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In this paper, we present a reconfigurable Physically Unclonable Functions (PUF) based on the Spin-Orbit-Torque Magnetic Random-Access Memory (SOT-MRAM), which exploits thermal noise as the true dynamic entropy source. Therefore, the MRAM cells could be configured to random final states with stochastic switching mechanism. The proposed PUF is constructed and reconfigured by combining the small-capacity true random number generator (TRNG) and high-reliability secure hash algorithm (SHA-512), realizing the dynamic transformation between SOT-MRAM based last level cache and PUF (In-Cache-MPUF). Thanks to the full reconfigurability and the high endurance of SOT-MRAM, the proposed In-Cache-MPUF can achieve $10^{\textbf {14}}$ maximum PUF bits per cell, which has greatly motivated the implementations compared with the traditional weak PUFs utilizing the static entropy source of process variations. The Monte-Carlo simulation results using 40 nm technology and a compact MTJ model show that the proposed PUF has desirable randomness as the digitized bit streams passing all the NIST tests, achieving 50.0428% uniqueness as well as 49.9236% uniformity. It also shows comparable reliability to the state-of-the-art works: a maximum bit error rate of 0.14% and 0.12% at 100 °C and 0.9 V, respectively. In addition, the system level performance is tested and validated by gem5. [ABSTRACT FROM AUTHOR]