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

Dmitry Kasatkin
2014-06-09T13:13:37Z

On 07/06/14 00:50, Dmitry Kasatkin wrote:
> On 3 June 2014 20:58, Mimi Zohar <zohar@linux.vnet.ibm.com> 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@linux.vnet.ibm.com>
>> Signed-off-by: David Howells <dhowells@redhat.com>
> 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
>>
>> --
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>
>