file: /proc/sys/fs/protected_hardlinks
variable: fs.protected_hardlinks
Official reference

A long-standing class of security issues is the hardlink-based time-of-check-time-of-use race, most commonly seen in world-writable directories like /tmp. The common method of exploitation of this flaw is to cross privilege boundaries when following a given hardlink (i.e. a root process follows a hardlink created by another user). Additionally, on systems without separated partitions, this stops unauthorized users from “pinning” vulnerable setuid/setgid files against being upgraded by the administrator, or linking to special files.

When set to “0”, hardlink creation behavior is unrestricted.

When set to “1” hardlinks cannot be created by users if they do not already own the source file, or do not have read/write access to it.

This protection is based on the restrictions in Openwall and grsecurity.