Re: [PATCH] usb: host: u132-hcd: Traverse u132_static_list under mutex lock in u132_hcd_exit

[Madhuparna Bhowmik]

On Thu, Apr 02, 2020 at 10:18:58AM -0400, Alan Stern wrote:
> On Thu, 2 Apr 2020 madhuparnabhowmik10@xxxxxxxxx wrote:
> 
> > From: Madhuparna Bhowmik <madhuparnabhowmik10@xxxxxxxxx>
> > 
> > The global list u132_static_list is protected by u132_module_lock.
> > Elements are added to this list in the probe function and this list is
> > traversed in u132_hcd_exit() to unregister devices.
> > 
> > If probe and exit execute simultaneously there can be a race condition
> > between writing to this list in probe and reading the list in exit as
> > u132_module_lock is not held in exit function.
> > 
> > Even though u132_exiting variable is used in probe to detect if the module is
> > exiting, it is ineffective as the probe function may read the value
> > before it is updated in exit and thus leading to a race condition.
> > 
> > Therefore, hold u132_module_lock while traversing u132_static_list in
> > exit function.
> > 
> > Found by Linux Driver Verification project (linuxtesting.org).
> > 
> > Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@xxxxxxxxx>
> > ---
> >  drivers/usb/host/u132-hcd.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/usb/host/u132-hcd.c b/drivers/usb/host/u132-hcd.c
> > index e9209e3e6248..1cadc4e0c9b2 100644
> > --- a/drivers/usb/host/u132-hcd.c
> > +++ b/drivers/usb/host/u132-hcd.c
> > @@ -3217,10 +3217,10 @@ static void __exit u132_hcd_exit(void)
> >  	struct u132 *temp;
> >  	mutex_lock(&u132_module_lock);
> >  	u132_exiting += 1;
> > -	mutex_unlock(&u132_module_lock);
> >  	list_for_each_entry_safe(u132, temp, &u132_static_list, u132_list) {
> >  		platform_device_unregister(u132->platform_dev);
> >  	}
> > +	mutex_unlock(&u132_module_lock);
> 
> How about just getting rid of this loop entirely, along with the 
> u132_static_list?  As far as I can see, that list doesn't do anything.
>
Yes, that makes sense. I will send an updated patch soon.

Thank you,
Madhuparna

> Not to mention that this driver has no business calling 
> platform_device_unregister() here, since it didn't call 
> platform_device_register() in the first place.  The call to 
> platform_driver_unregister() below should do all the necessary work.
> 
> Alan Stern
> 
> >  	platform_driver_unregister(&u132_platform_driver);
> >  	printk(KERN_INFO "u132-hcd driver deregistered\n");
> >  	wait_event(u132_hcd_wait, u132_instances == 0);
>