grsecurity is described as 'Grsecurity® is an extensive security enhancement to the Linux kernel that defends against a wide range of security threats through intelligent access control, memory corruption-based exploit prevention, and a host of other system hardening that generally require no' and is an app in the os & utilities category. There are six alternatives to grsecurity for Linux, Fedora and Xen. The best grsecurity alternative is Qubes OS, which is both free and Open Source. Other great apps like grsecurity are AppArmor, SELinux, CLIP OS and Smack.
Qubes is an open source operating system designed to provide strong security for desktop computing.
Qubes OS is the most popular Linux alternative to grsecurity.
Qubes OS is the most popular Open Source & free alternative to grsecurity.
A Mandatory Access Control (MAC) system which is a kernel (LSM) enhancement to confine programs to a limited set of resources.
Security Enhanced Linux (SELinux) is a role-based access control solution, developed by the US spying agency NSA and Red Hat, for internal and general use.
The CLIP OS project is an open source project maintained by the ANSSI (National Cybersecurity Agency of France) that aims to build a secure, multi-level operating system, based on the Linux kernel and a lot of free and open source software.
TOMOYO Linux is a Mandatory Access Control (MAC) implementation for Linux that can be used to increase the security of a system, while also being useful purely as a system analysis tool.
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While there is none replacement for grsecurity, at least qubes can separate things while grsecurity can prevent at kernel level which is the highest. Qubes OS security is based upon consciousness, the weakest link of the security where human is the factor. Qubes depends on several templates distros, it can't stop the attack like what grsecurity does but can separate which application vm (appvm) you should use, instead of mix everything together in one place.