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

[Eric Biggers <ebiggers@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>

Can you add a Fixes tag?

> ---
>  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);
>  	if (IS_ERR(tfm))
>  		return PTR_ERR(tfm);
>  
> @@ -414,8 +415,7 @@ int public_key_verify_signature(const struct public_key *pkey,
>  	if (ret)
>  		goto error_free_key;
>  
> -	if (sig->pkey_algo && strcmp(sig->pkey_algo, "sm2") == 0 &&
> -	    sig->data_size) {
> +	if (strcmp(pkey->pkey_algo, "sm2") == 0 && sig->data_size) {
>  		ret = cert_sig_digest_update(sig, tfm);
>  		if (ret)
>  			goto error_free_key;
> -- 

This is an improvement, but do you also have a plan to address the problem where
the code allows the "Za" hash step to be skipped?  The definitions of SM2 that I
could find require that step.  So, it is unclear that the algorithm with that
step skipped is still SM2, and how its security relates to that of the SM2
algorithm as actually defined.

- Eric