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



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