linux-kernel - Re: [PATCH 0/5] add printk specifier %px, unique identifier

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Date:   Mon, 27 Nov 2017 16:57:18 -0800
From:   Kees Cook <keescook@...omium.org>
To:     "Tobin C. Harding" <me@...in.cc>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        "Theodore Ts'o" <tytso@....edu>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Tycho Andersen <tycho@...ho.ws>,
        "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>,
        Radim Krčmář <rkrcmar@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>,
        kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH 0/5] add printk specifier %px, unique identifier

On Mon, Nov 27, 2017 at 3:40 PM, Tobin C. Harding <me@...in.cc> wrote:
> Linus,
>
> I know you are bored of this patch set already and this pits your vast
> experience against my eight months kernel dev experience ;)
>
> I humbly maintain that hashing %p and suggesting people use %x
> _correctly_ isn't a WIN solution.
>
> Please don't go easy on me because I'm new, if I'm out of line - say
> so.
>
> This set is based on the following assumptions.
>
> 1. We now have leaking_addresses.pl illuminating leaking addresses.
> 2. We have no _clear_ strategy for fixing leaks once found.
> 3. We do not have a proposed non opt-in solution.
> 4. There is a distinct use case for this specifier.
>
> Patch 1: Corrects the docs for %pK.
>
> Patch 2: Refactors %pK code out of pointer() into helper function.
>
> Patch 3: Adds specifier %px, small 'x' was chosen because the hashed hex
>          value is printed in lower case.
>
> Patch 4/5: Provides example usage of new specifier.
>
> The hashing code is based on the work done hashing %p during 4.14 dev
> cycle.
>
> Finally, with this patch set in place, we have the added benefit that
> newbies (me) can quietly go around the kernel 'sweeping up' after
> leaking addresses. This as apposed to using a hammer and hashing all
> %p. And if this is deemed too little and too slow we can always search
> and replace '%p' with '%px'.

How does this opt-in to %px help? We'll still have %p everywhere. :(
Why not invert this? %p is hashed and %px is the old %p? Then we can
move %x users to %px.

I'd still like to see a default-on solution for this class of leaks...

-Kees

>
> thanks,
> Tobin.
>
> Tobin C. Harding (5):
>   docs: correct documentation for %pK
>   vsprintf: refactor pK code out of pointer()
>   vsprintf: add specifier %px, unique identifier
>   KVM: use %px to print token identifier
>   vfio_pci: use %px to print token identifier
>
>  Documentation/printk-formats.txt  |   2 +-
>  drivers/vfio/pci/vfio_pci_intrs.c |   2 +-
>  lib/test_printf.c                 |  74 +++++++++++++++++
>  lib/vsprintf.c                    | 166 ++++++++++++++++++++++++++++----------
>  scripts/checkpatch.pl             |   2 +-
>  virt/kvm/eventfd.c                |   2 +-
>  6 files changed, 202 insertions(+), 46 deletions(-)
>
> --
> 2.7.4
>



-- 
Kees Cook
Pixel Security