Re: [PATCH] USB: USBHID: Fix race between disconnect and hiddev_ioctl

[Valentine Barshak]

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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