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

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, May 16, 2025 at 09:45:22AM -0700, Jacob Pan 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
> 
> Signed-off-by: Jacob Pan <jacob.pan@linux.microsoft.com>
> ---
>  drivers/vfio/vfio_main.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Jason