Re: [PATCH] KEYS: asymmetric: enforce SM2 signature use pkey algo

[Jarkko Sakkinen <jarkko@kernel.org>]

On Mon, Feb 21, 2022 at 10:43:39AM +0800, Tianjia Zhang wrote:
> Hi Jarkko,
> 
> On 2/21/22 9:49 AM, Jarkko Sakkinen wrote:
> > On Mon, Feb 07, 2022 at 07:43:27PM +0800, Tianjia Zhang wrote:
> > > The signature verification of SM2 needs to add the Za value and
> > > recalculate sig->digest, which requires the detection of the pkey_algo
> > > in public_key_verify_signature(). As Eric Biggers said, the pkey_algo
> > > field in sig is attacker-controlled and should be use pkey->pkey_algo
> > > instead of sig->pkey_algo, and secondly, if sig->pkey_algo is NULL, it
> > > will also cause signature verification failure.
> > > 
> > > The software_key_determine_akcipher() already forces the algorithms
> > > are matched, so the SM3 algorithm is enforced in the SM2 signature,
> > > although this has been checked, we still avoid using any algorithm
> > > information in the signature as input.
> > > 
> > > Reported-by: Eric Biggers <ebiggers@google.com>
> > > Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
> > > ---
> > >   crypto/asymmetric_keys/public_key.c | 6 +++---
> > >   1 file changed, 3 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/crypto/asymmetric_keys/public_key.c b/crypto/asymmetric_keys/public_key.c
> > > index a603ee8afdb8..ea9a5501f87e 100644
> > > --- a/crypto/asymmetric_keys/public_key.c
> > > +++ b/crypto/asymmetric_keys/public_key.c
> > > @@ -309,7 +309,8 @@ static int cert_sig_digest_update(const struct public_key_signature *sig,
> > >   	if (ret)
> > >   		return ret;
> > > -	tfm = crypto_alloc_shash(sig->hash_algo, 0, 0);
> > > +	/* SM2 signatures always use the SM3 hash algorithm */
> > > +	tfm = crypto_alloc_shash("sm3", 0, 0);
> > 
> > Why not simply fail when sig->hash_algo != "sm3"?
> > 
> > BR, Jarkko
> 
> This series of Eric's patch 2/2 has done this check.
> 
> Best regards,
> Tianjia

Hmm... So how does that make this legit?

BR, Jarkko