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

Mimi Zohar
2014-06-03T17:59:56Z

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>
---
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;
}

/* 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