linux-kernel - Re: [RFC PATCH] X.509: Don't check the signature on apparently self-signed keys [ver #2]

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Date:	Tue, 05 Jan 2016 11:08:18 -0500
From:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:	David Howells <dhowells@...hat.com>
Cc:	dwmw2@...radead.org, David Woodhouse <David.Woodhouse@...el.com>,
	linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org, keyrings@...r.kernel.org,
	Petko Manolov <petkan@...-labs.com>
Subject: Re: [RFC PATCH] X.509: Don't check the signature on apparently
 self-signed keys [ver #2]

On Tue, 2016-01-05 at 15:47 +0000, David Howells wrote:
> If a certificate is self-signed, don't bother checking the validity of the
> signature.  The cert cannot be checked by validation against the next one
> in the chain as this is the root of the chain.  Trust for this certificate
> can only be determined by whether we obtained it from a trusted location
> (ie. it was built into the kernel at compile time).
> 
> This also fixes a bug whereby certificates were being assumed to be
> self-signed if they had neither AKID nor SKID, the symptoms of which show
> up as an attempt to load a certificate failing with -ERANGE or -EBADMSG.
> This is produced from the RSA module when the result of calculating "m =
> s^e mod n" is checked.
> 
> Signed-off-by: David Howells <dhowells@...hat.com>
> cc: David Woodhouse <David.Woodhouse@...el.com>
> cc: Mimi Zohar <zohar@...ux.vnet.ibm.com>
> ---
> 
>  crypto/asymmetric_keys/x509_public_key.c |   25 ++++++++++++++++---------
>  1 file changed, 16 insertions(+), 9 deletions(-)
> 
> diff --git a/crypto/asymmetric_keys/x509_public_key.c b/crypto/asymmetric_keys/x509_public_key.c
> index 2a44b3752471..26e1937af7f4 100644
> --- a/crypto/asymmetric_keys/x509_public_key.c
> +++ b/crypto/asymmetric_keys/x509_public_key.c
> @@ -255,6 +255,9 @@ static int x509_validate_trust(struct x509_certificate *cert,
>  	struct key *key;
>  	int ret = 1;
> 
> +	if (!cert->akid_id || !cert->akid_skid)
> +		return 1;
> +
>  	if (!trust_keyring)
>  		return -EOPNOTSUPP;
> 
> @@ -312,17 +315,21 @@ static int x509_key_preparse(struct key_preparsed_payload *prep)
>  	cert->pub->algo = pkey_algo[cert->pub->pkey_algo];
>  	cert->pub->id_type = PKEY_ID_X509;
> 
> -	/* Check the signature on the key if it appears to be self-signed */
> -	if ((!cert->akid_skid && !cert->akid_id) ||
> -	    asymmetric_key_id_same(cert->skid, cert->akid_skid) ||
> -	    asymmetric_key_id_same(cert->id, cert->akid_id)) {
> -		ret = x509_check_signature(cert->pub, cert); /* self-signed */
> -		if (ret < 0)
> -			goto error_free_cert;
> -	} else if (!prep->trusted) {
> +	/* See if we can derive the trustability of this certificate.
> +	 *
> +	 * When it comes to self-signed certificates, we cannot evaluate
> +	 * trustedness except by the fact that we obtained it from a trusted
> +	 * location.  So we just rely on x509_validate_trust() failing in this
> +	 * case.
> +	 *
> +	 * Note that there's a possibility of a self-signed cert matching a
> +	 * cert that we have (most likely a duplicate that we already trust) -
> +	 * in which case it will be marked trusted.
> +	 */
> +	if (!prep->trusted) {
>  		ret = x509_validate_trust(cert, get_system_trusted_keyring());
>  		if (!ret)
> -			prep->trusted = 1;
> +			prep->trusted = true;
>  	}

You're missing Petko's patch:
41c89b6 IMA: create machine owner and blacklist keyrings

Mimi

> 
>  	/* Propose a description */
> 


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