vfio: use __aligned_u64 in struct vfio_device_gfx_plane_info

The memory layout of struct vfio_device_gfx_plane_info is
architecture-dependent due to a u64 field and a struct size that is not
a multiple of 8 bytes:
- On x86_64 the struct size is padded to a multiple of 8 bytes.
- On x32 the struct size is only a multiple of 4 bytes, not 8.
- Other architectures may vary.

Use __aligned_u64 to make memory layout consistent. This reduces the
chance of 32-bit userspace on a 64-bit kernel breakage.

This patch increases the struct size on x32 but this is safe because of
the struct's argsz field. The kernel may grow the struct as long as it
still supports smaller argsz values from userspace (e.g. applications
compiled against older kernel headers).

Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://lore.kernel.org/r/20230918205617.1478722-3-stefanha@redhat.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/gpu/drm/i915/gvt/kvmgt.c

@@ -1379,7 +1379,7 @@ static long intel_vgpu_ioctl(struct vfio_device *vfio_dev, unsigned int cmd,
 		intel_gvt_reset_vgpu(vgpu);
 		return 0;
 	} else if (cmd == VFIO_DEVICE_QUERY_GFX_PLANE) {
-		struct vfio_device_gfx_plane_info dmabuf;
+		struct vfio_device_gfx_plane_info dmabuf = {};
 		int ret = 0;
 
 		minsz = offsetofend(struct vfio_device_gfx_plane_info,

include/uapi/linux/vfio.h

@@ -816,7 +816,7 @@ struct vfio_device_gfx_plane_info {
 	__u32 drm_plane_type;	/* type of plane: DRM_PLANE_TYPE_* */
 	/* out */
 	__u32 drm_format;	/* drm format of plane */
-	__u64 drm_format_mod;   /* tiled mode */
+	__aligned_u64 drm_format_mod;   /* tiled mode */
 	__u32 width;	/* width of plane */
 	__u32 height;	/* height of plane */
 	__u32 stride;	/* stride of plane */
@@ -829,6 +829,7 @@ struct vfio_device_gfx_plane_info {
 		__u32 region_index;	/* region index */
 		__u32 dmabuf_id;	/* dma-buf id */
 	};
+	__u32 reserved;
 };
 
 #define VFIO_DEVICE_QUERY_GFX_PLANE _IO(VFIO_TYPE, VFIO_BASE + 14)

samples/vfio-mdev/mbochs.c

@@ -1262,7 +1262,7 @@ static long mbochs_ioctl(struct vfio_device *vdev, unsigned int cmd,
 
 	case VFIO_DEVICE_QUERY_GFX_PLANE:
 	{
-		struct vfio_device_gfx_plane_info plane;
+		struct vfio_device_gfx_plane_info plane = {};
 
 		minsz = offsetofend(struct vfio_device_gfx_plane_info,
 				    region_index);

samples/vfio-mdev/mdpy.c

@@ -591,7 +591,7 @@ static long mdpy_ioctl(struct vfio_device *vdev, unsigned int cmd,
 
 	case VFIO_DEVICE_QUERY_GFX_PLANE:
 	{
-		struct vfio_device_gfx_plane_info plane;
+		struct vfio_device_gfx_plane_info plane = {};
 
 		minsz = offsetofend(struct vfio_device_gfx_plane_info,
 				    region_index);