  Different from the traditional software vulnerability, the microarchitecture
side channel has three characteristics: extensive influence, potent threat, and
tough defense. The main reason for the micro-architecture side channel is
resource sharing. There are many reasons for resource sharing, one of which is
SMT (Simultaneous Multi-Threading) technology. In this paper, we define the SMT
Transient Channel, which uses the transient state of shared resources between
threads to steal information. To mitigate it, we designed a security
demand-based dynamic mitigation (DDM) to Mitigate the SMT transient channels.
The DDM writes the processes' security requirements to the CPU register sets,
and the operating system calls the HLT instruction to dynamically turn on and
off the hyper-threading according to the register values to avoid the side
channels caused by execution resource sharing. During the implementation of the
scheme, we modified the Linux kernel and used the MSR register groups of Intel
processor. The evaluation results show that DDM can effectively protect against
the transient side-channel attacks such as PortsMash that rely on SMT, and the
performance loss of DDM is less than 8%.
