The Linux kernel features an extensive suite of network protocols and supporting components. As networking is an important aspect of Linux, LSM extends the concept of a generalized security framework to this area of the kernel.
A key implementation challenge was to determine the initial requirements for the network hooks. The existing SELinux implementation was utilized as a model, as SELinux is itself a highly generalized security infrastructure which was to be ported to LSM. Other Linux security projects were reviewed, although none relevant to the version 2.5 kernel series were found with networking requirements in excess of SELinux. Potential requirements for IPSec and traditional labeled networking systems were also taken into account.
As the Linux network stack utilizes the Berkeley sockets model , LSM is able to provide coarse coverage for all socket-based protocols via the use of hooks within the socket layer.
Additional finer-grained hooks have been implemented for the IPv4, UNIX domain, and Netlink protocols, which were considered essential for the implementation of a minimally useful system. Similar hooks for other protocols may be implemented at a later stage.
Coverage of low level network support components such as routing tables and traffic classifiers is somewhat limited due to the invasiveness of the code which would be required to implement consistent fine-grained hooks. Accesses to these objects can be interposed at higher levels (e.g., via system calls such as ioctl(2)), although granularity may be reduced by TOCTTOU issues. The existing kernel code does however impose a CAP_NET_ADMIN capability requirement for tasks which attempt to write to important network support components.
The details of the network hooks are described in Sections 3.7.1 through 3.7.6.