security: define kernel_read_file hook

The kernel_read_file security hook is called prior to reading the file
into memory.

Changelog v4+:
- export security_kernel_read_file()

Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Luis R. Rodriguez <mcgrof@kernel.org>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>

security/integrity/ima/ima_main.c

@@ -337,6 +337,22 @@ int ima_module_check(struct file *file)
 	return process_measurement(file, NULL, 0, MAY_EXEC, MODULE_CHECK, 0);
 }
 
+/**
+ * ima_read_file - pre-measure/appraise hook decision based on policy
+ * @file: pointer to the file to be measured/appraised/audit
+ * @read_id: caller identifier
+ *
+ * Permit reading a file based on policy. The policy rules are written
+ * in terms of the policy identifier.  Appraising the integrity of
+ * a file requires a file descriptor.
+ *
+ * For permission return 0, otherwise return -EACCES.
+ */
+int ima_read_file(struct file *file, enum kernel_read_file_id read_id)
+{
+	return 0;
+}
+
 /**
  * ima_post_read_file - in memory collect/appraise/audit measurement
  * @file: pointer to the file to be measured/appraised/audit