KEYS: verify a certificate is signed by a 'trusted' key

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.

Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Signed-off-by: David Howells <dhowells@redhat.com>

1 files changed, 80 insertions, 1 deletions

diff --git a/crypto/asymmetric_keys/x509_public_key.c b/crypto/asymmetric_keys/x509_public_key.c
index c1540e8f454a..87612642f1b1 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 +151,33 @@ 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;
+
+        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 +230,13 @@ 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, system_trusted_keyring);
+                if (!ret)
+                        prep->trusted = 1;
         }
 
         /* Propose a description */