On Tue, 2016-01-19 at 11:30 +0000, David Howells wrote:
> Trust for a self-signed certificate can normally only be determined by
> whether we obtained it from a trusted location (ie. it was built into the
> kernel at compile time), so there's not really any point in checking it -
> we could verify that the signature is valid, but it doesn't really tell us
> anything if the signature checks out.
>
> However, there's a bug in the code determining whether a certificate is
> self-signed or not - if they have neither AKID nor SKID then we just assume
> that the cert is self-signed, which may not be true.
>
> Given this, remove the code that treats self-signed certs specially when it
> comes to evaluating trustability and attempt to evaluate them as ordinary
> signed certificates. We then expect self-signed certificates to fail the
> trustability check and be marked as untrustworthy in x509_key_preparse().
>
> Note that there is the possibility of the trustability check on a
> self-signed cert then succeeding. This is most likely to happen when a
> duplicate of the certificate is already on the trust keyring - in which
> case it shouldn't be a problem.
>
> Signed-off-by: David Howells <dhowe...@redhat.com>
> cc: David Woodhouse <david.woodho...@intel.com>
> cc: Mimi Zohar <zo...@linux.vnet.ibm.com>
Acked-by: Mimi Zohar <zo...@linux.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 c4f3c40a4ab9..630c1c331fe1 100644
> --- a/crypto/asymmetric_keys/x509_public_key.c
> +++ b/crypto/asymmetric_keys/x509_public_key.c
> @@ -265,6 +265,9 @@ 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;
>
> @@ -322,19 +325,23 @@ 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)
> ret = x509_validate_trust(cert, get_ima_mok_keyring());
> if (!ret)
> - prep->trusted = 1;
> + prep->trusted = true;
> }
>
> /* Don't permit addition of blacklisted keys */
>
> --
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