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From: Eric Biggers <ebiggers3@gmail.com>
To: Kees Cook <keescook@chromium.org>
Cc: David Howells <dhowells@redhat.com>,
	James Morris <jmorris@namei.org>,
	keyrings@vger.kernel.org,
	linux-security-module <linux-security-module@vger.kernel.org>,
	linux-crypto <linux-crypto@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	syzkaller-bugs@googlegroups.com, Tycho Andersen <tycho@tycho.ws>,
	Stephan Mueller <smueller@chronox.de>,
	Eric Biggers <ebiggers@google.com>
Subject: Re: [PATCH] dh key: fix rounding up KDF output length
Date: Thu, 7 Jun 2018 12:28:48 -0700	[thread overview]
Message-ID: <20180607192848.GD29665@gmail.com> (raw)
In-Reply-To: <CAGXu5jJf-cW8WkwMoPZT8zM2aUoO6P-EQs7MK6_uUTneZr0ehA@mail.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

  parent reply	other threads:[~2018-06-07 19:28 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-06-07 13:07 KASAN: slab-out-of-bounds Write in sha1_finup syzbot
2018-06-07 14:43 ` syzbot
2018-06-07 19:12 ` [PATCH] dh key: fix rounding up KDF output length Eric Biggers
2018-06-07 19:16   ` Kees Cook
2018-06-07 19:28     ` James Morris
2018-06-07 19:28     ` Eric Biggers [this message]
2018-06-07 20:28   ` Tycho Andersen
2018-06-08 15:37 ` David Howells
2018-06-25 17:14   ` Eric Biggers

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