[RFC PATCH v3 1/3] mm: make persistent huge zero folio read-only

[Xueyuan Chen <xueyuan.chen21@gmail.com>]

From: Xueyuan Chen <xueyuan.chen21@gmail.com>

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 persistent huge zero folio read-only
in the direct map turns such writes into faults instead of silent zero-page
corruption.

Add set_direct_map_ro_noflush() so mm code can make a direct-map range
read-only. Use an address-based signature to match ongoing direct-map
helper work[2], where existing page-based helpers may move the same way.
The helper is direct-map specific and does not flush TLBs. Architectures
without direct-map permission support keep existing behavior through the
generic stub.

Persistent huge zero folio setup happens during early boot, so no explicit
TLB flush is needed.

Inspired by Jann Horn's read-only zero page work[1] and follow-up
discussion[3] with Yang Shi.

[1] https://lore.kernel.org/linux-mm/20260508-ro-zeropage-v1-1-9808abc20b49@google.com/
[2] https://lore.kernel.org/linux-mm/0e5b23a6-4895-454a-9dfa-6dc21adc2991@kernel.org/
[3] https://lore.kernel.org/linux-mm/CAHbLzkrXXe7r3n3jXgDKtwZhRqj=jDx9E6dLOULohnhBguvi9A@mail.gmail.com/

Suggested-by: David Hildenbrand <david@kernel.org>
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 | 26 ++++++++++++++++++++++++++
 mm/huge_memory.c           |  9 ++++++++-
 2 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/include/linux/set_memory.h b/include/linux/set_memory.h
index 3030d9245f5a..a905074fb21d 100644
--- a/include/linux/set_memory.h
+++ b/include/linux/set_memory.h
@@ -40,6 +40,22 @@ static inline int set_direct_map_valid_noflush(struct page *page,
 	return 0;
 }
 
+/**
+ * set_direct_map_ro_noflush - make direct-map mappings read-only
+ * @addr: start address in the direct map
+ * @nr_pages: number of pages starting at @addr
+ *
+ * Make the direct-map mappings for @nr_pages pages starting at @addr
+ * read-only, without flushing TLBs.
+ *
+ * Return: 0 on success or when unsupported, negative error code on failure.
+ */
+static inline int set_direct_map_ro_noflush(const void *addr,
+					    unsigned long nr_pages)
+{
+	return 0;
+}
+
 static inline bool kernel_page_present(struct page *page)
 {
 	return true;
@@ -56,6 +72,16 @@ static inline bool can_set_direct_map(void)
 }
 #define can_set_direct_map can_set_direct_map
 #endif
+
+#ifndef set_direct_map_ro_noflush
+static inline int set_direct_map_ro_noflush(const void *addr,
+					    unsigned long nr_pages)
+{
+	return 0;
+}
+
+#define set_direct_map_ro_noflush set_direct_map_ro_noflush
+#endif
 #endif /* CONFIG_ARCH_HAS_SET_DIRECT_MAP */
 
 #ifdef CONFIG_X86_64
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index bdd8635922f9..6633217b10dc 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -41,6 +41,7 @@
 #include <linux/pgalloc.h>
 #include <linux/pgalloc_tag.h>
 #include <linux/pagewalk.h>
+#include <linux/set_memory.h>
 
 #include <asm/tlb.h>
 #include "internal.h"
@@ -981,8 +982,14 @@ static int __init thp_shrinker_init(void)
 		 * that get_huge_zero_folio() will most likely not fail as
 		 * thp_shrinker_init() is invoked early on during boot.
 		 */
-		if (!get_huge_zero_folio())
+		if (!get_huge_zero_folio()) {
 			pr_warn("Allocating persistent huge zero folio failed\n");
+			return 0;
+		}
+
+		/* Set up during early boot; no explicit TLB flush is needed here. */
+		set_direct_map_ro_noflush(folio_address(huge_zero_folio),
+					  HPAGE_PMD_NR);
 		return 0;
 	}
 
-- 
2.49.0