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SubjectRe: [PATCH] kprobes: disable preempt for module_text_address()
Lai Jiangshan wrote:
> Masami Hiramatsu wrote:
>> Hi Lai,
>>
>> Lai Jiangshan wrote:
>>> __register_kprobe() may be preempted after module_text_address()
>>> but before try_module_get(), and in this interval the module may be
>>> unloaded and try_module_get(probed_mod) will access to invalid address.
>>> this patch uses preempt_disable() to protect it.
>> Thank you for your work.
>>
>> I think this is the problem of module_text_address() because it can
>> return incorrect address of struct module if a preemption happens.
>> So, I think the module_text_address() would better to call try_module_get()
>> before returning its address, or at least they should comment that caller
>> needs disabling preemption.
>>
>> struct module *module_text_address(unsigned long addr)
>> {
>> struct module *mod;
>>
>> preempt_disable(); /*
>> * I also think this preemption disabling is not so useful
>> * without try_module_get(), because caller have to
>> * disable preemption...
>> */
>> mod = __module_text_address(addr);
>> /* here, try_module_get() is needed.
>> * (or commenting "caller must disable preemption!") */
>> preempt_enable();
>>
>> /*
>> * !!Here!! if the preemption happened, it could return invalid "mod".
>> * In that case, even if module_text_address() returns non-NULL,
>> * the addr is no longer in any module.
>> */
>> return mod;
>> }
>>
>> Thank you,
>>
>
> I don't think module_text_address() are needed to modified.
> only __register_kprobe() uses module_text_address()'s return value.
> other users of module_text_address() are just for testing it's
> return value. so only __register_kprobe() needs disabling preemption
> currently.

I guess, there is another issue if the module is unloaded before
module_text_address(). p->addr is no longer valid, but in
__register_kprobe(), module_text_address() returns NULL means
it's not a module.

How about this?
--------
From: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Subject: [PATCH] kprobe: fix module checking in __register_kprobe()
__register_kprobe() may be preempted before/after module_text_address().
In this preemption, the module may be unloaded.

If the module is unloaded before module_text_address(), kprobe address
becomes invalid. If the module is unloaded after module_text_address()
and before try_module_get(), try_module_get(probe_mod) will access invalid
address.

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
---
kernel/kprobes.c | 19 +++++++++++++------
1 files changed, 13 insertions(+), 6 deletions(-)
diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index 8b57a25..71dc2e9 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -605,7 +605,7 @@ static int __kprobes __register_kprobe(struct kprobe *p,
{
int ret = 0;
struct kprobe *old_p;
- struct module *probed_mod;
+ struct module *probed_mod = NULL;
kprobe_opcode_t *addr;

addr = kprobe_addr(p);
@@ -622,20 +622,27 @@ static int __kprobes __register_kprobe(struct kprobe *p,
/*
* Check if are we probing a module.
*/
- probed_mod = module_text_address((unsigned long) p->addr);
- if (probed_mod) {
- struct module *calling_mod = module_text_address(called_from);
+ if (!core_kernel_text((unsigned long) p->addr)) {
+ preempt_disable();
+ probed_mod = __module_text_address((unsigned long) p->addr);
+ if (!probed_mod) {
+ preempt_enable();
+ return -EINVAL;
+ }
/*
* 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 (__module_text_address(called_from) != 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;
--
1.5.6


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