linux-kernel - Re: [PATCH] USB: USBHID: Fix race between disconnect and hiddev

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Message-ID: <4CF97DEF.7020403@mvista.com>
Date:	Sat, 04 Dec 2010 02:31:59 +0300
From:	Valentine Barshak <vbarshak@...sta.com>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
CC:	Jiri Kosina <jkosina@...e.cz>, linux-usb@...r.kernel.org,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] USB: USBHID: Fix race between disconnect and hiddev_ioctl

Dmitry Torokhov wrote:
> On Fri, Dec 03, 2010 at 08:27:46PM +0300, Valentine Barshak wrote:
>   
>> A USB HID device can be disconnected at any time.
>> If this happens right before or while hiddev_ioctl is in progress,
>> the hiddev_ioctl tries to access invalid hiddev->hid pointer.
>> When the hid device is disconnected, the hiddev_disconnect()
>> ends up with a call to hid_device_release() which frees
>> hid_device, but doesn't set the hiddev->hid pointer to NULL.
>> If the deallocated memory region has been re-used by the kernel,
>> this can cause a crash or memory corruption.
>>
>> Since disconnect can happen at any time, we can't initialize
>> struct hid_device *hid = hiddev->hid at the beginning of ioctl
>> and then use it.
>>
>> This change checks hiddev->exist flag while holding
>> the existancelock and uses hid_device only if it exists.
>>     
>
> Why didn't you take the lock and check hiddev->exist at the beginning of
> ioctl handler instead of pushing it down into individual command
> handlers? I guess it would slow down HIDIOCGVERSION but I think we could
> pay this price for code that is more clear ;)
>
>   
Well, some of the commands were already using the lock, while a couple 
of them doesn't seem to need it.
I've just added locking to the other commands that needed it. I guess I 
didn't want to rework the whole stuff
in order not to forget to unlock and return.
But I agree, the code would look a bit cleaner though if did as you say.

Thanks,
Val.

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