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

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Date:	Mon, 06 Dec 2010 17:03:03 +0300
From:	Valentine Barshak <vbarshak@...sta.com>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
CC:	Oliver Neukum <oliver@...kum.org>, 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 Sat, Dec 04, 2010 at 09:22:23PM +0100, Oliver Neukum wrote:
>   
>> Am Samstag, 4. Dezember 2010, 00:16:12 schrieb Dmitry Torokhov:
>>     
>>>> 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 ;)
>>>       
>> Strictly speaking you'd change the semantics. Right now you can execute
>> the ioctl even if you know you are holding an fd to a disconnected device
>> open.
>>     
>
> No, I do not think I would. I do not believe that the availability for
> HIDIOCGVERSION on disconnected device is spelled out in API/ABI spec. We
> only know that ioctl will either succeed or appropriate error code is
> returned. The fact that right now HIDIOCGVERSION is available on
> disconnected devices is just an implementation detail subject to change.
>
>   
It's not just HIDIOCGVERSION. A couple of other commands 
(HIDIOCGFLAG/HIDIOCSFLAG) didn't check device existence in the first 
place either.
Current implementation depends on when the device is actually removed.
If it has been removed before the hiddev_ioctl(), hiddev_ioctl() returns 
-EIO.
If the device is removed while hiddev_ioctl() is in progress, we either 
do not notice that and handle HIDIOCGVERSION and HIDIOCGFLAG/HIDIOCSFLAG
just fine, or return -ENODEV.
I'll submit a patch in a bit that applies on top of the "[PATCH] USB: 
USBHID: Fix race between disconnect and hiddev_ioctl" and makes the 
hiddev_ioctl() check
device existence before processing the command and always return -ENODEV 
in case the device has been removed.
Thanks,
Val.

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