phc-discussions - Re: [PHC] Tortuga issues

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Date: Thu, 03 Apr 2014 12:44:25 -0700
From: Jeremi Gosney <epixoip@...dshell.nl>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Tortuga issues

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.