drm/vmwgfx: Fix possible invalid drm gem put calls

vmw_bo_unreference sets the input buffer to null on exit, resulting in
null ptr deref's on the subsequent drm gem put calls.

This went unnoticed because only very old userspace would be exercising
those paths but it wouldn't be hard to hit on old distros with brand
new kernels.

Introduce a new function that abstracts unrefing of user bo's to make
the code cleaner and more explicit.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reported-by: Ian Forbes <iforbes@vmware.com>
Fixes: 9ef8d83e8e25 ("drm/vmwgfx: Do not drop the reference to the handle too soon")
Cc: <stable@vger.kernel.org> # v6.4+
Reviewed-by: Maaz Mombasawala<mombasawalam@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230818041301.407636-1-zack@kde.org

1 files changed, 8 insertions, 0 deletions

diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_bo.h b/drivers/gpu/drm/vmwgfx/vmwgfx_bo.h
index 50a836e70994..1d433fceed3d 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_bo.h
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_bo.h
@@ -195,6 +195,14 @@ static inline struct vmw_bo *vmw_bo_reference(struct vmw_bo *buf)
 	return buf;
 }
 
+static inline void vmw_user_bo_unref(struct vmw_bo *vbo)
+{
+	if (vbo) {
+		ttm_bo_put(&vbo->tbo);
+		drm_gem_object_put(&vbo->tbo.base);
+	}
+}
+
 static inline struct vmw_bo *to_vmw_bo(struct drm_gem_object *gobj)
 {
 	return container_of((gobj), struct vmw_bo, tbo.base);