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

[Tianjia Zhang <tianjia.zhang@linux.alibaba.com>]

Hi Eric,

On 2/8/22 1:35 PM, Eric Biggers 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>
> 
> Can you add a Fixes tag?
> 

Thanks, the v2 patch with Fixes tag added has been appended to your
v2 series.

>> ---
>>   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
The design of this Za has indeed brought us a lot of trouble, which
makes the two separate steps of calculating the hash and signature
forced to be coupled together. At present, it is a better way to design
skipping Za as an option. I will try to do this, which of course also
includes application layer libraries, like openssl.

Best regards,
Tianjia