Walleij: New ARM32 Security Features in v6.10

Linus Walleij writes
about a pair of security features for 32-bit Arm systems; these landed
in 6.10, but, he says, have now stabilized to the point that distributors
may want to enable them.

PAN is an abbreviation for the somewhat grammatically incorrect
Privileged Access Never.

For modern ARM32 systems with large memories configured to use LPAE
nothing like PAN was available: this version of the MMU simply did
not implement a PAN option.

As of the patch originally developed by Catalin Marinas, we deploy
a scheme that will use the fact that LPAE has two separate
translation table base registers (TTBR:s): one for userspace
(TTBR0) and one for kernelspace (TTBR1).