ima: define kernel parameter 'ima_template=' to change configured default

This patch allows users to specify from the kernel command line the
template descriptor, among those defined, that will be used to generate
and display measurement entries. If an user specifies a wrong template,
IMA reverts to the template descriptor set in the kernel configuration.

Signed-off-by: Roberto Sassu <[e-mail]>
Signed-off-by: Mimi Zohar <[e-mail]>

Documentation/kernel-parameters.txt

···
 			programs exec'd, files mmap'd for exec, and all files
 			opened for read by uid=0.
 
+	ima_template=   [IMA]
+			Select one of defined IMA measurements template formats.
+			Formats: { "ima" | "ima-ng" }
+			Default: "ima-ng"
+
 	init=		[KNL]
 			Format: <full_path>
 			Run specified binary instead of /sbin/init as init

security/integrity/ima/ima_template.c

···
  * File: ima_template.c
  *      Helpers to manage template descriptors.
  */
+#include <crypto/hash_info.h>
+
 #include "ima.h"
 #include "ima_template_lib.h"
 
···
 };
 
 static struct ima_template_desc *ima_template;
+static struct ima_template_desc *lookup_template_desc(const char *name);
+
+static int __init ima_template_setup(char *str)
+{
+	struct ima_template_desc *template_desc;
+	int template_len = strlen(str);
+
+	/*
+	 * Verify that a template with the supplied name exists.
+	 * If not, use CONFIG_IMA_DEFAULT_TEMPLATE.
+	 */
+	template_desc = lookup_template_desc(str);
+	if (!template_desc)
+		return 1;
+
+	/*
+	 * Verify whether the current hash algorithm is supported
+	 * by the 'ima' template.
+	 */
+	if (template_len == 3 && strcmp(str, IMA_TEMPLATE_IMA_NAME) == 0 &&
+	    ima_hash_algo != HASH_ALGO_SHA1 && ima_hash_algo != HASH_ALGO_MD5) {
+		pr_err("IMA: template does not support hash alg\n");
+		return 1;
+	}
+
+	ima_template = template_desc;
+	return 1;
+}
+__setup("ima_template=", ima_template_setup);
 
 static struct ima_template_desc *lookup_template_desc(const char *name)
 {