Re: [PATCH v2 1/2] vfio: Prevent open_count decrement to negative

[Alex Williamson <alex.williamson@redhat.com>]

Hi Jacob,

On Tue,  3 Jun 2025 08:23:42 -0700
Jacob Pan <jacob.pan@linux.microsoft.com> wrote:

> When vfio_df_close() is called with open_count=0, it triggers a warning in
> vfio_assert_device_open() but still decrements open_count to -1. This
> allows a subsequent open to incorrectly pass the open_count == 0 check,
> leading to unintended behavior, such as setting df->access_granted = true.
> 
> For example, running an IOMMUFD compat no-IOMMU device with VFIO tests
> (https://github.com/awilliam/tests/blob/master/vfio-noiommu-pci-device-open.c)
> results in a warning and a failed VFIO_GROUP_GET_DEVICE_FD ioctl on the
> first run, but the second run succeeds incorrectly.
> 
> Add checks to avoid decrementing open_count below zero.

The example above suggests to me that this is a means by which we could
see this, but in reality it seems it is the only means by which we can
create this scenario, right?

Why does VFIO_GROUP_GET_DEVICE_FD fail on the first iteration?  It
seems like things are pretty broken, we won't have access_granted set,
but I don't spot why the ioctl fails.

Doesn't this also (begin) to fix 6086efe73498 as well?  I think that
introduced skipping vfio_df_open() entirely for noiommu devices.  It
seems like this should have a Fixes: tag and the warning in the
assertion was unreachable until 608... so maybe it should be linked
here.  Thanks,

Alex


> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> Reviewed-by: Yi Liu <yi.l.liu@intel.com>
> Signed-off-by: Jacob Pan <jacob.pan@linux.microsoft.com>
> ---
> v2: Added Reviewed-by tags
> ---
>  drivers/vfio/vfio_main.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> index 1fd261efc582..5046cae05222 100644
> --- a/drivers/vfio/vfio_main.c
> +++ b/drivers/vfio/vfio_main.c
> @@ -583,7 +583,8 @@ void vfio_df_close(struct vfio_device_file *df)
>  
>  	lockdep_assert_held(&device->dev_set->lock);
>  
> -	vfio_assert_device_open(device);
> +	if (!vfio_assert_device_open(device))
> +		return;
>  	if (device->open_count == 1)
>  		vfio_df_device_last_close(df);
>  	device->open_count--;