On Tue, 9 Sep 2014, Joe Lawrence wrote:
> Hello linux-usb,
>
> I've been testing surprise device hotlug removal with RHEL7 on Stratus
> hardware (fully redundant PCI branches) and ran into a crashing NULL-ptr
> bug during device initialization. The code looks the same upstream, so
> I'm reporting it here.
...
> Possible order of events and commentary:
>
> *** USB host controller inserted ***
>
> [ khubd ] [ hotplug driver ]
> ...
> hub_probe
> hub_configure
>
> /* setup maxchild */
>
> hdev->maxchild =
> hub->descriptor->bNbrPorts;
>
> usb_get_status
> ...
> usb_start_wait_urb
> zzzzzz...
>
> *** USB host controller is removed ***
>
> [ khubd ] [ hotplug driver ]
>
> pci_stop_bus_device
> ...
> usb_hcd_pci_remove
> usb_hcd_irq
> ehci_irq
> usb_hc_died
> usb_set_device_state
> recursively_mark_NOTATTACHED
>
> /* iterate over all
> maxchild */
> /* assume ports[x] are
> allocated */
>
> for (i = 0; i <
> udev->maxchild; ++i) {
> if
> (hub->ports[i]->child)
>
> *** crash on hub->ports[i]-> dereference ***
>
>
> recursively_mark_NOTATTACHED(hub->ports[i]->child);
> }
>
>
> [ khubd ]
>
> /* port_dev would have been
> allocated here */
>
> usb_hub_create_port_device
> port_dev = kzalloc(...)
> hub->ports[port1 - 1] = port_dev
>
>
> In summary, khubd has initialized the usb_device maxchild to 8 and
> provided backing-store for the usb_hub ports[] array. However, before
> it gets to fill in pointers for each port[] entry, the device is removed
> and the hotplug driver issues a pci_stop_bus_device. This removal
> percolates all the way down to ehci_irq and usb_hc_died. When that goes
> to mark the device as not attached, it expects that the ports[] pointers
> are valid... kaboom.
>
> I've been testing the following patch with the RHEL7 kernel with no
> crashes. I'm not sure if it completely solves the issue at hand, or
> simply covers up the crash. Comments welcome.
This has been fixed in the mainline kernel by commit d8521afe3586,
which is part of a large series involving port-power control for USB-3
ports. You may want to apply just the portion that is relevant to this
problem, namely, have hub_configure store the maxchild value in a
local variable and don't assign it to hdev->maxchild until the port
devices have safely been created.
> Regards,
>
> -- Joe
>
> -->8-- -->8--
>
> From abb49e02e2f56ed1528198dfe242a9dd3041dc79 Mon Sep 17 00:00:00 2001
> From: Joe Lawrence <[email protected]>
> Date: Fri, 5 Sep 2014 15:02:29 -0400
> Subject: [PATCH] usb: hub: protect recursively_mark_NOTATTACHED and
> half-initialized device
>
> The recursively_mark_NOTATTACHED assumes that hub->ports[] contains valid
> pointers. If a device was removed after udev->maxchild was set, but before
> hub->ports[] was populated, recursively_mark_NOTATTACHED may try to reach
> through an uninitialized pointer. Since the backing memory was kzalloc'd,
> verify that the pointer is not-NULL before dereferencing.
No, this is not a good fix. The assumption in
recursively_mark_NOTATTACHED is valid, and the problem was in the code
that violated the assumption.
Alan Stern
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