Re: [PATCH] usb-core: Fix potential null pointer dereference in xhci-debugfs.c

[Alexander Kappner]

Hi Mathias,

thanks for the patch! The system now resumes cleanly from hibernate even 
with usbmuxd doing its thing.

Tested-by: Alexander Kappner <agk@xxxxxxxxxxx>

While testing this I hit some other issues with xhci-debugfs.c but I'll 
write these up in a separate bug.

On 12/08/2017 09:01 AM, Mathias Nyman wrote:
> On 08.12.2017 13:06, Alexander Kappner wrote:
> > Hi,
> >
> > > I think we need to dig a bit deeper. It's good to check if spriv is
> > > valid
> > > but there are probably other reasons than kzalloc failing.
> >
> > I agree -- this small allocation is  unlikely to fail in practice.
> > Also, while my patch prevents the kernel oops, it also prevents the
> > debugfs entries from being created.
> >
> > I've been debugging this more trying to come up with a better
> > solution, but I might need some guidance as I'm not too familiar with
> > the USB subsystem. The immediate cause of the crash was usbmuxd
> > sending a USBDEVFS_SETCONFIGURATION ioctl to a device, which _only if
> > it fails_ calls usb_hcd_alloc_bandwidth to try and reset the device,
> > which in turn calls xhci_debugfs_create_endpoint. The ioctl handler
> > acquires a device-specific lock via usb_lock_device.
> >
> > When the system resumed from hibernate, xhci_resume was called. This
> > in turn called xhci_mem_cleanup to deallocate the device structures,
> > which include setting the debugfs_private pointer to NULL  (via
> > xhci_free_virt_devices_depth_first). It thus seems likely that the
> > ioctl is somehow racing with the hibernate. The call to xhci_resume
> > is protected by a host-controller specific lock (xhci->lock) but it
> > doesn't attempt to take the usb_lock_device device-specific lock.
> >
> > Now my suspicion is that xhci_resume freed and zeroed the device
> > structures while racing with the ioctl handler. I can't seem to find
> > any exclusion mechanism that would prevent xhci_resume from racing
> > with the USBDEVFS_SETCONFIGURATION ioctl (or any other ioctl, for
> > that matter). Am I missing something? If not, is there any reason why
> > an ioctl might need to execute in parallel with the xhci_resume? If
> > not, can we just do a busy wait in xhci_resume until all pending
> > ioctls have returned?
>
> I'm not sure, but if I recall correctly then power management is supposed
> to make sure a driver doesn't access usb devices while the host 
> controller
> is still resuming.
>
> The odd thing here is that
> xhci_debugfs_remove_slot(xhci, slot_id), and
> xhci_free_virt_device(xhci, slot_id) are called together when
> xhci_mem_cleanup() calls xhci_free_virt_devices_depth_first()
>
> That means both the xhci_virt_device *dev and dev->debugfs_private
> should both be freed and xhci->devs[slot_id] set NULL for that 
> virt_device.
>
> so xhci_add_endpoint() should fail a lot earlier because the 
> xhci->devs[slot_id]
> should be a null pointer as well.
>
> Allocation is also done together in xhci_alloc_dev()
>
> Looking at it more closely there is actually the .free_dev callback that
> first frees the dev->debugs_private but the virt_dev is only freed
> conditionally later
>
> Attached a patch that frees them together, can you try it out?
>
> If it doesn't help we need to add some elaborate tracing
>
> Thanks
> -Mathias