drm/vmwgfx: avoid calling vzalloc with a 0 size in vmw_get_cap_3d_ioctl()

commit 63774069 upstream.

In vmw_get_cap_3d_ioctl(), a user can supply 0 for a size that is
used in vzalloc(). This eventually calls dump_stack() (in warn_alloc()),
which can leak useful addresses to dmesg.

Add check to avoid a size of 0.
Signed-off-by: Murray McAllister <murray.mcallister@insomniasec.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/gpu/drm/vmwgfx/vmwgfx_ioctl.c

@@ -90,7 +90,7 @@ int vmw_get_cap_3d_ioctl(struct drm_device *dev, void *data,
 	void *bounce;
 	int ret;
 
-	if (unlikely(arg->pad64 != 0)) {
+	if (unlikely(arg->pad64 != 0 || arg->max_size == 0)) {
 		DRM_ERROR("Illegal GET_3D_CAP argument.\n");
 		return -EINVAL;
 	}