linux-kernel - Re: [PATCH V8 0/2] printk: hash addresses printed with %p

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Date:   Mon, 30 Oct 2017 15:03:21 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Tobin C. Harding" <me@...in.cc>
Cc:     kernel-hardening@...ts.openwall.com,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        "Theodore Ts'o" <tytso@....edu>,
        Linus Torvalds <torvalds@...ux-foundation.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 V8 0/2] printk: hash addresses printed with %p

On Wed, Oct 25, 2017 at 7:53 PM, Tobin C. Harding <me@...in.cc> wrote:
> Here is the behaviour that this set implements.
>
> For kpt_restrict==0
>
> Randomness not ready:
>   printed with %p:              (pointer)          # NOTE: with padding
> Valid pointer:
>   printed with %pK:             deadbeefdeadbeef
>   printed with %p:              0xdeadbeef
>   malformed specifier (eg %i):  0xdeadbeef

I really think we can't include SPECIAL unless _every_ callsite of %p
is actually doing "0x%p", and then we're replacing all of those. We're
not doing that, though...

$ git grep '%p\b' | wc -l
12766
$ git grep '0x%p\b' | wc -l
1837

If we need some kind of special marking that this is a hashed
variable, that should be something other than "0x". If we're using the
existing "(null)" and new "(pointer)" text, maybe "(hash:xxxxxx)"
should be used instead? Then the (rare) callers with 0x become
"0x(hash:xxxx)" and naked callers produce "(hash:xxxx)".

I think the first step for this is to just leave SPECIAL out.

-Kees

-- 
Kees Cook
Pixel Security