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

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Date:   Wed, 20 Jun 2018 21:09:49 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     "Tobin C. Harding" <me@...in.cc>
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 06/20/2018 08:15 PM, Tobin C. Harding wrote:
> 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.

Yes, that's good.  Thanks.

-- 
~Randy