linux-kernel - Re: [PATCH v2 2/2] certs: Add support for using elliptic curve keys for signing modules

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Date:   Wed, 21 Apr 2021 08:54:59 -0400
From:   Stefan Berger <stefanb@...ux.ibm.com>
To:     Jessica Yu <jeyu@...nel.org>
Cc:     keyrings@...r.kernel.org, dhowells@...hat.com, zohar@...ux.ibm.com,
        jarkko@...nel.org, nayna@...ux.ibm.com,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] certs: Add support for using elliptic curve keys
 for signing modules


On 4/21/21 8:52 AM, Jessica Yu wrote:
> +++ Stefan Berger [20/04/21 17:02 -0400]:
>>
>> On 4/20/21 10:03 AM, Jessica Yu wrote:
>>> +++ Stefan Berger [08/04/21 11:24 -0400]:
>>>>
>>>> diff --git a/crypto/asymmetric_keys/pkcs7_parser.c 
>>>> b/crypto/asymmetric_keys/pkcs7_parser.c
>>>> index 967329e0a07b..2546ec6a0505 100644
>>>> --- a/crypto/asymmetric_keys/pkcs7_parser.c
>>>> +++ b/crypto/asymmetric_keys/pkcs7_parser.c
>>>> @@ -269,6 +269,10 @@ int pkcs7_sig_note_pkey_algo(void *context, 
>>>> size_t hdrlen,
>>>>         ctx->sinfo->sig->pkey_algo = "rsa";
>>>>         ctx->sinfo->sig->encoding = "pkcs1";
>>>>         break;
>>>> +    case OID_id_ecdsa_with_sha256:
>>>> +        ctx->sinfo->sig->pkey_algo = "ecdsa";
>>>> +        ctx->sinfo->sig->encoding = "x962";
>>>> +        break;
>>>
>>> Hi Stefan,
>>>
>>> Does CONFIG_MODULE_SIG_KEY_TYPE_ECDSA have a dependency on 
>>> MODULE_SIG_SHA256?
>>
>> You are right, per the code above it does have a dependency on 
>> SHA256. ECDSA is using NIST p384 (secp384r1) for signing and per my 
>> tests it can be paired with all the sha hashes once the code above is 
>> extended. Now when it comes to module signing, should we pair it with 
>> a particular hash? I am not currently aware of a guidance document on 
>> this but sha256 and sha384 seem to be good choices these days, so 
>> maybe selecting ECDSA module signing should have a 'depends on' on 
>> these?
>
> Yeah, I would tack on the 'depends on' until the code above has been
> extended to cover more sha hashes - because currently if someone
> builds and signs a bunch of modules with an ECDSA key, they will fail
> to load if they picked something other than sha256. I am unfortunately
> not knowledgeable enough to suggest an official guideline on choice of
> hash, but for now it is reasonable to have a 'depends on' for which
> hashes the code currently supports, so that users don't run into
> module loading rejection issues.


I was going to repost this series now with the additional OIDs supported 
above and a recommendation to use sha384 in the help text for 
ECDSA-signed modules, but not enforcing this but instead trusting the 
user that they will choose a reasonable hash (probably >= sha256).


    Stefan


>
> Thanks!
>
> Jessica