[PATCH] kprobes: disable preempt for module_text_address() and kernel_text_address()

[Masami Hiramatsu <mhiramat@redhat.com>]

__register_kprobe() can be preempted after checking probing address
but before module_text_address() or try_module_get(), and in this
interval the module can be unloaded. In that case,
try_module_get(probed_mod) will access to invalid address,
or kprobe will probe invalid address.

this patch uses preempt_disable() to protect it and use
__module_text_address() and __kernel_text_address().

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
---

>> @@ -718,9 +725,11 @@ static void __kprobes __unregister_kprob
>>  	struct kprobe *old_p;
>>
>>  	if (p->mod_refcounted) {
>> -		mod = module_text_address((unsigned long)p->addr);
>> +		preempt_disable();
>> +		mod = __module_text_address((unsigned long)p->addr);
>>  		if (mod)
>>  			module_put(mod);
>> +		preempt_enable();
>
> this is bad fix, we have had a reference to mod. we don't need
> preempt_disable() before module_put(mod).

Hmm, Indeed.
So let's add a comment there.

 kernel/kprobes.c |   21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

Index: 2.6.28-rc3/kernel/kprobes.c
===================================================================
--- 2.6.28-rc3.orig/kernel/kprobes.c
+++ 2.6.28-rc3/kernel/kprobes.c
@@ -613,30 +613,37 @@ static int __kprobes __register_kprobe(s
 		return -EINVAL;
 	p->addr = addr;

-	if (!kernel_text_address((unsigned long) p->addr) ||
-	    in_kprobes_functions((unsigned long) p->addr))
+	preempt_disable();
+	if (!__kernel_text_address((unsigned long) p->addr) ||
+	    in_kprobes_functions((unsigned long) p->addr)) {
+		preempt_enable();
 		return -EINVAL;
+	}

 	p->mod_refcounted = 0;

 	/*
 	 * Check if are we probing a module.
 	 */
-	probed_mod = module_text_address((unsigned long) p->addr);
+	probed_mod = __module_text_address((unsigned long) p->addr);
 	if (probed_mod) {
-		struct module *calling_mod = module_text_address(called_from);
+		struct module *calling_mod;
+		calling_mod = __module_text_address(called_from);
 		/*
 		 * We must allow modules to probe themself and in this case
 		 * avoid incrementing the module refcount, so as to allow
 		 * unloading of self probing modules.
 		 */
 		if (calling_mod && calling_mod != probed_mod) {
-			if (unlikely(!try_module_get(probed_mod)))
+			if (unlikely(!try_module_get(probed_mod))) {
+				preempt_enable();
 				return -EINVAL;
+			}
 			p->mod_refcounted = 1;
 		} else
 			probed_mod = NULL;
 	}
+	preempt_enable();

 	p->nmissed = 0;
 	INIT_LIST_HEAD(&p->list);
@@ -718,6 +725,10 @@ static void __kprobes __unregister_kprob
 	struct kprobe *old_p;

 	if (p->mod_refcounted) {
+		/*
+		 * Since we've already incremented refcount,
+		 * we don't need to disable preemption.
+		 */
 		mod = module_text_address((unsigned long)p->addr);
 		if (mod)
 			module_put(mod);

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@redhat.com