[kernel-hardening] [PATCH v2 1/4] init: create cmdline param to disable readonly

[Kees Cook <keescook@chromium.org>]

It may be useful to debug writes to the readonly sections of memory,
so provide a cmdline "rodata=off" to allow for this.

Suggested-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 Documentation/kernel-parameters.txt |  4 ++++
 init/main.c                         | 31 +++++++++++++++++++++++++++----
 2 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 742f69d18fc8..21cf76dbba90 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3409,6 +3409,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 
 	ro		[KNL] Mount root device read-only on boot
 
+	rodata=		[KNL]
+		on	Mark read-only kernel memory as read-only (default).
+		off	Leave read-only kernel memory writable for debugging.
+
 	root=		[KNL] Root filesystem
 			See name_to_dev_t comment in init/do_mounts.c.
 
diff --git a/init/main.c b/init/main.c
index 9e64d7097f1a..06200d2fbf08 100644
--- a/init/main.c
+++ b/init/main.c
@@ -93,9 +93,6 @@ static int kernel_init(void *);
 extern void init_IRQ(void);
 extern void fork_init(void);
 extern void radix_tree_init(void);
-#ifndef CONFIG_DEBUG_RODATA
-static inline void mark_rodata_ro(void) { }
-#endif
 
 /*
  * Debug helper: via this flag we know that we are in 'early bootup code'
@@ -929,6 +926,32 @@ static int try_to_run_init_process(const char *init_filename)
 
 static noinline void __init kernel_init_freeable(void);
 
+#ifdef CONFIG_DEBUG_RODATA
+bool disable_mark_readonly;
+static int __init set_debug_rodata(char *str)
+{
+	if (!str)
+		return -EINVAL;
+	if (!strncmp(str, "on", 2))
+		disable_mark_readonly = false;
+	else if (!strncmp(str, "off", 3))
+		disable_mark_readonly = true;
+
+	return 0;
+}
+__setup("rodata=", set_debug_rodata);
+
+static void mark_readonly(void)
+{
+	if (disable_mark_readonly)
+		pr_info("Kernel read-only memory marking disabled.\n");
+	else
+		mark_rodata_ro();
+}
+#else
+static inline void mark_readonly(void) { };
+#endif
+
 static int __ref kernel_init(void *unused)
 {
 	int ret;
@@ -937,7 +960,7 @@ static int __ref kernel_init(void *unused)
 	/* need to finish all async __init code before freeing the memory */
 	async_synchronize_full();
 	free_initmem();
-	mark_rodata_ro();
+	mark_readonly();
 	system_state = SYSTEM_RUNNING;
 	numa_default_policy();
 
-- 
1.9.1