[PATCH] virtio: Don't access index after unregister.

[PATCH] virtio: Don't access index after unregister.

[Cornelia Huck]

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@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

Re: [PATCH] virtio: Don't access index after unregister.

[Sjur Brændeland]

On Thu, Nov 8, 2012 at 11:43 AM, Cornelia Huck <cornelia.huck@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@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 Brændeland <sjur.brandeland@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
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Re: [PATCH] virtio: Don't access index after unregister.

[Rusty Russell]

Cornelia Huck <cornelia.huck@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.

Re: [PATCH] virtio: Don't access index after unregister.

[Michael S. Tsirkin]

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@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