Re: [PATCH 4/7] crypto/hmac_sha256: Remove undefined memset()

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On Tue, Apr 04, 2017 at 11:52:39AM +0930, Paul 'Rusty' Russell wrote:
> David Gibson <[email protected]> writes:
> > A memset() with a zero length argument isn't well defined.  It *might* =
be
> > a no-op, but then it might not.  So, test for this case in
> > hmac_sha256_init().
>=20
> Huh?  That seems really odd.

Well.. more precisely, my understanding is that memset(p, x, 0) can't
be counted on not to dereference p.

In this case, k_ipad + ksize is past the end of the array, and so not
a valid pointer to dereference.
>=20
> Rusty.
>=20
> > Found by Coverity.
> >
> > Signed-off-by: David Gibson <[email protected]>
> > ---
> >  ccan/crypto/hmac_sha256/hmac_sha256.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/ccan/crypto/hmac_sha256/hmac_sha256.c b/ccan/crypto/hmac_s=
ha256/hmac_sha256.c
> > index 0392afe..70ca3b2 100644
> > --- a/ccan/crypto/hmac_sha256/hmac_sha256.c
> > +++ b/ccan/crypto/hmac_sha256/hmac_sha256.c
> > @@ -36,7 +36,9 @@ void hmac_sha256_init(struct hmac_sha256_ctx *ctx,
> >  	 *   appended with 44 zero bytes 0x00)
> >  	 */
> >  	memcpy(k_ipad, k, ksize);
> > -	memset((char *)k_ipad + ksize, 0, HMAC_SHA256_BLOCKSIZE - ksize);
> > +	if (ksize < HMAC_SHA256_BLOCKSIZE)
> > +		memset((char *)k_ipad + ksize, 0,
> > +		       HMAC_SHA256_BLOCKSIZE - ksize);
> > =20
> >  	/*
> >  	 * (2) XOR (bitwise exclusive-OR) the B byte string computed
> >
> > _______________________________________________
> > ccan mailing list
> > [email protected]
> > https://lists.ozlabs.org/listinfo/ccan
>=20

--=20
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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