Re: [PATCH] dh key: fix rounding up KDF output length

[Eric Biggers <ebiggers3@gmail.com>]

On Thu, Jun 07, 2018 at 12:16:16PM -0700, Kees Cook wrote:
> On Thu, Jun 7, 2018 at 12:12 PM, Eric Biggers <ebiggers3@gmail.com> wrote:
> > From: Eric Biggers <ebiggers@google.com>
> >
> > Commit 383203eff718 ("dh key: get rid of stack allocated array") changed
> > kdf_ctr() to assume that the length of key material to derive is a
> > multiple of the digest size.  The length was supposed to be rounded up
> > accordingly.  However, the round_up() macro was used which only gives
> > the correct result on power-of-2 arguments, whereas not all hash
> > algorithms have power-of-2 digest sizes.  In some cases this resulted in
> > a write past the end of the 'outbuf' buffer.
> >
> > Fix it by switching to roundup(), which works for non-power-of-2 inputs.
> 
> round_up() vs roundup(). Wow, that's not confusing. :( I wonder if we
> should rename the former to roundup_pow2() or something?

Yes, it's very confusing, and I wish the names were clearer, or that there was
one macro that just did the right thing (but then the power-of-2 optimization
could only be done for constants, where it might not be necessary anyway).
roundup_pow2() would still be confused with roundup_pow_of_two(), unfortunately.

Eric