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From: Kees Cook <keescook@chromium.org>
To: Jean-Philippe Aumasson <jeanphilippe.aumasson@gmail.com>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>,
	Kristen Carlson Accardi <kristen@linux.intel.com>,
	tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
	hpa@zytor.com, arjan@linux.intel.com, rick.p.edgecombe@intel.com,
	x86@kernel.org, LKML <linux-kernel@vger.kernel.org>,
	kernel-hardening@lists.openwall.com
Subject: Re: [RFC PATCH 04/11] x86/boot/KASLR: Introduce PRNG for faster shuffling
Date: Fri, 7 Feb 2020 01:05:03 -0800
Message-ID: <202002070100.2521E7563@keescook> (raw)
In-Reply-To: <CAGiyFdes26XnNeDfaz-vkm+bU7MBYQiK3xty2EigGjOXBGui2w@mail.gmail.com>

On Fri, Feb 07, 2020 at 08:23:53AM +0100, Jean-Philippe Aumasson wrote:
> 
> On Thu, Feb 6, 2020 at 4:10 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> 
> > Hey Kees,
> >
> > On Wed, Feb 05, 2020 at 02:39:43PM -0800, Kristen Carlson Accardi wrote:
> > > +#define rot(x, k) (((x)<<(k))|((x)>>(64-(k))))
> > > +static u64 prng_u64(struct prng_state *x)
> > > +{
> > > +     u64 e;
> > > +
> > > +     e = x->a - rot(x->b, 7);
> > > +     x->a = x->b ^ rot(x->c, 13);
> > > +     x->b = x->c + rot(x->d, 37);
> > > +     x->c = x->d + e;
> > > +     x->d = e + x->a;
> > > +
> > > +     return x->d;
> > > +}
> >
> > I haven't looked closely at where the original entropy sources are
> > coming from and how all this works, but on first glance, this prng
> > doesn't look like an especially cryptographically secure one. I realize
> > that isn't necessarily your intention (you're focused on speed), but
> > actually might this be sort of important? If I understand correctly, the
> > objective of this patch set is so that leaking the address of one
> > function doesn't leak the address of all other functions, as is the case
> > with fixed-offset kaslr. But if you leak the addresses of _some_ set of
> > functions, and your prng is bogus, might it be possible to figure out
> > the rest? For some prngs, if you give me the output stream of a few
> > numbers, I can predict the rest. For others, it's not this straight
> > forward, but there are some varieties of similar attacks. If any of that
> > set of concerns turns out to apply to your prng_u64 here, would that
> > undermine kaslr in similar ways as the current fixed-offset variety? Or
> > does it not matter because it's some kind of blinded fixed-size shuffle
> > with complex reasoning that makes this not a problem?
> 
> Let me share my 2 cents:
> 
> That permutation might be safe but afaict it hasn't been analyzed wrt
> modern cryptographic techniques and there might well be differential
> characteristics, statistical biases, etc.
> 
> What about just using SipHash's permutation, already in the kernel? It
> works on 4*u64 words too, and 6 rounds would be enough.
> 
> Doing a basic ops count, we currently have 5 group operations and 3
> rotations per round or 150 and 90 for the 30 init rounds. With SipHash it'd
> be 48 and 36 with the proposed 6 rounds. Probably insignificant speed wise
> as init is only done once but just to show that we'd get both better
> security assurance and better performance.

Yeah, this was never meant to be anything but a POC and after timing
tests, it seemed like an unneeded abstraction but was kept for this
RFC so it was possible to specify a stable seed at boot for debugging,
etc. I think this patch will not survive to v1. :)

-- 
Kees Cook

  reply index

Thread overview: 73+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-05 22:39 [RFC PATCH 00/11] Finer grained kernel address space randomization Kristen Carlson Accardi
2020-02-05 22:39 ` [RFC PATCH 01/11] modpost: Support >64K sections Kristen Carlson Accardi
2020-02-06 12:38   ` Kees Cook
2020-02-05 22:39 ` [RFC PATCH 02/11] x86: tools/relocs: Support >64K section headers Kristen Carlson Accardi
2020-02-06 12:39   ` Kees Cook
2020-02-05 22:39 ` [RFC PATCH 03/11] x86/boot: Allow a "silent" kaslr random byte fetch Kristen Carlson Accardi
2020-02-06  1:08   ` Andy Lutomirski
2020-02-06 11:48     ` Kees Cook
2020-02-06 16:58     ` Kristen Carlson Accardi
2020-02-05 22:39 ` [RFC PATCH 04/11] x86/boot/KASLR: Introduce PRNG for faster shuffling Kristen Carlson Accardi
2020-02-06  1:11   ` Andy Lutomirski
2020-02-06 15:10   ` Jason A. Donenfeld
2020-02-07  7:23     ` Jean-Philippe Aumasson
2020-02-07  9:05       ` Kees Cook [this message]
2020-02-07 16:52         ` Kristen Carlson Accardi
2020-02-05 22:39 ` [RFC PATCH 05/11] x86: Makefile: Add build and config option for CONFIG_FG_KASLR Kristen Carlson Accardi
2020-02-06 10:30   ` Peter Zijlstra
2020-02-06 11:52     ` Kees Cook
2020-02-25 17:55   ` Arvind Sankar
2020-02-26 19:13     ` Kristen Carlson Accardi
2020-03-24 21:24     ` Kristen Carlson Accardi
2020-03-25 15:34       ` Kees Cook
2020-02-05 22:39 ` [RFC PATCH 06/11] x86: make sure _etext includes function sections Kristen Carlson Accardi
2020-02-06 12:26   ` Kees Cook
2020-02-06 13:15     ` Jann Horn
2020-02-06 16:27       ` David Laight
2020-02-06 14:39     ` Arvind Sankar
2020-02-06 15:29       ` Arvind Sankar
2020-02-06 16:11         ` Andy Lutomirski
2020-02-06 14:57     ` Arvind Sankar
2020-02-06 15:45       ` Arvind Sankar
2020-02-06 19:41     ` Kristen Carlson Accardi
2020-02-06 20:02       ` Andy Lutomirski
2020-02-07  9:24         ` Peter Zijlstra
2020-02-10  1:43           ` Kees Cook
2020-02-10 10:51             ` Peter Zijlstra
2020-02-10 15:54               ` Arjan van de Ven
2020-02-10 16:36                 ` Arvind Sankar
2020-02-21 19:50                   ` Josh Poimboeuf
2020-02-21 23:05                     ` Arvind Sankar
2020-02-05 22:39 ` [RFC PATCH 07/11] x86/tools: Adding relative relocs for randomized functions Kristen Carlson Accardi
2020-02-06 12:37   ` Kees Cook
2020-02-05 22:39 ` [RFC PATCH 08/11] x86: Add support for finer grained KASLR Kristen Carlson Accardi
2020-02-06  1:17   ` Andy Lutomirski
2020-02-06 11:56     ` Kees Cook
2020-02-06 17:36       ` Kristen Carlson Accardi
2020-02-06 10:38   ` Peter Zijlstra
2020-02-06 12:06     ` Kees Cook
2020-02-06 14:52       ` Peter Zijlstra
2020-02-06 17:25         ` Kristen Carlson Accardi
2020-02-06 17:35           ` Peter Zijlstra
2020-02-06 17:43             ` Kristen Carlson Accardi
2020-02-25 17:49   ` Arvind Sankar
2020-02-26 19:26     ` Kristen Carlson Accardi
2020-02-05 22:39 ` [RFC PATCH 09/11] kallsyms: hide layout and expose seed Kristen Carlson Accardi
2020-02-06 12:32   ` Kees Cook
2020-02-06 17:51     ` Kristen Carlson Accardi
2020-02-06 19:27       ` Jann Horn
2020-03-02 19:01         ` Kristen Carlson Accardi
2020-03-02 19:08           ` Kees Cook
2020-03-02 19:19             ` Kristen Carlson Accardi
2020-02-27  2:42       ` Baoquan He
2020-02-27 16:02         ` Kees Cook
2020-02-28  3:36           ` Baoquan He
2020-02-05 22:39 ` [RFC PATCH 10/11] module: Reorder functions Kristen Carlson Accardi
2020-02-06 12:41   ` Kees Cook
2020-02-11 12:39     ` Jessica Yu
2020-02-05 22:39 ` [RFC PATCH 11/11] x86/boot: Move "boot heap" out of .bss Kristen Carlson Accardi
2020-02-06  0:11   ` Arvind Sankar
2020-02-06  0:33     ` Kristen Carlson Accardi
2020-02-06 11:13     ` Kees Cook
2020-02-06 14:25       ` Arvind Sankar
2020-02-06 21:32         ` Kees Cook

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