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

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Date:   Thu, 7 Jun 2018 12:16:16 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Eric Biggers <ebiggers3@...il.com>
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 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?

> Reported-by: syzbot+486f97f892efeb2075a3@...kaller.appspotmail.com
> Reported-by: syzbot+29d17b7898b41ee120a5@...kaller.appspotmail.com
> Reported-by: syzbot+8a608baf8751184ec727@...kaller.appspotmail.com
> Reported-by: syzbot+d04e58bd384f1fe0b112@...kaller.appspotmail.com
> Fixes: 383203eff718 ("dh key: get rid of stack allocated array")
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>

Regardless:

Acked-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
>  security/keys/dh.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/security/keys/dh.c b/security/keys/dh.c
> index f7403821db7f0..b203f7758f976 100644
> --- a/security/keys/dh.c
> +++ b/security/keys/dh.c
> @@ -142,6 +142,8 @@ static void kdf_dealloc(struct kdf_sdesc *sdesc)
>   * The src pointer is defined as Z || other info where Z is the shared secret
>   * from DH and other info is an arbitrary string (see SP800-56A section
>   * 5.8.1.2).
> + *
> + * 'dlen' must be a multiple of the digest size.
>   */
>  static int kdf_ctr(struct kdf_sdesc *sdesc, const u8 *src, unsigned int slen,
>                    u8 *dst, unsigned int dlen, unsigned int zlen)
> @@ -205,8 +207,8 @@ static int keyctl_dh_compute_kdf(struct kdf_sdesc *sdesc,
>  {
>         uint8_t *outbuf = NULL;
>         int ret;
> -       size_t outbuf_len = round_up(buflen,
> -                                    crypto_shash_digestsize(sdesc->shash.tfm));
> +       size_t outbuf_len = roundup(buflen,
> +                                   crypto_shash_digestsize(sdesc->shash.tfm));
>
>         outbuf = kmalloc(outbuf_len, GFP_KERNEL);
>         if (!outbuf) {
> --
> 2.17.1.1185.g55be947832-goog
>



-- 
Kees Cook
Pixel Security