Re: [PATCH v7 1/3] mm: make persistent huge zero folio read-only

[Mike Rapoport <rppt@kernel.org>]

On Thu, Sep 03, 2026 at 08:49:52PM +0800, Xueyuan Chen wrote:
> On Thu, Sep 3, 2026 at 5:22 PM Mike Rapoport <rppt@kernel.org> wrote:
> >
> > On Tue, Sep 01, 2026 at 11:18:16PM +0800, Xueyuan Chen wrote:
> > > The persistent huge zero folio is shared globally and should stay zero
> > > after initialization. As Jann Horn pointed out [1], kernel bugs have
> > > ended up writing to pages that were meant to be read-only, including in
> > > security-sensitive cases. Making the folio read-only in the direct map
> > > turns such writes into faults instead of silent zero-page corruption.
> > >
> > > Add a page-based helper consistent with the existing direct-map interfaces.
> > > Handle TLB invalidation in the architecture implementation; unsupported
> > > architectures retain their current behavior.
> > >
> > > Protect the folio after initialization. Skip highmem folios, which have no
> > > permanent direct-map mapping.
> > >
> > > Inspired by Jann Horn's read-only zero page work [1] and follow-up
> > > discussion [3] with Yang Shi.
> > >
> > > Link: https://lore.kernel.org/r/20260508-ro-zeropage-v1-1-9808abc20b49@google.com [1]
> > > Link: https://lore.kernel.org/r/0e5b23a6-4895-454a-9dfa-6dc21adc2991@kernel.org [2]
> > > Link: https://lore.kernel.org/r/CAHbLzkrXXe7r3n3jXgDKtwZhRqj=jDx9E6dLOULohnhBguvi9A@mail.gmail.com [3]
> > >
> > > Suggested-by: David Hildenbrand <david@kernel.org>
> > > Suggested-by: Usama Arif <usama.arif@linux.dev>
> > > Co-developed-by: Lance Yang <lance.yang@linux.dev>
> > > Signed-off-by: Lance Yang <lance.yang@linux.dev>
> > > Signed-off-by: Xueyuan Chen <xueyuan.chen21@gmail.com>
> > > ---
> > >  include/linux/set_memory.h | 17 +++++++++++++++++
> > >  mm/huge_memory.c           | 13 ++++++++++---
> > >  2 files changed, 27 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/include/linux/set_memory.h b/include/linux/set_memory.h
> > > index 3fe293cfed8c..ed9ce04b18a1 100644
> > > --- a/include/linux/set_memory.h
> > > +++ b/include/linux/set_memory.h
> > > @@ -54,6 +54,23 @@ static inline bool can_set_direct_map(void)
> > >  #endif
> > >  #endif /* CONFIG_ARCH_HAS_SET_DIRECT_MAP */
> > >
> > > +#ifndef set_direct_map_ro
> > > +/**
> > > + * set_direct_map_ro - make a direct-map range read-only
> > > + * @page: first page in the direct-map range
> > > + * @nr: number of pages in the range
> > > + *
> > > + * Make the direct-map range starting at @page read-only and invalidate stale
> > > + * writable translations before returning.
> > > + *
> > > + * Return: 0 on success, or a negative error code on failure.
> > > + */
> > > +static inline int set_direct_map_ro(struct page *page, unsigned int nr)
> > > +{
> > > +     return 0;
> > > +}
> > > +#endif
> > > +
> > >  #ifdef CONFIG_X86_64
> > >  int set_mce_nospec(unsigned long pfn);
> > >  int clear_mce_nospec(unsigned long pfn);
> > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > index 54494c3fa983..742283b36d74 100644
> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > > @@ -42,6 +42,7 @@
> > >  #include <linux/pgalloc_tag.h>
> > >  #include <linux/pagewalk.h>
> > >  #include <linux/cleanup.h>
> > > +#include <linux/set_memory.h>
> > >
> > >  #include <asm/tlb.h>
> > >  #include "internal.h"
> > > @@ -291,10 +292,16 @@ static int __init huge_zero_init(void)
> > >       huge_zero_folio = alloc_huge_zero_folio();
> > >       if (!huge_zero_folio) {
> > >               pr_warn("Allocating persistent huge zero folio failed\n");
> > > -     } else {
> > > -             huge_zero_pfn = folio_pfn(huge_zero_folio);
> > > -             count_vm_event(THP_ZERO_PAGE_ALLOC);
> > > +             return 0;
> > >       }
> > > +
> > > +     huge_zero_pfn = folio_pfn(huge_zero_folio);
> > > +     count_vm_event(THP_ZERO_PAGE_ALLOC);
> > > +
> > > +     /* Highmem folios have no permanent direct-map mapping to protect. */
> > > +     if (!folio_test_highmem(huge_zero_folio))
> > > +             set_direct_map_ro(folio_page(huge_zero_folio, 0), HPAGE_PMD_NR);
> >
> > Sorry, I don't remember if it was discussed previously, but why can't we
> > use the existing set_memory_ro() here?
> 
> Hi Mike,
> 
> We want to change the linear map here, but arm64 set_memory_ro() only
> works on vmalloc addresses.

I believe this is an historical artifact. change_memory_common() already
updates the linear map when a vmalloc mapping switches to RO and the system
supports it.

I believe arm64::set_memory_ro() can change the linear map in the general
case as well as long as can_set_direct_map() is true.

> So a new helper is needed on arm64, and x86 implements the same
> helper to keep the two architectures consistent. On x86 it is
> basically set_memory_ro() minus the alias check.
> 
> Thanks,
> Xueyuan
> 
> >
> > > +
> > >       return 0;
> > >  }
> > >
> > > --
> > > 2.47.3
> > >
> >
> > --
> > Sincerely yours,
> > Mike.

-- 
Sincerely yours,
Mike.