The Linux security module (LSM) mechanism was created as a result of the first Kernel Summit in 2001; it was
designed to allow the development of multiple approaches to Linux security.
That goal has been met; there are several security modules available with
current kernels. The LSM subsystem was not designed, though, to allow
multiple security modules to work together on the same system. Developers
have been working to rectify that problem almost since the LSM subsystem
was merged, but with limited success; some small security modules can be
stacked on top of the “major” ones, but arbitrary stacking is not possible.
Now, a full 20 years after
security-module support went into the 2.5 development kernel series, it
looks like a solution to the stacking problem may finally be getting
closer.

Source: LWN.net – [$] Still waiting for stackable security modules