Re: [Qemu-devel] [PATCH 1/6] crypto: make PBKDF iterations configurable for LUKS format

[Daniel P. Berrange]

On Thu, Sep 08, 2016 at 12:44:55PM -0500, Eric Blake wrote:
> On 09/08/2016 11:27 AM, Daniel P. Berrange wrote:
> > As protection against bruteforcing passphrases, the PBKDF
> > algorithm is tuned by counting the number of iterations
> > needed to produce 1 second of running time. If the machine
> > that the image will be used on is much faster than the
> > machine where the image is created, it can be desirable
> > to raise the number of limits. This adds a new 'iter-time'
> 
> s/limits/iterations/ ?
> 
> > property that allows the user to choose the iteration
> > wallclock time.
> > 
> > Signed-off-by: Daniel P. Berrange <address@hidden>
> > ---
> >  block/crypto.c      |  6 ++++++
> >  crypto/block-luks.c | 32 +++++++++++++++++++++++---------
> >  qapi/crypto.json    |  6 +++++-
> >  3 files changed, 34 insertions(+), 10 deletions(-)
> > 
> 
> > +++ b/crypto/block-luks.c
> > @@ -917,8 +917,12 @@ qcrypto_block_luks_create(QCryptoBlock *block,
> >      const char *hash_alg;
> >      char *cipher_mode_spec = NULL;
> >      QCryptoCipherAlgorithm ivcipheralg = 0;
> > +    uint64_t iters;
> >  
> >      memcpy(&luks_opts, &options->u.luks, sizeof(luks_opts));
> > +    if (!luks_opts.has_iter_time) {
> > +        luks_opts.iter_time = 1000;
> > +    }
> >      if (!luks_opts.has_cipher_alg) {
> >          luks_opts.cipher_alg = QCRYPTO_CIPHER_ALG_AES_256;
> >      }
> > @@ -1064,7 +1068,7 @@ qcrypto_block_luks_create(QCryptoBlock *block,
> >      /* Determine how many iterations we need to hash the master
> >       * key, in order to have 1 second of compute time used
> >       */
> > -    luks->header.master_key_iterations =
> > +    iters = luks_opts.iter_time *
> >          qcrypto_pbkdf2_count_iters(luks_opts.hash_alg,
> 
> luks_opts.iter_time is a user-provided 64-bit value, so this
> multiplication can overflow...

Oh doh, there I was thinkig it was just a 32bit int...

Regards,
Daniel
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