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Message-ID: <87lgox8gjc.fsf@kamboji.qca.qualcomm.com>
Date: Mon, 12 Jun 2017 16:46:47 +0300
From: Kalle Valo <kvalo@...eaurora.org>
To: Arend van Spriel <arend.vanspriel@...adcom.com>
Cc: Emil Lenngren <emil.lenngren@...il.com>, Emmanuel Grumbach <egrumbach@...il.com>, Kees Cook <keescook@...omium.org>, "Jason A. Donenfeld" <Jason@...c4.com>, LKML <linux-kernel@...r.kernel.org>, "kernel-hardening\@lists.openwall.com" <kernel-hardening@...ts.openwall.com>, Anna Schumaker <anna.schumaker@...app.com>, David Howells <dhowells@...hat.com>, David Safford <safford@...ibm.com>, "David S. Miller" <davem@...emloft.net>, Gilad Ben-Yossef <gilad@...yossef.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Gustavo Padovan <gustavo@...ovan.org>, "J. Bruce Fields" <bfields@...ldses.org>, Jeff Layton <jlayton@...chiereds.net>, Johan Hedberg <johan.hedberg@...il.com>, Johannes Berg <johannes@...solutions.net>, Marcel Holtmann <marcel@...tmann.org>, Mimi Zohar <zohar@...ux.vnet.ibm.com>, Trond Myklebust <trond.myklebust@...marydata.com>, keyrings@...r.kernel.org, Bluez mailing list <linux-bluetooth@...r.kernel.org>, "open list\:NFS\, SUNRPC\, AND..." <linu
x-nfs@...r.kernel.org>, linux-wireless <linux-wireless@...r.kernel.org>, Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH 0/6] Constant Time Memory Comparisons Are Important
Arend van Spriel <arend.vanspriel@...adcom.com> writes:
> On 6/11/2017 11:30 PM, Emil Lenngren wrote:
>> 2017-06-11 22:48 GMT+02:00 Emmanuel Grumbach <egrumbach@...il.com>:
>>> On Sun, Jun 11, 2017 at 4:36 PM, Kees Cook <keescook@...omium.org> wrote:
>>>>
>>>> On Sun, Jun 11, 2017 at 1:13 AM, Kalle Valo <kvalo@...eaurora.org> wrote:
>>>>> "Jason A. Donenfeld" <Jason@...c4.com> writes:
>>>>>
>>>>>> Whenever you're comparing two MACs, it's important to do this using
>>>>>> crypto_memneq instead of memcmp. With memcmp, you leak timing information,
>>>>>> which could then be used to iteratively forge a MAC.
>>>>>
>>>>> Do you have any pointers where I could learn more about this?
>>>>
>>>> While not using C specifically, this talks about the problem generally:
>>>> https://www.chosenplaintext.ca/articles/beginners-guide-constant-time-cryptography.html
>>>>
>>>
>>> Sorry for the stupid question, but the MAC address is in plaintext in
>>> the air anyway or easily accessible via user space tools. I fail to
>>> see what it is so secret about a MAC address in that code where that
>>> same MAC address is accessible via myriads of ways.
>>
>> I think you're mixing up Media Access Control (MAC) addresses with
>> Message Authentication Code (MAC). The second one is a cryptographic
>> signature of a message.
>
> While this may be obvious to those who are in the know this mixup is
> easily made outside the crypto domain and especially in the (wireless)
> networking domain (my mind wandered towards the same error path).
I did realise that this was about Message Authentication Code (yay!) but
I got lost because I thought this is somehow related to timestamps :)
Thanks to Kees I now understand this is about revealing execution time
to the attacker, not timestamps or anything like that.
> As this series is touching stuff outside crypto it is good to be
> explicit and not use such abbreviations that can be misinterpreted.
> The article Kees referred to is also useful to get into the proper
> context here and at least worth mentioning this or other useful
> references in the cover letter.
And the kernel documentation we have is not really helping much:
/**
* crypto_memneq - Compare two areas of memory without leaking
* timing information.
*
* @a: One area of memory
* @b: Another area of memory
* @size: The size of the area.
*
* Returns 0 when data is equal, 1 otherwise.
*/
For most people "leaking timing information" does not tell much. Adding
a sentence or two _why_ this function should be used would be very
helpful.
--
Kalle Valo
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