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

[Mike Rapoport <rppt@kernel.org>]

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?

> +
>  	return 0;
>  }
>  
> -- 
> 2.47.3
> 

-- 
Sincerely yours,
Mike.