casts
>> >> using the standard kernel.h macros.
>> >> 
>> >> 	void *ptr;
>> >> 
>> >> 	ptr = maybe_hide_ptr(sk);
>> >> 	r->id.idiag_cookie[0] = lower_32_bits(ptr);
>> >> 	r->id.idiag_cookie[1] = upper_32_bits(ptr);
>> >> 
>> > 
>> > I am not sure I want to patch lower_32_bits() and upper_32_bits() for
>> > this.
>> > 
>> > They dont work on pointers, but on "numbers", according to kerneldoc
>> > Andrew wrote years ago. gcc agrees :
>> > 
>> > net/ipv4/inet_diag.c: In function ‘inet_csk_diag_fill’:
>> > net/ipv4/inet_diag.c:119: warning: cast from pointer to integer of different size
>> > net/ipv4/inet_diag.c:120: error: invalid operands to binary >>
>> > make[1]: *** [net/ipv4/inet_diag.o] Error 1
>> 
>> Also you can't do this, the "cookie" is used by the kernel future
>> lookups to find sockets.
>> 
>> The kernel pointer is part of the API, so sorry you can't "hide"
>> kernel pointers in this case without really breaking user visible
>> things.
> 
> But this is precisely what we're trying to control with kptr_restrict.
> Setting kptr_restrict will make inet_diag (and some details of similar
> things in /proc) meaningless. Based on the name, "diag" isn't going to be
> used in normal operation, and kptr_restrict is 0 by default, so only system
> owners interested in this will enable it and effectively disable inet_diag.

Are you kidding me?

inet_diag is the standard way to dump sockets using netlink.
It's not a special obscure debugging facility, it's for real
users.

And the encoded kernel pointer here is used as a shortcut to looking
up precise sockets.