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

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Date:	Fri, 3 Dec 2010 15:16:12 -0800
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Valentine Barshak <vbarshak@...sta.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

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 ;)

-- 
Dmitry
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