Re: [PATCH RFC v9 11/25] mm: kpkeys: Introduce kpkeys_hardened_pgtables feature

Hi Kevin,

kpkeys_hardened_pgtables is a hardening feature based on kpkeys. It
aims to prevent the corruption of page tables by: 1. mapping all
page table pages, both kernel and user, with a privileged pkey
(KPKEYS_PKEY_PGTABLES), and 2. granting write access to that pkey
only when running at in a privileged kpkeys context
(KPKEYS_CTX_PGTABLES). This patch introduces basic infrastructure;
the implementation of both aspects will follow.

The feature is exposed as CONFIG_KPKEYS_HARDENED_PGTABLES; it
requires explicit architecture opt-in by selecting
ARCH_HAS_KPKEYS_HARDENED_PGTABLES, since much of the page table
handling is arch-specific.

Because this feature relies on kpkeys being supported and modifies
attributes of the linear map, it must be inactive on boot.
kpkeys_hardened_pgtables_init() enables it by toggling a static key;
this function must be called by supported architectures in
mem_init(), before any call to pagetable_alloc() is made.

Presuming other architecures will use kpkeys framework it's better to
call kpkeys_hardened_pgtables_init() from mm_core_init() rather than
stick into each arch's mem_init().

-- 
Sincerely yours,
Mike.