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

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Message-ID: <20161113030422.ijgbudyagdytvzjz@jeyu>
Date:   Sat, 12 Nov 2016 19:04:22 -0800
From:   Jessica Yu <jeyu@...hat.com>
To:     AKASHI Takahiro <takahiro.akashi@...aro.org>
Cc:     rusty@...tcorp.com.au, kernel-hardening@...ts.openwall.com,
        linux-kernel@...r.kernel.org
Subject: Re: module: extend 'rodata=off' boot cmdline parameter to module
 mappings

+++ 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@....com>
>Signed-off-by: AKASHI Takahiro <takahiro.akashi@...aro.org>
>Reviewed-by: Kees Cook <keescook@...omium.org>
>Cc: Rusty Russell <rusty@...tcorp.com.au>
>---
>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