Re: [RFC PATCH v5 2/4] KEYS: verify a certificate is signed by a 'trusted' key

[Dmitry Kasatkin]

On 07/06/14 00:50, Dmitry Kasatkin wrote:
> On 3 June 2014 20:58, Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx> wrote:
>> Only public keys, with certificates signed by an existing
>> 'trusted' key on the system trusted keyring, should be added
>> to a trusted keyring.  This patch adds support for verifying
>> a certificate's signature.
>>
>> This is derived from David Howells pkcs7_request_asymmetric_key() patch.
>>
>> Changelog:
>> - define get_system_trusted_keyring() to fix kbuild issues
>>
>> Signed-off-by: Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx>
>> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> Acked-by: me
>
>
>> ---
>>  crypto/asymmetric_keys/x509_public_key.c | 84 +++++++++++++++++++++++++++++++-
>>  include/keys/system_keyring.h            | 10 +++-
>>  2 files changed, 92 insertions(+), 2 deletions(-)
>>
>> diff --git a/crypto/asymmetric_keys/x509_public_key.c b/crypto/asymmetric_keys/x509_public_key.c
>> index 382ef0d..1af8a30 100644
>> --- a/crypto/asymmetric_keys/x509_public_key.c
>> +++ b/crypto/asymmetric_keys/x509_public_key.c
>> @@ -18,12 +18,60 @@
>>  #include <linux/asn1_decoder.h>
>>  #include <keys/asymmetric-subtype.h>
>>  #include <keys/asymmetric-parser.h>
>> +#include <keys/system_keyring.h>
>>  #include <crypto/hash.h>
>>  #include "asymmetric_keys.h"
>>  #include "public_key.h"
>>  #include "x509_parser.h"
>>
>>  /*
>> + * Find a key in the given keyring by issuer and authority.
>> + */
>> +static struct key *x509_request_asymmetric_key(
>> +       struct key *keyring,
>> +       const char *signer, size_t signer_len,
>> +       const char *authority, size_t auth_len)
>> +{
>> +       key_ref_t key;
>> +       char *id;
>> +
>> +       /* Construct an identifier. */
>> +       id = kmalloc(signer_len + 2 + auth_len + 1, GFP_KERNEL);
>> +       if (!id)
>> +               return ERR_PTR(-ENOMEM);
>> +
>> +       memcpy(id, signer, signer_len);
>> +       id[signer_len + 0] = ':';
>> +       id[signer_len + 1] = ' ';
>> +       memcpy(id + signer_len + 2, authority, auth_len);
>> +       id[signer_len + 2 + auth_len] = 0;
>> +
>> +       pr_debug("Look up: \"%s\"\n", id);
>> +
>> +       key = keyring_search(make_key_ref(keyring, 1),
>> +                            &key_type_asymmetric, id);
>> +       if (IS_ERR(key))
>> +               pr_debug("Request for module key '%s' err %ld\n",
>> +                        id, PTR_ERR(key));
>> +       kfree(id);
>> +
>> +       if (IS_ERR(key)) {
>> +               switch (PTR_ERR(key)) {
>> +                       /* Hide some search errors */
>> +               case -EACCES:
>> +               case -ENOTDIR:
>> +               case -EAGAIN:
>> +                       return ERR_PTR(-ENOKEY);
>> +               default:
>> +                       return ERR_CAST(key);
>> +               }
>> +       }
>> +
>> +       pr_devel("<==%s() = 0 [%x]\n", __func__, key_serial(key_ref_to_ptr(key)));
>> +       return key_ref_to_ptr(key);
>> +}
>> +
>> +/*
>>   * Set up the signature parameters in an X.509 certificate.  This involves
>>   * digesting the signed data and extracting the signature.
>>   */
>> @@ -103,6 +151,36 @@ int x509_check_signature(const struct public_key *pub,
>>  EXPORT_SYMBOL_GPL(x509_check_signature);
>>
>>  /*
>> + * Check the new certificate against the ones in the trust keyring.  If one of
>> + * those is the signing key and validates the new certificate, then mark the
>> + * new certificate as being trusted.
>> + *
>> + * Return 0 if the new certificate was successfully validated, 1 if we couldn't
>> + * find a matching parent certificate in the trusted list and an error if there
>> + * is a matching certificate but the signature check fails.
>> + */
>> +static int x509_validate_trust(struct x509_certificate *cert,
>> +                              struct key *trust_keyring)
>> +{
>> +       const struct public_key *pk;
>> +       struct key *key;
>> +       int ret = 1;
>> +
>> +       if (!trust_keyring)
>> +               return -EOPNOTSUPP;
>> +
>> +       key = x509_request_asymmetric_key(trust_keyring,
>> +                                         cert->issuer, strlen(cert->issuer),
>> +                                         cert->authority,
>> +                                         strlen(cert->authority));
>> +       if (!IS_ERR(key))  {
>> +               pk = key->payload.data;
>> +               ret = x509_check_signature(pk, cert);
>> +       }
>> +       return ret;
>> +}
>> +
>> +/*
>>   * Attempt to parse a data blob for a key as an X509 certificate.
>>   */
>>  static int x509_key_preparse(struct key_preparsed_payload *prep)
>> @@ -155,9 +233,13 @@ static int x509_key_preparse(struct key_preparsed_payload *prep)
>>         /* Check the signature on the key if it appears to be self-signed */
>>         if (!cert->authority ||
>>             strcmp(cert->fingerprint, cert->authority) == 0) {
>> -               ret = x509_check_signature(cert->pub, cert);
>> +               ret = x509_check_signature(cert->pub, cert); /* self-signed */
>>                 if (ret < 0)
>>                         goto error_free_cert;
>> +       } else {
>> +               ret = x509_validate_trust(cert, get_system_trusted_keyring());
>> +               if (!ret)
>> +                       prep->trusted = 1;

Actually this can be like this

>> +       } else if (!prep->trusted)
>> +               ret = x509_validate_trust(cert,
get_system_trusted_keyring());
>> +               if (!ret)
>> +                       prep->trusted = 1;

>>         }
>>
>>         /* Propose a description */
>> diff --git a/include/keys/system_keyring.h b/include/keys/system_keyring.h
>> index 8dabc39..72665eb 100644
>> --- a/include/keys/system_keyring.h
>> +++ b/include/keys/system_keyring.h
>> @@ -17,7 +17,15 @@
>>  #include <linux/key.h>
>>
>>  extern struct key *system_trusted_keyring;
>> -
>> +static inline struct key *get_system_trusted_keyring(void)
>> +{
>> +       return system_trusted_keyring;
>> +}
>> +#else
>> +static inline struct key *get_system_trusted_keyring(void)
>> +{
>> +       return NULL;
>> +}
>>  #endif
>>
>>  #endif /* _KEYS_SYSTEM_KEYRING_H */
>> --
>> 1.8.1.4
>>
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-security-module" in
>> the body of a message to majordomo@xxxxxxxxxxxxxxx
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>
>

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/