[PATCH] drm/vmwgfx: Fix kernel NULL pointer dereference on older hardware

[Thomas Hellstrom]

The commit "drm/vmwgfx: Fix up user_dmabuf refcounting", while fixing a
kernel crash introduced a NULL pointer dereference on older hardware.
Fix this.

Cc: <stable at vger.kernel.org>
Signed-off-by: Thomas Hellstrom <thellstrom at vmware.com>
Reviewed-by: Sinclair Yeh <syeh at vmware.com>
Reviewed-by: Brian Paul <brianp at vmware.com>
---
 drivers/gpu/drm/vmwgfx/vmwgfx_surface.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c b/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c
index 64b5040..03f63c7 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c
@@ -657,7 +657,8 @@ static void vmw_user_surface_base_release(struct ttm_base_object **p_base)
 	struct vmw_resource *res = &user_srf->srf.res;
 
 	*p_base = NULL;
-	ttm_base_object_unref(&user_srf->backup_base);
+	if (user_srf->backup_base)
+		ttm_base_object_unref(&user_srf->backup_base);
 	vmw_resource_unreference(&res);
 }
 
-- 
2.1.0