linux-kernel - Re: [PATCH] usb: host: u132-hcd: Traverse u132_static_list under mutex lock in u132_hcd

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Date:   Thu, 2 Apr 2020 10:18:58 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>
cc:     gregkh@...uxfoundation.org, <hariprasad.kelam@...il.com>,
        <colin.king@...onical.com>, <linux-usb@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <ldv-project@...uxtesting.org>,
        <andrianov@...ras.ru>
Subject: Re: [PATCH] usb: host: u132-hcd: Traverse u132_static_list under
 mutex lock in u132_hcd_exit

On Thu, 2 Apr 2020 madhuparnabhowmik10@...il.com wrote:

> From: Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>
> 
> 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@...il.com>
> ---
>  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.

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);