ACPI / hotplug / PCI: Fix bridge removal race vs dock events

If a PCI bridge with an ACPIPHP context attached is removed via
sysfs, the code path executed as a result is the following:

pci_stop_and_remove_bus_device_locked
 pci_remove_bus
  pcibios_remove_bus
   acpi_pci_remove_bus
    acpiphp_remove_slots
     cleanup_bridge
      unregister_hotplug_dock_device (drops dock references to the bridge)
     put_bridge
      free_bridge
       acpiphp_put_context (for each child, under context lock)
        kfree (context)

Now, if a dock event affecting one of the bridge's child devices
occurs (roughly at the same time), it will lead to the following code
path:

acpi_dock_deferred_cb
 dock_notify
  handle_eject_request
   hot_remove_dock_devices
    dock_hotplug_event
     hotplug_event (dereferences context)

That may lead to a kernel crash in hotplug_event() if it is executed
after the last kfree() in the bridge removal code path.

To prevent that from happening, add a wrapper around hotplug_event()
called dock_event() and point the .handler pointer in acpiphp_dock_ops
to it.  Make that wrapper retrieve the device's ACPIPHP context using
acpiphp_get_context() (instead of taking it from the data argument)
under acpiphp_context_lock and check if the parent bridge's
is_going_away flag is set.  If that flag is set, it will return
immediately and if it is not set it will grab a reference to the
device's parent bridge before executing hotplug_event().

Then, in the above scenario, the reference to the parent bridge
held by dock_event() will prevent free_bridge() from being executed
for it until hotplug_event() returns.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/pci/hotplug/acpiphp_glue.c

@@ -210,10 +210,29 @@ static void post_dock_fixups(acpi_handle not_used, u32 event, void *data)
 	}
 }
 
+static void dock_event(acpi_handle handle, u32 type, void *data)
+{
+	struct acpiphp_context *context;
+
+	mutex_lock(&acpiphp_context_lock);
+	context = acpiphp_get_context(handle);
+	if (!context || WARN_ON(context->handle != handle)
+	    || context->func.parent->is_going_away) {
+		mutex_unlock(&acpiphp_context_lock);
+		return;
+	}
+	get_bridge(context->func.parent);
+	acpiphp_put_context(context);
+	mutex_unlock(&acpiphp_context_lock);
+
+	hotplug_event(handle, type, data);
+
+	put_bridge(context->func.parent);
+}
 
 static const struct acpi_dock_ops acpiphp_dock_ops = {
 	.fixup = post_dock_fixups,
-	.handler = hotplug_event,
+	.handler = dock_event,
 };
 
 /* Check whether the PCI device is managed by native PCIe hotplug driver */