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Date:	Mon, 10 Mar 2014 13:42:48 -0700
From:	Kees Cook <keescook@...omium.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Andy Honig <ahonig@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jianguo Wu <wujianguo@...wei.com>,
	David Rientjes <rientjes@...gle.com>,
	Fengguang Wu <fengguang.wu@...el.com>
Subject: [PATCH] x86, kaslr: fix module lock ordering problem

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@...el.com>
Signed-off-by: Andy Honig <ahonig@...gle.com>
Signed-off-by: Kees Cook <keescook@...omium.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
--
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