Smack: getting the Smack security context of keys

With this commit, the LSM Smack implements the LSM
side part of the system call keyctl with the action
code KEYCTL_GET_SECURITY.

It is now possible to get the context of, for example,
the user session key using the command "keyctl security @s".

The original patch has been modified for merge.
Signed-off-by: José Bollo <jose.bollo@open.eurogiciel.org>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>

security/smack/smack_lsm.c

@@ -4000,6 +4000,36 @@ static int smack_key_permission(key_ref_t key_ref,
 	rc = smk_bu_note("key access", tkp, keyp->security, request, rc);
 	return rc;
 }
+
+/*
+ * smack_key_getsecurity - Smack label tagging the key
+ * @key points to the key to be queried
+ * @_buffer points to a pointer that should be set to point to the
+ * resulting string (if no label or an error occurs).
+ * Return the length of the string (including terminating NUL) or -ve if
+ * an error.
+ * May also return 0 (and a NULL buffer pointer) if there is no label.
+ */
+static int smack_key_getsecurity(struct key *key, char **_buffer)
+{
+	struct smack_known *skp = key->security;
+	size_t length;
+	char *copy;
+
+	if (key->security == NULL) {
+		*_buffer = NULL;
+		return 0;
+	}
+
+	copy = kstrdup(skp->smk_known, GFP_KERNEL);
+	if (copy == NULL)
+		return -ENOMEM;
+	length = strlen(copy) + 1;
+
+	*_buffer = copy;
+	return length;
+}
+
 #endif /* CONFIG_KEYS */
 
 /*
@@ -4324,6 +4354,7 @@ struct security_operations smack_ops = {
 	.key_alloc = 			smack_key_alloc,
 	.key_free = 			smack_key_free,
 	.key_permission = 		smack_key_permission,
+	.key_getsecurity =		smack_key_getsecurity,
 #endif /* CONFIG_KEYS */
 
  /* Audit hooks */