linux-kernel - Re: [kernel-hardening] [PATCH] printk: hash addresses printed with %p

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Message-ID: <20171012025518.GD30753@eros>
Date:   Thu, 12 Oct 2017 13:55:18 +1100
From:   "Tobin C. Harding" <me@...in.cc>
To:     Theodore Ts'o <tytso@....edu>, kernel-hardening@...ts.openwall.com,
        linux-kernel@...r.kernel.org,
        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 <will.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>
Subject: Re: [kernel-hardening] [PATCH] printk: hash addresses printed with %p

Removing kvm@...r.kernel.org from the CC list.

On Wed, Oct 11, 2017 at 01:48:58PM -0400, Theodore Ts'o wrote:
> On Wed, Oct 11, 2017 at 02:48:16PM +1100, Tobin C. Harding wrote:
> > +/*
> > + * Obfuscates pointer (algorithm taken from kptr_obfuscate(). See kernel/kcmp.c)
> > + * v is the pointer value, randval is some random value, oddval is some random
> > + * odd value.
> > + *
> > + * The obfuscation is done in two steps. First we xor the kernel pointer with
> > + * a random value, which puts pointer into a new position in a reordered space.
> > + * Secondly we multiply the xor production with a large odd random number to
> > + * permute its bits even more (the odd multiplier guarantees that the product
> > + * is unique ever after the high bits are truncated, since any odd number is
> > + * relative prime to 2^n).
> > + */
> 
> Why not just expose kptr_obfusecate() and use it, instead of copying
> code?
> 
> Also, I'm nervous about the obfuscation.  If the attacker can get a
> handful of known "real kernel pointer" and "obfuscated kernel pointer"
> values, it wouldn't be that hard for them to be able to reverse
> engineer the two secret values.
> 
> Perhaps the argument is "if the attacker can get a _single_ real
> kernel address, it's all over anyway", which is probably true for
> KASLR, but which might not be true for all attacks.
> 
> Anyway, if you use kptr_obfuscate in kernel/kcmp.c, then if we later
> decide that we should change the obfuscation algorithm to something
> stronger, we only need to do it in one place.
> 
> 					- Ted

Thanks Ted, others have misgivings about this method also. The email threads are all a bit mixed up
(thansk to my ineptness at posting patches :).

FYI, in the other threads Jason A. Donenfel and Linus Torvalds have discussed SipHash as a solution.

thanks,
Tobin.