Re: [PATCH] usb: usbtest: fix NULL pointer dereference in usbtest_probe

Hi Greg,

Thanks for the review!


On Mon, Aug 3, 2026 at 5:15 PM Greg Kroah-Hartman
<[email protected]> wrote:
>
> On Mon, Aug 03, 2026 at 05:13:00PM +0200, Greg Kroah-Hartman wrote:
> > On Wed, Jul 29, 2026 at 09:19:54AM +0000, syzbot wrote:
> > > From: Aleksandr Nogikh <[email protected]>
> > >
> > > The usbtest driver relies on the driver_info field of the usb_device_id
> > > structure to hold a pointer to a struct usbtest_info, which contains
> > > necessary configuration for the device. When a new device ID is dynamically
> > > added via the sysfs new_id interface without specifying a reference device,
> > > the driver_info field defaults to 0. When the USB core binds the device
> > > using this dynamic ID, usbtest_probe() unconditionally casts the 0 to a
> > > pointer, resulting in a NULL pointer dereference when attempting to access
> > > the autoconf bitfield (at offset 0xa) of the struct usbtest_info.
> > >
> > > This leads to the following crash:
> > >
> > > Oops: general protection fault, probably for non-canonical address
> > > 0xdffffc0000000001: 0000 [#1] SMP KASAN NOPTI
> > > KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f]
> > > RIP: 0010:usbtest_probe+0x522/0xcc0 drivers/usb/misc/usbtest.c:2822
> > > Call Trace:
> > >  <TASK>
> > >  usb_probe_interface+0x653/0xc60 drivers/usb/core/driver.c:396
> > >  call_driver_probe drivers/base/dd.c:-1 [inline]
> > >  really_probe+0x254/0xae0 drivers/base/dd.c:706
> > >  __driver_probe_device+0x1e8/0x360 drivers/base/dd.c:868
> > >  driver_probe_device+0x4f/0x240 drivers/base/dd.c:898
> > >  __device_attach_driver+0x270/0x410 drivers/base/dd.c:1026
> > >  bus_for_each_drv+0x258/0x2f0 drivers/base/bus.c:500
> > >  __device_attach+0x2c4/0x450 drivers/base/dd.c:1098
> > >  device_initial_probe+0xa1/0xd0 drivers/base/dd.c:1153
> > >  bus_probe_device+0x12a/0x220 drivers/base/bus.c:620
> > >  device_add+0x7d7/0xb80 drivers/base/core.c:3772
> > >  usb_set_configuration+0x1ad8/0x2180 drivers/usb/core/message.c:2268
> > >  usb_generic_driver_probe+0x8d/0x150 drivers/usb/core/generic.c:250
> > >  usb_probe_device+0x1c3/0x3b0 drivers/usb/core/driver.c:291
> > >
> > > To fix this, validate that id->driver_info is not NULL before proceeding
> > > with the probe. If it is NULL, gracefully return -ENODEV. The check is
> > > placed before any memory allocation to avoid unnecessary work and simplify
> > > error handling.
> > >
> > > Fixes: 733260ff9c45 ("[PATCH] USB: add dynamic id functionality to USB core")
>
> Not really, but I can see why the LLM thinks this.  It's "just how this
> driver works" instead, right?
>
> > > Assisted-by: Gemini:gemini-3.5-flash Gemini:gemini-3.1-pro-preview syzbot
> > > Reported-by: [email protected]
> > > Closes: https://syzkaller.appspot.com/bug?extid=7e1e5911f9eac50bedc7
> > > Link: https://syzkaller.appspot.com/ai_job?id=9e476ca5-99d1-47d2-b9d1-f3b501897179
> > > Signed-off-by: Aleksandr Nogikh <[email protected]>
> > >
> > > ---
> > > diff --git a/drivers/usb/misc/usbtest.c b/drivers/usb/misc/usbtest.c
> > > index 98071b25a..6410b3e26 100644
> > > --- a/drivers/usb/misc/usbtest.c
> > > +++ b/drivers/usb/misc/usbtest.c
> > > @@ -2786,10 +2786,13 @@ usbtest_probe(struct usb_interface *intf, const struct usb_device_id *id)
> > >     }
> > >  #endif
> > >
> > > +   info = (struct usbtest_info *) id->driver_info;
> >
> > Extra space there, right?  Didn't checkpatch catch that?

The checkpatch was happy with the diff:
total: 0 errors, 0 warnings, 14 lines checked

You mean the whitespace after `*)`?
I've just looked at the other code in `drivers/usb/misc/usbtest.c`,
and it's inconsistent: in some cases there's a whiltespace, in some
isn't.

> >
> > And this really isn't the proper solution, just don't allow dynamic
> > binding for this driver.  That should resolve it, right?

I'll double-check that and send a v2, thanks for the suggestion.

>
> If you don't want to do that, you need a BIG comment here as to what you
> are trying to prevent, as a NULL check just looks like any other normal
> check, while you are trying to prevent a specific use case from being
> possible, and it WILL have user-visable affect.
>
> thanks,
>
> greg k-h

-- 
Aleksandr