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

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Date:   Tue, 08 Mar 2022 07:45:03 -0500
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Stefan Berger <stefanb@...ux.ibm.com>,
        Eric Snowberg <eric.snowberg@...cle.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 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;

Agreed, as long as the other two criteria are also met: CA and keyUsage
should be required and limited to keyCertSign.

thanks,

Mimi