Message-ID: <e3a64c96-18b7-f940-8da5-ced3786c33b8@linux.ibm.com>
Date:   Fri, 11 Mar 2022 15:23:15 -0500
From:   Stefan Berger <stefanb@...ux.ibm.com>
To:     Eric Snowberg <eric.snowberg@...cle.com>
Cc:     Mimi Zohar <zohar@...ux.ibm.com>,
        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 3/11/22 13:44, Eric Snowberg wrote:
> 
> 
>> On Mar 9, 2022, at 12:02 PM, Stefan Berger <stefanb@...ux.ibm.com> wrote:
>>
>>
>>
>> On 3/9/22 13:13, Eric Snowberg wrote:
>>>> On Mar 9, 2022, at 10:12 AM, Stefan Berger <stefanb@...ux.ibm.com> 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
>>> 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?
> 
> The information to determine if the key is a CA can not be derived without help from
> the specific key type.  Up to this point, no one has needed it.
> 
>>
>> +		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 ]
> 
> The information to determine if a key is self signed can be derived without help
> from the specific key type.  This can be achieved without modification to a generic
> public header file.  Adding a field to the public header would need to either be
> x509 specific or generic for all key types.  Adding a x509 specific field seems to
> go against the goal outlined in the kernel documentation.  Adding a generic
> self_signed field impacts all key types, now each needs to be modified to fill in
> the new field.
> 

If we now called the generic field cert_self_signed we could let it 
indicate whether the certificate the key was extracted from was 
self-self signed. The next question then is how many different types of 
certificates does the key subsystem support besides x509 so we know 
where to set this field if necessary? I don't know of any other...  x509 
seems to be the only type of certificate associated with struct public_key.
What seems to be the case is that pkcs7 also runs the x509 cert parser 
to extract an x509 certificate, thus the flag will be set down this call 
path as well.

https://elixir.bootlin.com/linux/latest/source/crypto/asymmetric_keys/pkcs7_parser.c#L408

Further, the public_key struct is only used in a few places and only in 
the crypto/asymmetric_keys directory filled in. Its usage in pkcs8 seems 
not relevant for certs, so leaving cert_self_signed there uninitialized 
seems just right. The code in public_key.c seems to not deal with 
certificates. So what's left is the x509_cert_parser.c and the function 
x509_cert_parse() allocates it and then calls 
x509_check_for_self_signed(cert), which can set the flag.

It looks to me giving it a generic name and only ever setting it to true 
iin x509_check_for_self_sign(cert) should work.

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