linux-kernel - RE: Re: Re: Re: Subject: [PATCH v1] USB:Core: BugFix: Proper handling of Race Condition when two USB class drivers try to call init_usb

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Date:   Thu, 02 Feb 2017 12:36:40 +0000
From:   Ajay Kaher <ajay.kaher@...sung.com>
To:     "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
Cc:     "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        AMAN DEEP <aman.deep@...sung.com>,
        HEMANSHU SRIVASTAVA <hemanshu.s@...sung.com>
Subject: RE: Re: Re: Re: Subject: [PATCH v1] USB:Core: BugFix: Proper handling
 of Race Condition when two USB class drivers try to call init_usb_class
 simultaneously


>>>> At boot time, probe function of multiple connected devices
>>>> (proprietary devices) execute simultaneously.
>>>
>>> What exactly do you mean here?  How can probe happen "simultaneously"?
>>> The USB core prevents this, right?
>> 
>> I have observed two scenarios to call probe function:
>> 
>> Scenario #1: Driver inserted and attaching USB Device:
>> Yes, you are right, two probes at same time is not happening
>> in this scenario.
>> 
>> Scenario #2: USB Device attached and inserting Driver:
>> In this case probe has been called in context of insmod,
>> refer following code flow:
>> init -> usb_register_driver -> driver_register -> bus_add_driver ->
>> driver_attach -> bus_for_each_dev -> __driver_attach ->
>> driver_probe_device -> usb_probe_interface -> probe -> usb_register_dev
>> 
>> I have observed the crash in Scenario #2, as two probes executes at
>> same time in this scenario. And init_usb_class_mutex lock require to
>> prevent race condition.
> 
> What about the fact that in __driver_attach() we call device_lock() so
> that probe never gets called at the same time for the same device?

Devices are different.

> Or are you saying that you can load multiple USB modules at the same
> time?  If so, how is insmod running on multiple cpus at the same time?
> I thought we had a global lock there to prevent that from happening
> (i.e. only one module can be loaded at a time.)  Or is that what has
> recently changed?

Yes, we can load multiple USB modules at the same time using insmod.
Tested on ARM Architecture with Linux kernel 4.1.10, that we can have 
multiple insmod on multiple cpus at same time. Also reviewed load_module and 
do_init_module functions and couldn't find any global lock.

> 
> What is causing your modules to be loaded from userspace?  What type of
> device is this happening for?  And why haven't we seen this before?
> What kernel versions have you had a problem with this?

Earlier we execute insmod in foreground as "insmod aaa.ko ; insmod bbb.ko"
and that's why insmod executed sequentially. Now we modified to execute in 
parallel/background as "insmod aaa.ko & ; insmod bbb.ko &". 

> And what for what drivers specifically?

This problem observed for drivers in which usb_register_dev called from their
probe function, and there are many such drivers.

As per my opinion, usb_class structure is global and allocated + initialized
in usb_register_dev->init_usb_class function. Also as per scenario #2
concurrency is possible, so protection using init_usb_class_mutex lock requires.
Don't you think so?

>>>> And because of the following code path race condition happens:
>>>> probe->usb_register_dev->init_usb_class
>>>
>>> Why is this just showing up now, and hasn't been an issue for the decade
>>> or so this code has been around?  What changed?
>>>
>>>> Tested with these changes, and problem has been solved.
>>>
>>> What changes?
>> 
>> Tested with my patch (i.e. locking with init_usb_class_mutex).
> 
> I don't see a patch here :(

Sorry, appending the patch again with this mail.
 
thanks,
 
ajay kaher


Signed-off-by: Ajay Kaher

---

 drivers/usb/core/file.c |    4 ++++
 1 file changed, 4 insertions(+)


diff --git a/drivers/usb/core/file.c b/drivers/usb/core/file.c
index 822ced9..dedc47c 100644
--- a/drivers/usb/core/file.c
+++ b/drivers/usb/core/file.c
@@ -156,6 +156,7 @@ int usb_register_dev(struct usb_interface *intf,
  int minor_base = class_driver->minor_base;
  int minor;
  char name[20];
+ static DEFINE_MUTEX(init_usb_class_mutex);
 
 #ifdef CONFIG_USB_DYNAMIC_MINORS
  /*
@@ -171,7 +172,10 @@ int usb_register_dev(struct usb_interface *intf,
  if (intf->minor >= 0)
   return -EADDRINUSE;
 
+ mutex_lock(&init_usb_class_mutex);
  retval = init_usb_class();
+ mutex_unlock(&init_usb_class_mutex);
+
  if (retval)
   return retval;