linux-kernel - Re: Possible data-race related bug in u132

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Date:   Mon, 30 Mar 2020 14:02:07 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     madhuparnabhowmik10@...il.com
Cc:     hariprasad.kelam@...il.com, colin.king@...onical.com,
        tony.olech@...ndigitalsystems.com, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, andrianov@...ras.ru
Subject: Re: Possible data-race related bug in u132_hcd module.

On Mon, Mar 30, 2020 at 05:22:43PM +0530, madhuparnabhowmik10@...il.com wrote:
> Hi,
> 
> This bug is found by  Linux Driver Verification project (linuxtesting.org).
> 
> The bug is related to the parallel execution of u132_probe() function and u132_hcd_exit() function in u132_hcd.c. In case the module is unloaded when the probe function is executing there can be data race as the mutex lock u132_module_lock is not used properly. 

Please note that module unloading, while a nice thing to have, is never
something that happens automatically :)

> i) Usage of mutex lock only when writing into the u132_exiting variable in u132_hcd_exit(). The lock is not used when this variable is read in u132_probe().
> 
> Moreover, this variable does not serve its purpose, as even if locking is used while the u132_exiting variable is read in probe(), the function may still miss that exit function is executing if it acquires the mutex before exit() function does.
> 
> How to fix this?
> 
> ii) Usage of mutex while adding entries in u132_static_list in probe function but not in exit function while unregistering.
> This should be easy to fix by holding the mutex in the exit function as well.
> 
> There can be other synchronization problems related to the usage of u132_module_lock in this module, I have only spotted these so far.
> 
> Please let me know if this bug report is helpful and I can send a patch fixing it.

Please just send a patch, no need to ever ask if you should, that's the
best way to report and fix anything.

thanks,

greg k-h