grsecurity is a set of patches for the Linux kernel with an emphasis on enhancing security. Its typical application is in web servers and systems that accept remote connections from untrusted locations, such as systems offering shell access to its users.
Work on grsecurity began in February 2001 as a port of Openwall Project's security-enhancing patches for Linux 2.4. The first release of grsecurity was for Linux 2.4.1.
A major component bundled with grsecurity is PaX, which is a patch that, amongst other things, flags data memory, such as that on the stack, as non-executable, and program memory as non-writable. The aim is to prevent executable memory pages from being overwritten with injected machine code, which prevents exploitation of many types of security vulnerabilities, such as buffer overflows. PaX also provides address space layout randomization (ASLR), which randomizes important memory addresses to hinder attacks that rely on such addresses being easily known. PaX is not itself developed by the grsecurity developers, and is also available independently from grsecurity .
Role-based Access Control
Another notable component of grsecurity is that it provides a full role-based access control (RBAC) system. RBAC is intended to restrict access to the system further than what is normally provided by Unix access control lists, with the aim of creating a fully least-privilege system, where users and processes have the absolute minimum privileges to work correctly and nothing more. This way, if the system is compromised, the ability by the attacker to damage or gain sensitive information on the system can be drastically reduced. RBAC works through a collection of "roles". Each role can have individual restrictions on what they can or cannot do, and these roles and restrictions form a "policy" which can be amended as needed.
grsecurity restricts chroot in a variety of ways to prevent a variety of vulnerabilities, privilege escalation attacks, and to add additional checks and balances.
- No attaching shared memory outside of chroot
- No kill outside of chroot
- No ptrace outside of chroot (architecture independent)
grsecurity also adds enhanced auditing to the Linux kernel. It can be configured to audit a specific group of users, audit mounts/unmounts of devices, changes to the system time and date, chdir logging, amongst other things. Some of these other things allow the admin to also log denied resource attempts, failed fork attempts, and exec logging with arguments.
Trusted path execution is another optional feature that can be used to prevent users from executing binaries that are not owned by the root user, or are world-writable. This is useful to prevent users from executing their own malicious binaries or accidentally executing system binaries that could have been modified by a malicious user (being world-writable).
grsecurity also hardens the way chroot "jails" work. A chroot jail can be used to isolate a particular process from the rest of the system, which can be used to minimise the potential for damage, should the service be compromised. However, there are ways to "break out" of a chroot jail. grsecurity attempts to prevent this.
List of additional features and security improvements:
- /proc restrictions that don't leak information about process owners
- Symlink/hardlink restrictions to prevent /tmp races
- Hardlink restrictions to prevent users from hardlinking to files they do not own
- FIFO/Named pipe restrictions
- dmesg(8) restriction
- Enhanced implementation of Trusted Path Execution
- Group-based socket restrictions