Kosuke Higuchi, Ryotaro Kobayashi
Ransomware continues encrypting files during the delay between attack onset and detection. ROFBS mitigates this problem by backing up pre-modification files in real time upon file-open events. However, because the Linux file-open path traverses multiple kernel functions, it remains unclear how the choice of hook point affects defense effectiveness. In this study, we kept the ROFBS mechanism fixed and changed only the hook points on the Linux file-open path. We compared may_open, inode_permission, do_dentry_open, security_file_open, and xfs_file_open on AlmaLinux with XFS using three ransomware families: AvosLocker, Conti, and IceFire. We used Backup Ratio as the main metric and also compared the number of encrypted files with backups and the total number of encrypted files. The results showed that hook-point selection substantially affected both recoverability and damage scale. For AvosLocker, security_file_open achieved the highest Backup Ratio (82.5%). For Conti and IceFire, xfs_file_open achieved the highest values (100.0% and 63.2%, respectively). Moreover, xfs_file_open minimized the total number of encrypted files for all three ransomware families. These results indicate that, in ROFBS, the layer at which file-open events are observed is a key design factor. In particular, on XFS, hooking the filesystem-specific callback xfs_file_open may be advantageous when the goal is to reduce overall damage.