In computer systems, the memory overflow attack is a long-existing security problem and is still common nowadays, which can be effectively hindered by pointer encryption. Nevertheless, the implementation of the technique by software significantly lowers the program running efficiency and leads to additional memory overhead. In this study, we develop an encrypted/decrypted pointer coprocessor PEC-V based on the Rocket Custom Coprocessor (RoCC) interface of RocketChip. The overflow attack can be prevented through the control of encryption/decryption of the return address and function pointer by the coprocessor under the user-defined instruction of RISC-V. PEC-V mainly depends on Physical Unclonable Function (PUF) to avoid storing the key value of the encrypted pointer in memory. Thus, this mechanism not only ensures the randomness of the key value, but also reduces the times of accessing memory. The experimental results show that PEC-V is defensive against various buffer overflow attacks while the program running efficiency is only reduced by approximately 3% on average, which is better than previous mechanisms.