From: Kees Cook Subject: Re: [PATCH] dh key: fix rounding up KDF output length Date: Thu, 7 Jun 2018 12:16:16 -0700 Message-ID: References: <0000000000009c221d056e0cf53a@google.com> <20180607191201.97080-1-ebiggers3@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Cc: David Howells , James Morris , keyrings@vger.kernel.org, linux-security-module , linux-crypto , LKML , syzkaller-bugs@googlegroups.com, Tycho Andersen , Stephan Mueller , Eric Biggers To: Eric Biggers Return-path: In-Reply-To: <20180607191201.97080-1-ebiggers3@gmail.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-crypto.vger.kernel.org On Thu, Jun 7, 2018 at 12:12 PM, Eric Biggers wrote: > From: Eric Biggers > > 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@syzkaller.appspotmail.com > Reported-by: syzbot+29d17b7898b41ee120a5@syzkaller.appspotmail.com > Reported-by: syzbot+8a608baf8751184ec727@syzkaller.appspotmail.com > Reported-by: syzbot+d04e58bd384f1fe0b112@syzkaller.appspotmail.com > Fixes: 383203eff718 ("dh key: get rid of stack allocated array") > Signed-off-by: Eric Biggers Regardless: Acked-by: Kees Cook -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