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

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Date:   Mon, 7 Mar 2022 23:38:48 +0000
From:   Eric Snowberg <eric.snowberg@...cle.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>
CC:     Stefan Berger <stefanb@...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 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.