linux-kernel - RE: RE: Re: Re: Re: Subject: [PATCH v2] USB:Core: BugFix: Proper handling of Race Condition when two USB class drivers try to call init_usb

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Date:   Thu, 16 Feb 2017 09:52:22 -0500 (EST)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Ajay Kaher <ajay.kaher@...sung.com>
cc:     "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "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: Re: Subject: [PATCH v2] USB:Core: BugFix: Proper
 handling of Race Condition when two USB class drivers try to call init_usb_class
 simultaneously

On Thu, 16 Feb 2017, Ajay Kaher wrote:

> > On Thu, 14 Feb 2017, Alan Stern wrote:
> > 
> > I think Ajay's argument is correct and a patch is needed.  But this
> > patch misses the race between init_usb_class() and release_usb_class().  
> 
> Thanks Alan for your comments, in patch v2 I have taken care for
> release_usb_class() also. Please review again.
> 
> > The basic problem is that reference counting doesn't work when you try
> > to use the same global pointer (usb_class) to refer to multiple
> > generations of a dynamically allocated entity.  We had the same sort of
> > problem many years ago with the usb_interface structure (and we
> > ultimately fixed it by creating a separate usb_interface_cache
> > structure).
> >  
> > The best approach here would be to forget about all the reference
> > counting.  Get rid of usb_class entirely, and create the "usbmisc"
> > class structure just once, when usbcore initializes.  Or, if you
> > prefer, use a mutex to protect a routine that allocates the class
> > structure dynamically, just once.  Either way, don't deallocate it
> > until usbcore is unloaded.
> 
> usbmisc class creation should not require everytime when USB core
> initializes. So better to keep usbmisc class creation as it is. 
> And to prevent the race conditions just protect it with Mutex locking
> as per patch v2.
> 
> thanks,
> ajay kaher
> 
> Signed-off-by: Ajay Kaher
> 
> ---
> 
>  drivers/usb/core/file.c |    6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/usb/core/file.c b/drivers/usb/core/file.c
> index 822ced9..56a151b 100644
> --- a/drivers/usb/core/file.c
> +++ b/drivers/usb/core/file.c
> @@ -27,6 +27,7 @@
>  #define MAX_USB_MINORS 256
>  static const struct file_operations *usb_minors[MAX_USB_MINORS];
>  static DECLARE_RWSEM(minor_rwsem);
> +static DEFINE_MUTEX(init_usb_class_mutex);
> 
>  static int usb_open(struct inode *inode, struct file *file)
>  {
> @@ -102,9 +103,11 @@ static int init_usb_class(void)
>  static void release_usb_class(struct kref *kref)
>  {
>         /* Ok, we cheat as we know we only have one usb_class */
> +       mutex_lock(&init_usb_class_mutex);
>         class_destroy(usb_class->class);
>         kfree(usb_class);
>         usb_class = NULL;
> +       mutex_unlock(&init_usb_class_mutex);
>  }
> 
>  static void destroy_usb_class(void)
> @@ -171,7 +174,10 @@ 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;

This is not right.  What happens if usb_register_dev() runs just before
release_usb_class() calls mutex_lock()?

Alan Stern