In this lesson, you will learn how to use SELinux security contexts to restrict which shared libraries (.so files) a process is permitted to load, effectively preventing unauthorized code injection.
The Problem: Arbitrary Library Injection
Under standard Linux Discretionary Access Control (DAC), if a malicious user can write to a directory, they can often place a malicious shared library there and trick a privileged service into loading it via LD_PRELOAD or by manipulating RPATH. Even if the service is running, the kernel generally allows any process to load any library it has filesystem-level permission to read.
Defining Security Contexts for Libraries
To enforce security, we apply SELinux labels to our shared objects. By treating libraries as distinct objects with specific types, we create a "deny-by-default" gatekeeper. We define a policy that dictates which domains (processes) are allowed to execute or map files labeled with a specific library type.
For example, if you have a sensitive service running in the my_service_t domain, you should label its legitimate dependencies with a specialized type, such as my_service_lib_t.
# Label the libraries specifically
semanage fcontext -a -t my_service_lib_t "/opt/my_app/lib(/.*)?"
restorecon -Rv /opt/my_app/lib
# In your policy module, allow only the specific domain to load them
allow my_service_t my_service_lib_t:file { execute map read open getattr };
Why This Matters
When the service attempts to load a library, the SELinux subsystem intercepts the mmap or exec system call. Even if a user places a malicious library in a global directory (like /tmp), the kernel will check the context of that file. If the file is labeled tmp_t instead of my_service_lib_t, the kernel will deny the map permission for the my_service_t domain.
By enforcing these constraints, you ensure that even if an attacker manages to place a malicious object on the disk, the service remains hardened because it cannot transition its execution state to code that lacks the authorized security label.
Key Takeaway: By assigning specific SELinux types to shared libraries and restricting map and execute permissions to only authorized service domains, you effectively prevent arbitrary code injection through untrusted shared objects.