In a mixed-criticality system, applications with different safety criticality levels are usually required to be implemented upon one platform for several reasons( reducing hardware cost, space, power consumption). Partitioning technology is used to enable the integration of mixed-criticality applications with reduced certification cost. In the partitioning architecture of strong spatial and temporal isolation, fault propagation can be prevented among mixed-criticality applications (regarded as partitions). However, I/O sharing between partitions could be the path of fault propagation that hinders the partitioning. E.g. a crashed partition generates incorrect outputs to shared I/Os, which affects the functioning of another partition. This paper focuses on a message-based approach of I/O sharing in the HARTEX real-time kernel on a multi-core platform. Based on a simple multi-core partitioning architecture, a certifiable I/O sharing approach is implemented based on a safe message mechanism, in order to support the partitioning architecture, enable individual certification of mixed-criticality applications and thus achieve minimized total certification cost of the entire system.