phc-discussions - Re: [PHC] Tortuga issues

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Date: Thu, 3 Apr 2014 12:49:52 -0700
From: Andy Lutomirski <luto@...capital.net>
To: discussions <discussions@...sword-hashing.net>
Subject: Re: [PHC] Tortuga issues

On Thu, Apr 3, 2014 at 12:44 PM, Jeremi Gosney <epixoip@...dshell.nl> wrote:
> On 4/3/2014 12:40 PM, Andy Lutomirski wrote:
>> On Thu, Apr 3, 2014 at 4:03 AM, Jeremi Gosney <epixoip@...dshell.nl> wrote:
>>> On 4/2/2014 9:26 PM, Bill Cox wrote:
>>>> Tortuga fails on both windows and Linux for > 1MiB m_cost, due to
>>>> allocating hashing memory on the stack.
>>>
>>> Just a heads-up, the optimized implementation of Pufferfish has this
>>> `issue' as well, as it calls alloca() to dynamically allocate the sbox
>>> buffers on the stack. The reference implementation allocates memory on
>>> the heap with calloc() so this is not a problem there, but you'll blow
>>> out the stack on the optimized implementation if using an m_cost > 10
>>> (it doesn't "go to 11.")
>>>
>>> And yes, this was done intentionally. Since it is unlikely that anyone
>>> will be using an m_cost > 10, it's a mostly-safe optimization
>>> (especially for attackers, which is largely what the optimized
>>> implementation was, rewriting the algorithm from an attacker's perspective.)
>>>
>>> For optimized defender code, where one might just be crazy enough to use
>>> an m_cost of 11, there might be some benefit in writing a custom malloc
>>> implementation that can quickly allocate heap memory without the
>>> unnecessary overhead, not unlike JTR's mem_calloc_tiny(). But I think
>>> this is implementation-specific detail that is outside the scope of the
>>> PHC. Ideally implementers should be coding to the reference
>>> implementation and making their own optimizations, using the optimized
>>> code only as, erm, a reference.
>> Remember that it's entirely possible that a PHC winner will be asked
>> to compare an untrusted password to an unsalted hash, salt, and
>> parameters.  Crashing isn't nice.
>> [...]
>>
>> There's no probe, so, depending on the order in which the memory is
>> accessed, this can shoot all the way past the guard page and turn into
>> a standard buffer overflow.  (Of course, the data being written may
>> not be easy to control, so it's mitigated a bit.)
>>
>> If you compile with -fstack-probe, you may get far better behavior.
>> The code execution risk is gone (assuming that your threading library
>> doesn't suck), and you can actually safely use a much larger amount of
>> memory if you're in the main thread.
>>
>> On the other hand, using alloca for a one-time thing like this seems
>> completely pointless.  A decent malloc can allocate a buffer in a few
>> tens of ns.
>>
>> --Andy
>
>
> You literally just re-stated everything that I said. Which is why I
> bothered to say that the reference implementation does not use alloca(),
> only the attacker-optimized code does.

Sorry, I missed that defenders aren't supposed to use the optimized
implementation.  I suppose that should be obvious, given that the code
is a patch to JtR.

--Andy

>
>



-- 
Andy Lutomirski
AMA Capital Management, LLC