[RFC PATCH] riscv: mm: allocate split linear mapping page tables via the standard page table allocation APIs

[li.hanting]

Hi,

Commit 9e4a3ec3411b ("x86/mm/pat: Allocate split page tables as kernel
page tables") recently fixed a latent inconsistency in the x86 CPA code
(assigned CVE-2026-97525): __split_large_page() allocated the new PTE
page with a bare pagetable_alloc(), bypassing pte_alloc_one_kernel(), so
the page table constructor never ran and the resulting page was neither
flagged PG_table, nor accounted in NR_PAGETABLE, nor marked as a kernel
page table.

While looking at the fallout of that commit, I noticed that RISC-V
open-codes the same pattern when splitting the linear mapping on behalf
of set_memory_*(): __split_linear_mapping_{pmd,pud,p4d}() allocate the
new page table pages with bare alloc_page(GFP_KERNEL) - three call
sites in arch/riscv/mm/pageattr.c, introduced by 311cd2f6e253 ("riscv:
Fix set_memory_XX() and set_direct_map_XX() by splitting huge linear
mappings").

The consequences are the same as on pre-fix x86: page tables that live
in init_mm without the PG_table flag, without NR_PAGETABLE accounting
and without the kernel ptdesc marking.

To be clear about severity: this is a latent inconsistency, not an
exploitable bug today. RISC-V never collapses the split linear mapping
back, and these page tables are never freed, so there is no free-path
mismatch of the kind the x86 fix was concerned about. But the
accounting mismatch is real (/proc/vmstat under-counts kernel page
tables), and any future work that frees these tables - linear map
memory hotplug, collapse support or anything similar - would inherit
the exact bug class the x86 fix addressed.

This RFC aligns the three sites with the standard allocation paths:

 - the PTE level switches to pte_alloc_one_kernel(), exactly like the
   x86 fix;

 - the PMD and PUD levels have no *_alloc_one_kernel() variant today,
   so a small local helper wraps pagetable_alloc() +
   __pagetable_ctor() + ptdesc_set_kernel(), mirroring what the
   generic PTE path does. GFP_PGTABLE_KERNEL also brings in
   __GFP_ZERO, which the open-coded allocation was missing.

No functional change is intended; the observable difference is that
split linear mapping page tables now show up in
/proc/vmstat:nr_page_table_pages.

Open questions:

 1. Would it make sense to provide generic pmd_alloc_one_kernel() /
    pud_alloc_one_kernel() variants in asm-generic/pgalloc.h so that
    architectures do not have to open-code this dance? This is the
    main reason this is an RFC rather than a regular patch.

 2. Since these tables are never freed today, is it preferable to take
    this alignment now (my preference: it removes a trap for any
    future free-path work), or defer it until such work actually
    lands?

    (Note: the generic CPA rework series "[PATCH RFC 00/18] arch, mm:
    create a generic set_memory/change_page_attr core" would eventually
    replace this code entirely; this patch is intended as a small fix in
    the meantime. The series author has said he is waiting for such
    fixes to settle before posting a non-RFC version.)

Comments welcome.

Signed-off-by: Hanting Li <li.hanting@xxxxxxxxxx>

---
 arch/riscv/mm/pageattr.c | 65 ++++++++++++++++++++++++++++-------------
 1 file changed, 44 insertions(+), 21 deletions(-)

diff --git a/arch/riscv/mm/pageattr.c b/arch/riscv/mm/pageattr.c
--- a/arch/riscv/mm/pageattr.c
+++ b/arch/riscv/mm/pageattr.c
@@ -3,9 +3,11 @@
  * Copyright (C) 2019 SiFive
  */

+#include <linux/mm.h>
 #include <linux/pagewalk.h>
 #include <linux/pgtable.h>
 #include <linux/vmalloc.h>
+#include <asm/pgalloc.h>
 #include <asm/tlbflush.h>
 #include <asm/bitops.h>
 #include <asm/set_memory.h>
@@ -93,6 +95,26 @@
 };

 #ifdef CONFIG_64BIT
+/*
+ * Kernel page tables allocated for the split linear mapping are never
+ * freed today, but they must still go through the page table constructor
+ * so that they are flagged PG_table, accounted in NR_PAGETABLE and marked
+ * as kernel page tables, like any other kernel page table allocated via
+ * the standard allocation APIs.
+ */
+static struct ptdesc *alloc_kernel_pgtable(void)
+{
+	struct ptdesc *ptdesc = pagetable_alloc(GFP_PGTABLE_KERNEL, 0);
+
+	if (!ptdesc)
+		return NULL;
+
+	__pagetable_ctor(ptdesc);
+	ptdesc_set_kernel(ptdesc);
+
+	return ptdesc;
+}
+
 static int __split_linear_mapping_pmd(pud_t *pudp,
 				      unsigned long vaddr, unsigned long end)
 {
@@ -109,23 +131,22 @@
 			continue;

 		if (pmd_leaf(pmdp_get(pmdp))) {
-			struct page *pte_page;
 			unsigned long pfn = _pmd_pfn(pmdp_get(pmdp));
 			pgprot_t prot = __pgprot(pmd_val(pmdp_get(pmdp)) & ~_PAGE_PFN_MASK);
 			pte_t *ptep_new;
 			int i;

-			pte_page = alloc_page(GFP_KERNEL);
-			if (!pte_page)
+			ptep_new = pte_alloc_one_kernel(&init_mm);
+			if (!ptep_new)
 				return -ENOMEM;

-			ptep_new = (pte_t *)page_address(pte_page);
-			for (i = 0; i < PTRS_PER_PTE; ++i, ++ptep_new)
-				set_pte(ptep_new, pfn_pte(pfn + i, prot));
+			for (i = 0; i < PTRS_PER_PTE; ++i)
+				set_pte(ptep_new + i, pfn_pte(pfn + i, prot));

 			smp_wmb();

-			set_pmd(pmdp, pfn_pmd(page_to_pfn(pte_page), PAGE_TABLE));
+			set_pmd(pmdp, pfn_pmd(page_to_pfn(virt_to_page(ptep_new)),
+					      PAGE_TABLE));
 		}
 	} while (pmdp++, vaddr = next, vaddr != end);

@@ -149,24 +170,25 @@
 			continue;

 		if (pud_leaf(pudp_get(pudp))) {
-			struct page *pmd_page;
+			struct ptdesc *ptdesc;
 			unsigned long pfn = _pud_pfn(pudp_get(pudp));
 			pgprot_t prot = __pgprot(pud_val(pudp_get(pudp)) & ~_PAGE_PFN_MASK);
 			pmd_t *pmdp_new;
 			int i;

-			pmd_page = alloc_page(GFP_KERNEL);
-			if (!pmd_page)
+			ptdesc = alloc_kernel_pgtable();
+			if (!ptdesc)
 				return -ENOMEM;

-			pmdp_new = (pmd_t *)page_address(pmd_page);
-			for (i = 0; i < PTRS_PER_PMD; ++i, ++pmdp_new)
-				set_pmd(pmdp_new,
+			pmdp_new = (pmd_t *)ptdesc_address(ptdesc);
+			for (i = 0; i < PTRS_PER_PMD; ++i)
+				set_pmd(pmdp_new + i,
 					pfn_pmd(pfn + ((i * PMD_SIZE) >> PAGE_SHIFT), prot));

 			smp_wmb();

-			set_pud(pudp, pfn_pud(page_to_pfn(pmd_page), PAGE_TABLE));
+			set_pud(pudp, pfn_pud(page_to_pfn(ptdesc_page(ptdesc)),
+					      PAGE_TABLE));
 		}

 		ret = __split_linear_mapping_pmd(pudp, vaddr, next);
@@ -198,23 +220,23 @@
 			continue;

 		if (p4d_leaf(p4dp_get(p4dp))) {
-			struct page *pud_page;
+			struct ptdesc *ptdesc;
 			unsigned long pfn = _p4d_pfn(p4dp_get(p4dp));
 			pgprot_t prot = __pgprot(p4d_val(p4dp_get(p4dp)) & ~_PAGE_PFN_MASK);
 			pud_t *pudp_new;
 			int i;

-			pud_page = alloc_page(GFP_KERNEL);
-			if (!pud_page)
+			ptdesc = alloc_kernel_pgtable();
+			if (!ptdesc)
 				return -ENOMEM;

 			/*
 			 * Fill the pud level with leaf puds that have the same
 			 * protections as the leaf p4d.
 			 */
-			pudp_new = (pud_t *)page_address(pud_page);
-			for (i = 0; i < PTRS_PER_PUD; ++i, ++pudp_new)
-				set_pud(pudp_new,
+			pudp_new = (pud_t *)ptdesc_address(ptdesc);
+			for (i = 0; i < PTRS_PER_PUD; ++i)
+				set_pud(pudp_new + i,
 					pfn_pud(pfn + ((i * PUD_SIZE) >> PAGE_SHIFT), prot));

 			/*
@@ -224,7 +246,8 @@
 			 */
 			smp_wmb();

-			set_p4d(p4dp, pfn_p4d(page_to_pfn(pud_page), PAGE_TABLE));
+			set_p4d(p4dp, pfn_p4d(page_to_pfn(ptdesc_page(ptdesc)),
+					      PAGE_TABLE));
 		}

 		ret = __split_linear_mapping_pud(p4dp, vaddr, next);