Re: [PATCH v2 RESEND] KEYS: asymmetric: enforce SM2 signature use pkey algo

[Jarkko Sakkinen <jarkko@kernel.org>]

On Mon, Jun 27, 2022 at 05:20:27PM +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.
> 
> Fixes: 215525639631 ("X.509: support OSCCA SM2-with-SM3 certificate verification")
> Reported-by: Eric Biggers <ebiggers@google.com>
> Cc: stable@vger.kernel.org # v5.10+
> 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 7c9e6be35c30..3f17ee860f89 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);

So, why this should not validate sig->hash_alog *to be* "sm3"?

I.e. add instead guard before crypto_alloc_hash:

        if (strncmp(sig->hash_algo, "sm3") != 0) {
                /* error */
        }
        /* continue */

>  	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;
> -- 
> 2.24.3 (Apple Git-128)
> 

BR, Jarkko