phc-discussions - Re: [PHC] Tortuga issues

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Date: Thu, 3 Apr 2014 12:40:53 -0700
From: Andy Lutomirski <luto@...capital.net>
To: discussions <discussions@...sword-hashing.net>
Subject: Re: [PHC] Tortuga issues

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.

Alas, this is even worse than a DoS.  This code:

#include <alloca.h>

extern void use(void *ptr);

void test(size_t size)
{
  use(alloca(size));
}

Generates this assembly with gcc -O2 -S:

        .file   "alloca_probe.c"
        .text
        .p2align 4,,15
        .globl  test
        .type   test, @function
test:
.LFB0:
        .cfi_startproc
        pushq   %rbp
        .cfi_def_cfa_offset 16
        .cfi_offset 6, -16
        addq    $30, %rdi
        andq    $-16, %rdi
        movq    %rsp, %rbp
        .cfi_def_cfa_register 6
        subq    %rdi, %rsp
        leaq    15(%rsp), %rdi
        andq    $-16, %rdi
        call    use
        leave
        .cfi_def_cfa 7, 8
        ret
        .cfi_endproc
.LFE0:
        .size   test, .-test
        .ident  "GCC: (GNU) 4.8.2 20131212 (Red Hat 4.8.2-7)"
        .section        .note.GNU-stack,"",@progbits


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