linux-kernel - Re: [PATCH 3/4] KEYS: CA link restriction

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Message-ID: <501b3be6-34fe-1519-53dc-5bfd2a5b21f5@linux.ibm.com>
Date:   Wed, 9 Mar 2022 12:17:06 -0500
From:   Stefan Berger <stefanb@...ux.ibm.com>
To:     Eric Snowberg <eric.snowberg@...cle.com>,
        Mimi Zohar <zohar@...ux.ibm.com>
Cc:     Jarkko Sakkinen <jarkko@...nel.org>,
        David Howells <dhowells@...hat.com>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "jmorris@...ei.org" <jmorris@...ei.org>,
        "serge@...lyn.com" <serge@...lyn.com>,
        "nayna@...ux.ibm.com" <nayna@...ux.ibm.com>,
        "mic@...ux.microsoft.com" <mic@...ux.microsoft.com>,
        Konrad Wilk <konrad.wilk@...cle.com>,
        "keyrings@...r.kernel.org" <keyrings@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH 3/4] KEYS: CA link restriction



On 3/9/22 12:12, Stefan Berger wrote:
> 
> 
> On 3/8/22 13:02, Eric Snowberg wrote:
>>
>>
>>> On Mar 8, 2022, at 5:45 AM, Mimi Zohar <zohar@...ux.ibm.com> wrote:
>>>
>>> On Mon, 2022-03-07 at 21:31 -0500, Stefan Berger wrote:
>>>>
>>>> On 3/7/22 18:38, Eric Snowberg wrote:
>>>>>
>>>>>
>>>>>> On Mar 7, 2022, at 4:01 PM, Mimi Zohar <zohar@...ux.ibm.com> wrote:
>>>>>>
>>>>>> On Mon, 2022-03-07 at 18:06 +0000, Eric Snowberg wrote:
>>>>>>>
>>>>>>>>> diff --git a/crypto/asymmetric_keys/restrict.c 
>>>>>>>>> b/crypto/asymmetric_keys/restrict.c
>>>>>>>>> index 6b1ac5f5896a..49bb2ea7f609 100644
>>>>>>>>> --- a/crypto/asymmetric_keys/restrict.c
>>>>>>>>> +++ b/crypto/asymmetric_keys/restrict.c
>>>>>>>>> @@ -108,6 +108,49 @@ int restrict_link_by_signature(struct key 
>>>>>>>>> *dest_keyring,
>>>>>>>>>     return ret;
>>>>>>>>> }
>>>>>>>>> +/**
>>>>>>>>> + * restrict_link_by_ca - Restrict additions to a ring of CA keys
>>>>>>>>> + * @dest_keyring: Keyring being linked to.
>>>>>>>>> + * @type: The type of key being added.
>>>>>>>>> + * @payload: The payload of the new key.
>>>>>>>>> + * @trust_keyring: Unused.
>>>>>>>>> + *
>>>>>>>>> + * Check if the new certificate is a CA. If it is a CA, then 
>>>>>>>>> mark the new
>>>>>>>>> + * certificate as being ok to link.
>>>>>>>>
>>>>>>>> CA = root CA here, right?
>>>>>>>
>>>>>>> Yes, I’ll update the comment
>>>>>>
>>>>>> Updating the comment is not enough.  There's an existing function 
>>>>>> named
>>>>>> "x509_check_for_self_signed()" which determines whether the 
>>>>>> certificate
>>>>>> is self-signed.
>>>>>
>>>>> Originally I tried using that function.  However when the restrict 
>>>>> link code is called,
>>>>> all the necessary x509 information is no longer available.   The 
>>>>> code in
>>>>> restrict_link_by_ca is basically doing the equivalent to 
>>>>> x509_check_for_self_signed.
>>>>> After verifying the cert has the CA flag set, the call to 
>>>>> public_key_verify_signature
>>>>> validates the cert is self signed.
>>>>>
>>>> Isn't x509_cert_parse() being called as part of parsing the 
>>>> certificate?
>>>> If so, it seems to check for a self-signed certificate every time. You
>>>> could add something like the following to 
>>>> x509_check_for_self_signed(cert):
>>>> pub->x509_self_signed = cert->self_signed = true;
>>>>
>>>> This could then reduce the function in 3/4 to something like:
>>>>
>>>> return payload->data[asym_crypto]->x509_self_signed;
>>
>> When I was studying the restriction code, before writing this patch, 
>> it looked like
>> it was written from the standpoint to be as generic as possible.  All 
>> code contained
>> within it works on either a public_key_signature or a public_key.  I 
>> had assumed it
>> was written this way to be used with different asymmetrical key types 
>> now and in
>> the future. I called the public_key_verify_signature function instead 
>> of interrogating
>> the x509 payload to keep in line with what I thought was the original 
>> design. Let me
>> know if I should be carrying x509 code in here to make the change above.
> 
> It does not seem right if there were two functions trying to determine 
> whether an x509 cert is self-signed. The existing is invoked as part of 
> loading a key onto the machine keyring from what I can see. It has 
> access to more data about the cert and therefore can do stronger tests, 
> yours doesn't have access to the data. So I guess I would remember in a 
> boolean in the public key structure that the x509 cert it comes from was 
> self signed following the existing test. Key in your function may be 
> that that payload->data[] array is guaranteed to be from the x509 cert 
> as set in x509_key_preparse().
> 
> https://elixir.bootlin.com/linux/v5.17-rc7/source/crypto/asymmetric_keys/x509_public_key.c#L236 
> 
> 
>     Stefan

Sorry for the mess in the response. The first version is the good one :-)