[PATCHv2] virtio: Don't access device data after unregistration.

[sjur . brandeland]

From: Sjur BrÃndeland <sjur.brandeland@xxxxxxxxxxxxxx>

Fix panic in virtio.c when CONFIG_DEBUG_SLAB is set.
device_unregister() drops reference to device so put_device()
could invoke release callback. In this case the release
callback will free the device. Make sure we don't access
device after unregister by fetching the device index
before calling unregister.

Signed-off-by: Sjur BrÃndeland <sjur.brandeland@xxxxxxxxxxxxxx>
cc: Guzman Lugo, Fernadndo <fernando.lugo@xxxxxx>
cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
cc: Ohad Ben-Cohen <ohad@xxxxxxxxxx>
cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
cc: linux-kernel@xxxxxxxxxxxxxxx
---
 drivers/virtio/virtio.c |    9 ++++++++-
 1 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
index c3b3f7f..faee112 100644
--- a/drivers/virtio/virtio.c
+++ b/drivers/virtio/virtio.c
@@ -225,8 +225,15 @@ EXPORT_SYMBOL_GPL(register_virtio_device);
 
 void unregister_virtio_device(struct virtio_device *dev)
 {
+	/*
+	 * device_unregister() drops reference to device so put_device could
+	 * invoke release callback. In case that callback will free the device,
+	 * make sure we don't access device after this call.
+	 */
+
+	int index = dev->index;
 	device_unregister(&dev->dev);
-	ida_simple_remove(&virtio_index_ida, dev->index);
+	ida_simple_remove(&virtio_index_ida, index);
 }
 EXPORT_SYMBOL_GPL(unregister_virtio_device);
 
-- 
1.7.5.4

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