linux-kernel - Re: [PATCH v8 4/4] vsprintf: Add command line option debug_boot_weak

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Date:   Thu, 21 Jun 2018 13:15:05 +1000
From:   "Tobin C. Harding" <me@...in.cc>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     Theodore Ts'o <tytso@....edu>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Kees Cook <keescook@...omium.org>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 4/4] vsprintf: Add command line option
 debug_boot_weak_hash

On Wed, Jun 20, 2018 at 04:38:05PM -0700, Randy Dunlap wrote:
> On 06/20/2018 04:22 PM, Tobin C. Harding wrote:
> > On Wed, Jun 20, 2018 at 03:36:44PM -0700, Randy Dunlap wrote:
> >> On 06/20/2018 03:30 PM, Tobin C. Harding wrote:
> >>> On Wed, Jun 20, 2018 at 09:09:49AM -0700, Randy Dunlap wrote:
> >>>> On 06/19/2018 09:20 PM, Tobin C. Harding wrote:
> >>>>> Currently printing [hashed] pointers requires enough entropy to be
> >>>>> available.  Early in the boot sequence this may not be the case
> >>>>> resulting in a dummy string '(____ptrval____)' being printed.  This
> >>>>> makes debugging the early boot sequence difficult.  We can relax the
> >>>>> requirement to use cryptographically secure hashing during debugging.
> >>>>> This enables debugging while keeping development/production kernel
> >>>>> behaviour the same.
> >>>>>
> >>>>> If new command line option debug_boot_weak_hash is enabled use
> >>>>> cryptographically insecure hashing and hash pointer value immediately.
> >>>>>
> >>>>> Signed-off-by: Tobin C. Harding <me@...in.cc>
> >>>>> Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> >>>>> ---
> >>>>>  Documentation/admin-guide/kernel-parameters.txt |  9 +++++++++
> >>>>>  lib/vsprintf.c                                  | 17 +++++++++++++++++
> >>>>>  2 files changed, 26 insertions(+)
> >>>>>
> >>>>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> >>>>> index 638342d0a095..a116fc0366b0 100644
> >>>>> --- a/Documentation/admin-guide/kernel-parameters.txt
> >>>>> +++ b/Documentation/admin-guide/kernel-parameters.txt
> >>>>> @@ -748,6 +748,15 @@
> >>>>>  
> >>>>>  	debug		[KNL] Enable kernel debugging (events log level).
> >>>>>  
> >>>>> +	debug_boot_weak_hash
> >>>>> +			[KNL] Enable printing pointers early in the boot
> >>>>> +			sequence.  If enabled, we use a weak hash instead of
> >>>>> +			siphash to hash pointers.  Use this option if you need
> >>>>> +			to see pointer values during early boot (i.e you are
> >>>>
> >>>> maybe:
> >>>> 			to see hashed pointer values
> >>>> i.e., not raw pointers.
> >>>
> >>> You cannot see 'raw pointers' anyways?
> >>
> >> only if using %px ?
> >>
> >> Maybe it's just terminology.  I don't consider a hashed value as a pointer value.
> >> It's just a key or handle or some other number, but it's not a pointer.
> >>
> >>>>
> >>>>> +			seeing instances of '(___ptrval___)').
> >>>>> +			Cryptographically insecure, please do not use on
> >>>>> +			production kernels.
> >>>
> >>> thanks for the review, I don't quiet see how to use your suggestion to
> >>> make the text clearer.  If you still feel this change is needed perhaps
> >>> you could write so I understand i.e 'Use this option if ...'
> >>
> >>
> >> OK, if you are good with it, I am too.  :)
> > 
> > I get you know.  I agree, how about this
> > 
> > 			[KNL] Enable printing pointers early in the boot
> > 			sequence.  If enabled, we use a weak hash instead of
> > 			siphash to hash pointers.  Use this option if you need
> > 			to print pointers with %px during early boot
> > 			(i.e you are seeing instances of '(___ptrval___)').
> > 			Cryptographically insecure, please do not use on
> > 			production kernels.
> 
> Sorry, I'm still confused by this paragraph.  It seems to say two different
> things.

My bad, I got totally confused myself.  After all this time you would
think I knew which specifier hashed and which didn't.  My apologies,
how about this:

 			[KNL] Enable printing [hashed] pointers early in
			the boot sequence.  If enabled, we use a weak hash
			instead of siphash to hash pointers.  Use this option if
			you are seeing instances of '(___ptrval___)') and need
			to see a value (hashed pointer) instead. Cryptographically
			insecure, please do not use on production kernels.
 			 			

thanks for your patience,
Tobin.