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Date:   Sat, 9 Dec 2017 14:38:44 -0800
From:   Alexander Kappner <>
To:     Mathias Nyman <>,
        Mathias Nyman <>,
        Greg Kroah-Hartman <>,,,
        Lu Baolu <>
Subject: Re: [PATCH] usb-core: Fix potential null pointer dereference in

Hi Mathias,

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

Tested-by: Alexander Kappner <>

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

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