[PATCH RFC v9 12/25] mm: kpkeys: Protect regular page tables

[Mike Rapoport]

On Mon, Sep 07, 2026 at 05:52:32PM +0200, Kevin Brodsky wrote:
> On 07/09/2026 12:54, Mike Rapoport wrote:
> >> [...]
> >>
> >>  static inline struct ptdesc *pagetable_alloc_noprof(gfp_t gfp, unsigned int order)
> >>  {
> >> -	struct page *page = alloc_pages_noprof(gfp | __GFP_COMP, order);
> >> +	struct page *page;
> >> +
> >> +	if (kpkeys_hardened_pgtables_enabled())
> >> +		page = kpkeys_pgtable_alloc(gfp | __GFP_COMP, order);
> >> +	else
> >> +		page = alloc_pages_noprof(gfp | __GFP_COMP, order);
> > Can we make it a sequence rahter than a branch?
> >
> > kpkeys_pgtable_alloc() does alloc_pages and then sets their pkeys, so I
> > think something like this should work:
> >
> > 	page = alloc_pages_noprof(gfp | __GFP_COMP, order);
> > 	if (!page)
> > 		return NULL;
> > 	err = kpkeys_pgtable_alloc(page);
> > 	if (err) {
> > 		__free_pages(page, order);
> > 		return NULL;
> >
> > with if (kpkeys_hardened_pgtables_enabled()) folded into
> > kpkeys_pgtable_alloc().
> 
> I agree this would be less ugly. In fact this is pretty much what this
> series did up to RFC v5 (albeit in the ctor/dtor instead of alloc/free).
> We could go back to this API, *but* the big issue is that it makes it
> impossible to use a smarter allocation strategy for protected pages.
> Patch 14 in RFC v6 [1] will give you an idea of what such an allocator
> (with support for large blocks and splitting) would look like.
> 
> As discussed with David H we're first trying to land this feature
> without large block support (fully PTE-mapped direct map), but we do
> want to support large blocks eventually [2] and I would prefer the core
> API to be already compatible with that objective. Very happy to hear
> about suggestions as to how to avoid the explicit condition in
> pagetable_alloc() though!

We could unconditionally replace alloc_pages_noprof() with
kpkeys_pgtable_alloc() and make the latter choose the right allocator, but
that's not very nice either :/
 
> - Kevin
> 
> [1]
> https://lore.kernel.org/linux-hardening/20260227175518.3728055-15-kevin.brodsky@arm.com/
> [2] https://lore.kernel.org/all/aMwd7IJVECEy8mzf@willie-the-truck/

-- 
Sincerely yours,
Mike.