[PATCH 2/4] vfio: use __aligned_u64 in struct vfio_device_gfx_plane_info

[Stefan Hajnoczi]

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 holes that result in an information leak and the chance that
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@xxxxxxxx>
Signed-off-by: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
---
 include/uapi/linux/vfio.h        | 3 ++-
 drivers/gpu/drm/i915/gvt/kvmgt.c | 4 +++-
 samples/vfio-mdev/mbochs.c       | 6 ++++--
 samples/vfio-mdev/mdpy.c         | 4 +++-
 4 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
index b1dfcf3b7665..45db62d74064 100644
--- a/include/uapi/linux/vfio.h
+++ b/include/uapi/linux/vfio.h
@@ -746,7 +746,7 @@ 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 */
@@ -759,6 +759,7 @@ 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)
diff --git a/drivers/gpu/drm/i915/gvt/kvmgt.c b/drivers/gpu/drm/i915/gvt/kvmgt.c
index de675d799c7d..ffab3536dc8a 100644
--- a/drivers/gpu/drm/i915/gvt/kvmgt.c
+++ b/drivers/gpu/drm/i915/gvt/kvmgt.c
@@ -1382,7 +1382,7 @@ 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,
@@ -1392,6 +1392,8 @@ static long intel_vgpu_ioctl(struct vfio_device *vfio_dev, unsigned int cmd,
 		if (dmabuf.argsz < minsz)
 			return -EINVAL;
 
+		minsz = min(minsz, sizeof(dmabuf));
+
 		ret = intel_vgpu_query_plane(vgpu, &dmabuf);
 		if (ret != 0)
 			return ret;
diff --git a/samples/vfio-mdev/mbochs.c b/samples/vfio-mdev/mbochs.c
index c6c6b5d26670..ee42a780041f 100644
--- a/samples/vfio-mdev/mbochs.c
+++ b/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);
@@ -1273,11 +1273,13 @@ static long mbochs_ioctl(struct vfio_device *vdev, unsigned int cmd,
 		if (plane.argsz < minsz)
 			return -EINVAL;
 
+		outsz = min_t(unsigned long, plane.argsz, sizeof(plane));
+
 		ret = mbochs_query_gfx_plane(mdev_state, &plane);
 		if (ret)
 			return ret;
 
-		if (copy_to_user((void __user *)arg, &plane, minsz))
+		if (copy_to_user((void __user *)arg, &plane, outsz))
 			return -EFAULT;
 
 		return 0;
diff --git a/samples/vfio-mdev/mdpy.c b/samples/vfio-mdev/mdpy.c
index a62ea11e20ec..1500b120de04 100644
--- a/samples/vfio-mdev/mdpy.c
+++ b/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);
@@ -602,6 +602,8 @@ static long mdpy_ioctl(struct vfio_device *vdev, unsigned int cmd,
 		if (plane.argsz < minsz)
 			return -EINVAL;
 
+		minsz = min_t(unsigned long, plane.argsz, sizeof(plane));
+
 		ret = mdpy_query_gfx_plane(mdev_state, &plane);
 		if (ret)
 			return ret;
-- 
2.41.0