A flaw was found in the X.Org server. The GLX PBuffer code does not call the XACE hook when creating the buffer, leaving it unlabeled. When the client issues another request to access that resource (as with a GetGeometry) or when it creates another resource that needs to access that buffer, such as a GC, the XSELINUX code will try to use an object that was never labeled and crash because the SID is NULL.
Workaround:
The product receives input from an upstream component, but it does not neutralize or incorrectly neutralizes NUL characters or null bytes when they are sent to a downstream component.