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Date: Fri, 3 Dec 2010 15:16:12 -0800
From: Dmitry Torokhov <dmitry.torokhov@gmail.com>
To: Valentine Barshak <vbarshak@mvista.com>
Cc: Jiri Kosina <jkosina@suse.cz>, linux-usb@vger.kernel.org,
        linux-input@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] USB: USBHID: Fix race between disconnect and hiddev_ioctl
Message-ID: <20101203231611.GB22969@core.coreip.homeip.net>
References: <20101203172746.GA31045@mvista.com>
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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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