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

[David Hildenbrand (Arm)]

On 9/8/26 16:48, Xueyuan Chen wrote:
> On Tue, Sep 8, 2026 at 4:30 PM David Hildenbrand (Arm) <david@xxxxxxxxxx> wrote:
>>
>> On 9/1/26 17:18, 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@xxxxxxxxxx [1]
>>> Link: https://lore.kernel.org/r/0e5b23a6-4895-454a-9dfa-6dc21adc2991@xxxxxxxxxx [2]
>>> Link: https://lore.kernel.org/r/CAHbLzkrXXe7r3n3jXgDKtwZhRqj=jDx9E6dLOULohnhBguvi9A@xxxxxxxxxxxxxx [3]
>>>
>>> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
>>> Suggested-by: Usama Arif <usama.arif@xxxxxxxxx>
>>> Co-developed-by: Lance Yang <lance.yang@xxxxxxxxx>
>>> Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>
>>> Signed-off-by: Xueyuan Chen <xueyuan.chen21@xxxxxxxxx>
>>> ---
>>>  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);
>>
>> Assuming we keep the page-based approach, can't we just move the highmem test in
>> there?
> 
> Hi David,
> 
> We can move the check into set_direct_map_ro(). However, on x86,
> set_direct_map_invalid_noflush() and set_direct_map_default_noflush()
> also expect callers to exclude highmem pages.
> 
> Would it make sense to make highmem a documented no-op for those
> helpers as well, so the direct-map APIs handle it consistently?

I think getting something consistent for now is the most important thing. :)

-- 
Cheers,

David