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

[Jarkko Sakkinen <jarkko@kernel.org>]

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