In this lesson, you will learn how to ensure that Mandatory Access Control (MAC) security labels remain intact and correctly applied across various file systems and mount operations.
The Challenge of Persistence
When you apply security labels (like SELinux contexts or AppArmor profiles) to files, the kernel stores this metadata in extended attributes (xattrs). However, security context persistence is not guaranteed simply by labeling a file. If a file system does not support extended attributes, or if the mount configuration explicitly ignores them, your security policy will effectively vanish, leaving those files unprotected by your MAC framework.
Ensuring Integrity Across Mounts
Persistence issues frequently arise when moving files between partitions or mounting external storage. For example, if you mount a storage device without the context or fscontext mount options, the kernel may treat all files on that device as having a generic file_t label, overriding the specific policies you spent time crafting.
To maintain persistence when mounting a file system that might not natively support persistent labeling (like FAT32 or some network shares), you can force a security context at the mount point. This ensures that every file accessed through that mount path is treated as if it possessed the specified label:
# Mounting a drive and forcing a security context for all its contents
mount -t vfat /dev/sdb1 /mnt/data -o context="system_u:object_r:data_t:s0"
Verifying Persistence
Always verify that your labels survive after a remount or reboot. You can check the current context of a file using ls -Z. If you notice that labels are reverting to a default state, check your /etc/fstab configuration. If your file system type (like Ext4 or XFS) supports xattrs but they are disappearing, verify that the selinux=1 or apparmor=1 boot parameters were passed to the kernel and that the file system was mounted with the seclabel option enabled. Without these, the kernel will refuse to read or store your security metadata, rendering your MAC enforcement incomplete.
Key Takeaway: Security labels are only as persistent as the file system’s ability to store xattrs and the mount options that govern how the kernel interprets those attributes at runtime.