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

Kees Cook
2015-11-25T23:32:24Z

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