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SubjectHIDDEV: potential NULL dereference

I've a question for you :). How could this patch help:

commit 7032269e87ade34cc12891675371fa2ac150a620
Author: Chris Ball <>
Date: Thu Aug 12 19:07:40 2010 -0400

HID: hiddev: protect against disconnect/NULL-dereference race

One of our users reports consistently hitting a NULL dereference that
resolves to the "hid_to_usb_dev(hid);" call in hiddev_ioctl(), when
disconnecting a Lego WeDo USB HID device from an OLPC XO running
Scratch software. There's a FIXME comment and a guard against the
dereference, but that happens farther down the function than the
initial dereference does.

This patch moves the call to be below the guard, and the user reports
that it fixes the problem for him. OLPC bug report:

Signed-off-by: Chris Ball <>
Signed-off-by: Jiri Kosina <>

when the code now looks like:
struct usb_device *dev; // here was the assignment before the patch
struct usbhid_device *usbhid = hid->driver_data;
if (!hiddev->exist || !hid)
return -EIO;
dev = hid_to_usb_dev(hid);

If hid was ever NULL at this phase, the check couldn't improve anything
due to hid->driver_data dereference being still before the check. So
again my question, how this could change anything?

Above that, it just makes the window shorter, but the bug is still
there, isn't it? Is the following scenario reasonable?

A (hiddev_ioctl) | B
if (!hid) |
return -EIO; |
| hid = NULL
| kfree(hid);
dev = hid_to_usb_dev(hid); |

Actually who's the process B you are trying to catch the race against by
the patch? hid-core?

suse labs

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