[PATCH v7 1/3] mm: make persistent huge zero folio read-only
From: Xueyuan Chen <hidden>
Date: 2026-09-01 15:18:52
Also in:
linux-mm, lkml
Subsystem:
memory management, memory management - thp (transparent huge page), the rest · Maintainers:
Andrew Morton, David Hildenbrand, Linus Torvalds
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 (local) [1] Link: https://lore.kernel.org/r/0e5b23a6-4895-454a-9dfa-6dc21adc2991@kernel.org (local) [2] Link: https://lore.kernel.org/r/CAHbLzkrXXe7r3n3jXgDKtwZhRqj=jDx9E6dLOULohnhBguvi9A@mail.gmail.com (local) [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 <redacted> --- 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); + return 0; }
--
2.47.3