Pair Based Cryptography library (PBC) Heap corruption on call to element_to_mpz()

Question

I'm working in Visual Studio 2012. In Debug mode the program works fine, however, when changed to release mode the program fails on the call to element_to_mpz() which is called by the PBC function element_pow_zn()

I'm using MS VC++ Convertion of PBC from their download page.

The call stack seems to indicate there is an error on a call to _realloc(). I believe I found the line responsible. In the PBC source, line 133 of montfp.c (the _mpz_realloc() call)

static void fp_to_mpz(mpz_ptr z, element_ptr e) {
  eptr ep = e->data;
  if (!ep->flag) mpz_set_ui(z, 0);
  else {
    // x is stored as xR.
    // We must divide out R to convert to standard representation.
    fptr p = e->field->data;
    mp_limb_t tmp[2 * p->limbs];

    memcpy(tmp, ep->d, p->limbs * sizeof(mp_limb_t));
    memset(&tmp[p->limbs], 0, p->limbs * sizeof(mp_limb_t));

    /**************************************************************************
     * The line I believe to be failing - However I can't step into PBC.dll as
     * I do not have the symbols.
     **************************************************************************/
    _mpz_realloc(z, p->limbs);//This is a call into the GMP library

    mont_reduce(z->_mp_d, tmp, p);
    // Remove leading zero limbs.
    for (z->_mp_size = p->limbs; !z->_mp_d[z->_mp_size - 1]; z->_mp_size--);
  }
}

The code I am using to actually call element_to_mpz() is:

bswabe_cph_t*
bswabe_enc( bswabe_pub_t* pub, element_t m, char* policy )
{
    bswabe_cph_t* cph;
    element_t s;

    cph = (bswabe_cph_t*)malloc(sizeof(bswabe_cph_t));

    element_init_Zr(s, pub->p);
    element_init_GT(m, pub->p);
    element_init_GT(cph->cs, pub->p);
    element_init_G1(cph->c,  pub->p);
    cph->p = parse_policy_postfix(policy);

    element_random(m);
    element_random(s);

    /****************************************************
    * The call to element_to_mpz() is in element_pow_zn()
    *****************************************************/
    element_pow_zn(cph->cs, pub->g_hat_alpha, s);

    element_mul(cph->cs, cph->cs, m);

    element_pow_zn(cph->c, pub->h, s);

    fill_policy(cph->p, pub, s);

    return cph;
}

The above code is from CPABE - Which is a library for policy based encryption. The arguments pub, m, and policy are all properly initialized from the calling environment.

I'm not sure if this is a bug in the CPABE code, the PBC, code or a bug in GMP. (since that's where _mpz_realloc() is defined)

Any help would be greatly appreciated.

Answer 1

To fix this issue I replaced the GMP requirement with MPIR. It can be built with VS2012 and used as a drop-in replacement for GMP. After switching to this library the issue went away.

I'm guessing the cause of my issue was a bug in GMP-4.1. Since I also couldn't get GMP-5.1 compiled and working with VC++ I could never verify that the bug is actually gone from GMP.