Kees Cook has posted a
long list of security-related improvements that made it into the 5.9
kernel release. “Sasha Levin, Andy Lutomirski, Chang S. Bae, Andi
Kleen, Tony Luck, Thomas Gleixner, and others landed the long-awaited
FSGSBASE series. This provides task switching performance improvements
while keeping the kernel safe from modules accidentally (or maliciously)
trying to use the features directly (which exposed an unprivileged direct
kernel access hole).“

Source: LWN.net – Cook: Security things in Linux v5.9