Increasing chip power densities allied to the continuous technology shrink are making emerging multiprocessor embedded systems more vulnerable to radiation-induced transient faults (i.e., soft errors). Due the high cost and design time inherent to board-based fault injection approaches, more appropriate and efficient simulation-based fault injection frameworks become crucial to guarantee the adequate design exploration support at the early design phases. Virtual platforms emerge as a solution to early reliability explorations as it offers a flexible modeling environment, acceptable simulation speed, and different accuracy levels. This work introduces a fast and flexible fault injector framework (gem5-FIM) developed using a state-of-the-art cycle-accurate virtual platform (i.e., gem5 simulator), which accelerates the analysis of complex and large-scale systems composed of commercial processors under different classes of fault campaigns. Further, the gem5-FIM supports multicore ARM processor models running sophisticated software stacks comprising a Linux OS and parallelization libraries (e.g., MPI, OpenMP). Authors also propose different techniques to boost up the fault injection process.