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 <[email protected]>
>> Signed-off-by: Tianjia Zhang <[email protected]>
>
> 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