答复: [PATCH] USB:Fix kernel NULL pointer when unbind UHCI form vfio-pci

[Weitao Wang(BJ-RD)]

On Wed, Jul 22, 2020 at 02:44:14PM +0200, Greg KH wrote:
> On Wed, Jul 22, 2020 at 07:57:48PM +0800, WeitaoWangoc wrote:
> > This bug is found in Zhaoxin platform, but it's a commom code bug.
> > Fail sequence:
> > step1: Unbind UHCI controller from native driver;
> > step2: Bind UHCI controller to vfio-pci, which will put UHCI controller in one
> vfio
> >        group's device list and set UHCI's dev->driver_data to struct
> vfio-pci(for UHCI)
>
> Hah, that works?  How do you do that properly?  What code does that?

Drivers/vfio/vfio.c
The function vfio_group_create_device will set UHCI dev->driver_data
to vfio-device struct.

> > step3: Unbind EHCI controller from native driver, will try to tell UHCI native
> driver
> >        that "I'm removed by set companion_hcd->self.hs_companion to
> NULL. However,
> >        companion_hcd get from UHCI's dev->driver_data that has modified
> by vfio-pci
> >        already.So, the vfio-pci structure will be damaged!
>
> Because you are messing around with bind/unbind "by hand", which is
> never guaranteed to work properly.
>
> It's amazing any of that works at all...

If adjust the sequence of UHCI/EHCI unbind form native driver, that
can avoid Null Pointer dereference. Eg. Step3-->stet4-->step1-->step2.
Our experiments prove that this sequence is indeed good as expected.
However, some application software such as virt-manager/qemu assign
/EHCI to guest OS has the same bind/unbind sequence as test “by hand”.

> 4.19.65-2020051917-rainos #1
>
> 4.19 is a bit old, what about 5.7?

5.7.7 has the same issue.

> > +#define PCI_DEV_DRV_FLAG       2
>
> Why is the USB core allowed to touch a private PCI structure field?
>
> I do not think this is allowed.
>
> And why is this a USB host controller-specific issue, shouldn't this
> type of thing be fixed in the PCI core?

When ehci hcd_driver bind or unbind to ehci, it will touch/modify UHCI usb_hcd
that get from UHCI's dev->driver_data. However, UHCI maybe bind to a
another driver(vfio-pci) and it's driver_data will be rewritten. what we
should do is to prevent ehci_hcd to modify UHCI's dev->driver_data when UHCI
has already bound to vfio-pci.

Thanks
weitaowang



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