Linux Virtualization - Nov 2012 - [PATCH] virtio: Don't access index after unregister.

Virtio wants to release used indices after the corresponding
virtio device has been unregistered. However, virtio does not
hold an extra reference, giving up its last reference with
device_unregister(), making accessing dev->index afterwards
invalid.

I actually saw problems when testing my (not-yet-merged)
virtio-ccw code:

- device_add virtio-net,id=xxx
-> creates device virtio<n> with n>0

- device_del xxx
-> deletes virtio<n>, but calls ida_simple_remove with an
   index of 0

- device_add virtio-net,id=xxx
-> tries to add virtio0, which is still in use...

So let's save the index we want to release before calling
device_unregister().

Signed-off-by: Cornelia Huck <cornelia.huck at de.ibm.com>
---
 drivers/virtio/virtio.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
index 1e8659c..809b0de 100644
--- a/drivers/virtio/virtio.c
+++ b/drivers/virtio/virtio.c
@@ -225,8 +225,10 @@ EXPORT_SYMBOL_GPL(register_virtio_device);
 
 void unregister_virtio_device(struct virtio_device *dev)
 {
+	int index = dev->index; /* save for after device release */
+
 	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);
 
-- 
1.7.12.4

On Thu, Nov 8, 2012 at 11:43 AM, Cornelia Huck <cornelia.huck at
de.ibm.com> wrote:
> Virtio wants to release used indices after the corresponding
> virtio device has been unregistered. However, virtio does not
> hold an extra reference, giving up its last reference with
> device_unregister(), making accessing dev->index afterwards
> invalid.
>
> I actually saw problems when testing my (not-yet-merged)
> virtio-ccw code:
>
> - device_add virtio-net,id=xxx
> -> creates device virtio<n> with n>0
>
> - device_del xxx
> -> deletes virtio<n>, but calls ida_simple_remove with an
>    index of 0
>
> - device_add virtio-net,id=xxx
> -> tries to add virtio0, which is still in use...
>
> So let's save the index we want to release before calling
> device_unregister().
>
> Signed-off-by: Cornelia Huck <cornelia.huck at de.ibm.com>
> ---
>  drivers/virtio/virtio.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> index 1e8659c..809b0de 100644
> --- a/drivers/virtio/virtio.c
> +++ b/drivers/virtio/virtio.c
> @@ -225,8 +225,10 @@ EXPORT_SYMBOL_GPL(register_virtio_device);
>
>  void unregister_virtio_device(struct virtio_device *dev)
>  {
> +       int index = dev->index; /* save for after device release */
> +
>         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);
Acked-by: Sjur Br?ndeland <sjur.brandeland at stericsson.com>

Great minds think alike! I discovered issues with this implementation
a while back and Michael suggested an identical patch:
https://lkml.org/lkml/2012/9/4/173
https://lkml.org/lkml/2012/9/7/105

The issue I ran into was that when virtio devices are created by remoteproc
the device memory might be freed when calling  device_unregister(), and
the value of dev->index is then undefined. So this bug bites when
unregistering a Virtio devices from remoteproc with CONFIG_DEBUG_SLAB
enabled. However this bug is not triggered by virtio_pci as it implements a
non-standard device release-function that does not free the device memory.

Thanks,
Sjur

Cornelia Huck <cornelia.huck at de.ibm.com>
writes:
> Virtio wants to release used indices after the corresponding
> virtio device has been unregistered. However, virtio does not
> hold an extra reference, giving up its last reference with
> device_unregister(), making accessing dev->index afterwards
> invalid.
>
> I actually saw problems when testing my (not-yet-merged)
> virtio-ccw code:
>
> - device_add virtio-net,id=xxx
> -> creates device virtio<n> with n>0
>
> - device_del xxx
> -> deletes virtio<n>, but calls ida_simple_remove with an
>    index of 0
>
> - device_add virtio-net,id=xxx
> -> tries to add virtio0, which is still in use...
>
> So let's save the index we want to release before calling
> device_unregister().
Great catch!  I've add a CC:stable.

Applied,
Rusty.

On Thu, Nov 08, 2012 at 11:43:47AM +0100, Cornelia Huck
wrote:
> Virtio wants to release used indices after the corresponding
> virtio device has been unregistered. However, virtio does not
> hold an extra reference, giving up its last reference with
> device_unregister(), making accessing dev->index afterwards
> invalid.
> 
> I actually saw problems when testing my (not-yet-merged)
> virtio-ccw code:
> 
> - device_add virtio-net,id=xxx
> -> creates device virtio<n> with n>0
> 
> - device_del xxx
> -> deletes virtio<n>, but calls ida_simple_remove with an
>    index of 0
> 
> - device_add virtio-net,id=xxx
> -> tries to add virtio0, which is still in use...
> 
> So let's save the index we want to release before calling
> device_unregister().
> 
> Signed-off-by: Cornelia Huck <cornelia.huck at de.ibm.com>
> ---
>  drivers/virtio/virtio.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> index 1e8659c..809b0de 100644
> --- a/drivers/virtio/virtio.c
> +++ b/drivers/virtio/virtio.c
> @@ -225,8 +225,10 @@ EXPORT_SYMBOL_GPL(register_virtio_device);
>  
>  void unregister_virtio_device(struct virtio_device *dev)
>  {
> +	int index = dev->index; /* save for after device release */
It's obvious from code that we safe for after release,
I think a better comment would explain *why* we do this.

Something like
	/*
	   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);
>  
> -- 
> 1.7.12.4