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

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Message-ID: <20180607192848.GD29665@gmail.com>
Date:   Thu, 7 Jun 2018 12:28:48 -0700
From:   Eric Biggers <ebiggers3@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     David Howells <dhowells@...hat.com>,
        James Morris <jmorris@...ei.org>, keyrings@...r.kernel.org,
        linux-security-module <linux-security-module@...r.kernel.org>,
        linux-crypto <linux-crypto@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs@...glegroups.com, Tycho Andersen <tycho@...ho.ws>,
        Stephan Mueller <smueller@...onox.de>,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH] dh key: fix rounding up KDF output length

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@...il.com> wrote:
> > From: Eric Biggers <ebiggers@...gle.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