[PATCH 3/4] KEYS: CA link restriction

[Stefan Berger]

On 3/9/22 13:13, Eric Snowberg wrote:
> 
> 
>> On Mar 9, 2022, at 10:12 AM, Stefan Berger <stefanb at linux.ibm.com> wrote:
>>
>>
>>
>> On 3/8/22 13:02, Eric Snowberg wrote:
>>>> On Mar 8, 2022, at 5:45 AM, Mimi Zohar <zohar at linux.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 at linux.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
> 
> I could add another bool to the public key structure to designate if the key was self signed,
> but this seems to go against what the kernel document states. "Asymmetric / Public-key
> Cryptography Key Type” [1] states:
> 
> "The “asymmetric” key type is designed to be a container for the keys used in public-key
> cryptography, without imposing any particular restrictions on the form or mechanism of
> the cryptography or form of the key.
> 
> The asymmetric key is given a subtype that defines what sort of data is associated with
> the key and provides operations to describe and destroy it. However, no requirement is
> made that the key data actually be stored in the key."
> 
> Now every public key type would need to fill in the information on whether the key is self
> signed or not.  Instead of going through the public_key_verify_signature function currently
> used in this patch.

Every public key extracted from a x509 certificate would have to set 
this field to true if the public key originates from a self-signed x509 
cert. Is this different from this code here where now every public key 
would have to set the key_is_ca field?

+		if (v[1] != 0 && v[2] == ASN1_BOOL && v[3] == 1)
+			ctx->cert->pub->key_is_ca = true;

The extension I would have suggested looked similar:

cert->pub->x509_self_sign = cert->self_signed = true

[ to be put here: 
https://elixir.bootlin.com/linux/v5.17-rc7/source/crypto/asymmetric_keys/x509_public_key.c#L147 
]



> 
> https://docs.kernel.org/crypto/asymmetric-keys.html
>