Re: [PATCH v2] vfio: Fix null pointer dereference bug in vfio_bars_finalize()

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

On Tue, 4 Jul 2023 16:39:27 +0300
Avihai Horon <avihaih@nvidia.com> wrote:

> vfio_realize() has the following flow:
> 1. vfio_bars_prepare() -- sets VFIOBAR->size.
> 2. msix_early_setup().
> 3. vfio_bars_register() -- allocates VFIOBAR->mr.
> 
> After vfio_bars_prepare() is called msix_early_setup() can fail. If it
> does fail, vfio_bars_register() is never called and VFIOBAR->mr is not
> allocated.
> 
> In this case, vfio_bars_finalize() is called as part of the error flow
> to free the bars' resources. However, vfio_bars_finalize() calls
> object_unparent() for VFIOBAR->mr after checking only VFIOBAR->size, and
> thus we get a null pointer dereference.
> 
> Fix it by checking VFIOBAR->mr in vfio_bars_finalize().
> 
> Fixes: 89d5202edc50 ("vfio/pci: Allow relocating MSI-X MMIO")
> Signed-off-by: Avihai Horon <avihaih@nvidia.com>
> ---
> 
>  Changes from v1:
>  * Assert VFIOBAR->size and set VFIOBAR->mr to NULL to make the code
>    more accurate. (Philippe)
>  * Small reword in the last paragraph of the commit message.
> 
>  hw/vfio/pci.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> index ab6645ba60..bc98791cbb 100644
> --- a/hw/vfio/pci.c
> +++ b/hw/vfio/pci.c
> @@ -1752,9 +1752,11 @@ static void vfio_bars_finalize(VFIOPCIDevice *vdev)
>  
>          vfio_bar_quirk_finalize(vdev, i);
>          vfio_region_finalize(&bar->region);
> -        if (bar->size) {
> +        if (bar->mr) {
> +            assert(bar->size);
>              object_unparent(OBJECT(bar->mr));
>              g_free(bar->mr);
> +            bar->mr = NULL;
>          }
>      }
>  

Reviewed-by: Alex Williamson <alex.williamson@redhat.com>