[PATCH] x86, kaslr: fix module lock ordering problem

[PATCH] x86, kaslr: fix module lock ordering problem

[Kees Cook]

There was a potential lock ordering problem with the module kASLR patch
("x86, kaslr: randomize module base load address"). This patch removes
the usage of the module_mutex and creates a new mutex to protect the
module base address offset value.

Chain exists of:
  text_mutex --> kprobe_insn_slots.mutex --> module_mutex

[    0.515561]  Possible unsafe locking scenario:
[    0.515561]
[    0.515561]        CPU0                    CPU1
[    0.515561]        ----                    ----
[    0.515561]   lock(module_mutex);
[    0.515561]                                lock(kprobe_insn_slots.mutex);
[    0.515561]                                lock(module_mutex);
[    0.515561]   lock(text_mutex);
[    0.515561]
[    0.515561]  *** DEADLOCK ***

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Andy Honig <ahonig@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 arch/x86/kernel/module.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c
index 49483137371f..e69f9882bf95 100644
--- a/arch/x86/kernel/module.c
+++ b/arch/x86/kernel/module.c
@@ -48,6 +48,9 @@ do {							\
 static unsigned long module_load_offset;
 static int randomize_modules = 1;
 
+/* Mutex protects the module_load_offset. */
+static DEFINE_MUTEX(module_kaslr_mutex);
+
 static int __init parse_nokaslr(char *p)
 {
 	randomize_modules = 0;
@@ -58,7 +61,7 @@ early_param("nokaslr", parse_nokaslr);
 static unsigned long int get_module_load_offset(void)
 {
 	if (randomize_modules) {
-		mutex_lock(&module_mutex);
+		mutex_lock(&module_kaslr_mutex);
 		/*
 		 * Calculate the module_load_offset the first time this
 		 * code is called. Once calculated it stays the same until
@@ -67,7 +70,7 @@ static unsigned long int get_module_load_offset(void)
 		if (module_load_offset == 0)
 			module_load_offset =
 				(get_random_int() % 1024 + 1) * PAGE_SIZE;
-		mutex_unlock(&module_mutex);
+		mutex_unlock(&module_kaslr_mutex);
 	}
 	return module_load_offset;
 }
-- 
1.7.9.5


-- 
Kees Cook
Chrome OS Security

Re: [PATCH] x86, kaslr: fix module lock ordering problem

[Masami Hiramatsu]

(2014/03/11 5:42), Kees Cook wrote:
> There was a potential lock ordering problem with the module kASLR patch
> ("x86, kaslr: randomize module base load address"). This patch removes
> the usage of the module_mutex and creates a new mutex to protect the
> module base address offset value.
> 
> Chain exists of:
>   text_mutex --> kprobe_insn_slots.mutex --> module_mutex
> 
> [    0.515561]  Possible unsafe locking scenario:
> [    0.515561]
> [    0.515561]        CPU0                    CPU1
> [    0.515561]        ----                    ----
> [    0.515561]   lock(module_mutex);
> [    0.515561]                                lock(kprobe_insn_slots.mutex);
> [    0.515561]                                lock(module_mutex);
> [    0.515561]   lock(text_mutex);
> [    0.515561]
> [    0.515561]  *** DEADLOCK ***
> 
> Reported-by: Fengguang Wu <fengguang.wu@intel.com>
> Signed-off-by: Andy Honig <ahonig@google.com>
> Signed-off-by: Kees Cook <keescook@chromium.org>

Reviewed-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>

Thanks for fix that ! :)


> ---
>  arch/x86/kernel/module.c |    7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c
> index 49483137371f..e69f9882bf95 100644
> --- a/arch/x86/kernel/module.c
> +++ b/arch/x86/kernel/module.c
> @@ -48,6 +48,9 @@ do {							\
>  static unsigned long module_load_offset;
>  static int randomize_modules = 1;
>  
> +/* Mutex protects the module_load_offset. */
> +static DEFINE_MUTEX(module_kaslr_mutex);
> +
>  static int __init parse_nokaslr(char *p)
>  {
>  	randomize_modules = 0;
> @@ -58,7 +61,7 @@ early_param("nokaslr", parse_nokaslr);
>  static unsigned long int get_module_load_offset(void)
>  {
>  	if (randomize_modules) {
> -		mutex_lock(&module_mutex);
> +		mutex_lock(&module_kaslr_mutex);
>  		/*
>  		 * Calculate the module_load_offset the first time this
>  		 * code is called. Once calculated it stays the same until
> @@ -67,7 +70,7 @@ static unsigned long int get_module_load_offset(void)
>  		if (module_load_offset == 0)
>  			module_load_offset =
>  				(get_random_int() % 1024 + 1) * PAGE_SIZE;
> -		mutex_unlock(&module_mutex);
> +		mutex_unlock(&module_kaslr_mutex);
>  	}
>  	return module_load_offset;
>  }
> 


-- 
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com