Re: module: extend 'rodata=off' boot cmdline parameter to module mappings

[Jessica Yu]

+++ AKASHI Takahiro [21/10/16 10:13 +0900]:
> The current "rodata=off" parameter disables read-only kernel mappings
> under CONFIG_DEBUG_RODATA:
>    commit d2aa1acad22f ("mm/init: Add 'rodata=off' boot cmdline parameter
>    to disable read-only kernel mappings")
>
> This patch is a logical extension to module mappings ie. read-only mappings
> at module loading can be disabled even if CONFIG_DEBUG_SET_MODULE_RONX
> (mainly for debug use). Please note, however, that it only affects RO/RW
> permissions, keeping NX set.
>
> This is the first step to make CONFIG_DEBUG_SET_MODULE_RONX mandatory
> (always-on) in the future as CONFIG_DEBUG_RODATA on x86 and arm64.
>
> Suggested-by: Mark Rutland <mark.rutland@xxxxxxx>
> Signed-off-by: AKASHI Takahiro <takahiro.akashi@xxxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
> ---
> v2:
>  * use CONFIG_DEBUG_RODATA/SET_MODULE_RONX guards better where appropriate
>  * make "rodata_enabled" variable as __ro_after_init
> v1:
>  * remove RFC's "module_ronx=" and merge it with "rodata="
>  * always keep NX set if CONFIG_SET_MODULE_RONX
>
> include/linux/init.h |  3 +++
> init/main.c          |  7 +++++--
> kernel/module.c      | 21 ++++++++++++++++++---
> 3 files changed, 26 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/init.h b/include/linux/init.h
> index e30104c..885c3e6 100644
> --- a/include/linux/init.h
> +++ b/include/linux/init.h
> @@ -126,6 +126,9 @@ void prepare_namespace(void);
> void __init load_default_modules(void);
> int __init init_rootfs(void);
>
> +#if defined(CONFIG_DEBUG_RODATA) || defined(CONFIG_DEBUG_SET_MODULE_RONX)
> +extern bool rodata_enabled;
> +#endif
> #ifdef CONFIG_DEBUG_RODATA
> void mark_rodata_ro(void);
> #endif
> diff --git a/init/main.c b/init/main.c
> index 2858be7..959a242 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -81,6 +81,7 @@
> #include <linux/integrity.h>
> #include <linux/proc_ns.h>
> #include <linux/io.h>
> +#include <linux/cache.h>
>
> #include <asm/io.h>
> #include <asm/bugs.h>
> @@ -914,14 +915,16 @@ static int try_to_run_init_process(const char *init_filename)
>
> static noinline void __init kernel_init_freeable(void);
>
> -#ifdef CONFIG_DEBUG_RODATA
> -static bool rodata_enabled = true;
> +#if defined(CONFIG_DEBUG_RODATA) || defined(CONFIG_SET_MODULE_RONX)
> +bool rodata_enabled __ro_after_init = true;
> static int __init set_debug_rodata(char *str)
> {
> 	return strtobool(str, &rodata_enabled);
> }
> __setup("rodata=", set_debug_rodata);
> +#endif
>
> +#ifdef CONFIG_DEBUG_RODATA
> static void mark_readonly(void)
> {
> 	if (rodata_enabled)
> diff --git a/kernel/module.c b/kernel/module.c
> index f57dd63..34d1880 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -1910,6 +1910,9 @@ static void frob_writable_data(const struct module_layout *layout,
> /* livepatching wants to disable read-only so it can frob module. */
> void module_disable_ro(const struct module *mod)
> {
> +	if (!rodata_enabled)
> +		return;
> +
> 	frob_text(&mod->core_layout, set_memory_rw);
> 	frob_rodata(&mod->core_layout, set_memory_rw);
> 	frob_ro_after_init(&mod->core_layout, set_memory_rw);
> @@ -1919,6 +1922,9 @@ void module_disable_ro(const struct module *mod)
>
> void module_enable_ro(const struct module *mod, bool after_init)
> {
> +	if (!rodata_enabled)
> +		return;
> +
> 	frob_text(&mod->core_layout, set_memory_ro);
> 	frob_rodata(&mod->core_layout, set_memory_ro);
> 	frob_text(&mod->init_layout, set_memory_ro);
> @@ -1951,6 +1957,9 @@ void set_all_modules_text_rw(void)
> {
> 	struct module *mod;
>
> +	if (!rodata_enabled)
> +		return;
> +
> 	mutex_lock(&module_mutex);
> 	list_for_each_entry_rcu(mod, &modules, list) {
> 		if (mod->state == MODULE_STATE_UNFORMED)
> @@ -1967,6 +1976,9 @@ void set_all_modules_text_ro(void)
> {
> 	struct module *mod;
>
> +	if (!rodata_enabled)
> +		return;
> +
> 	mutex_lock(&module_mutex);
> 	list_for_each_entry_rcu(mod, &modules, list) {
> 		if (mod->state == MODULE_STATE_UNFORMED)
> @@ -1980,10 +1992,13 @@ void set_all_modules_text_ro(void)
>
> static void disable_ro_nx(const struct module_layout *layout)
> {
> -	frob_text(layout, set_memory_rw);
> -	frob_rodata(layout, set_memory_rw);
> +	if (rodata_enabled) {
> +		frob_text(layout, set_memory_rw);
> +		frob_rodata(layout, set_memory_rw);
> +	}
> 	frob_rodata(layout, set_memory_x);
> -	frob_ro_after_init(layout, set_memory_rw);
> +	if (rodata_enabled)
> +		frob_ro_after_init(layout, set_memory_rw);

Why not merge this hunk with the previous if (rodata_enabled) check?
I think it's more readable that way. In any case, the rest of the
patch looks good to me.

Jessica