[PATCH RFC v3 2/4] mm/pgtable: Make pfn_pte() filter out huge page attributes

[Yin Tirui]

A fundamental principle of page table type safety is that `pte_t` represents
the lowest level page table entry and should never carry huge page attributes.

Currently, passing a pgprot with huge page bits (e.g., extracted via
pmd_pgprot()) into pfn_pte() creates a malformed PTE that retains the huge
attribute, leading to the necessity of the ugly `pte_clrhuge()` anti-pattern.

Enforce type safety by making `pfn_pte()` inherently filter out huge page
attributes:
- On x86: Strip the `_PAGE_PSE` bit.
- On ARM64: Mask out the block descriptor bits in `PTE_TYPE_MASK` and
  enforce the `PTE_TYPE_PAGE` format.
- On RISC-V: No changes required, as RISC-V leaf PMDs and PTEs share the
  exact same hardware format and do not use a distinct huge bit.

Signed-off-by: Yin Tirui <yintirui at huawei.com>
---
 arch/arm64/include/asm/pgtable.h | 4 +++-
 arch/x86/include/asm/pgtable.h   | 4 ++++
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index b3e58735c49b..f2a7a40106d2 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -141,7 +141,9 @@ static inline pteval_t __phys_to_pte_val(phys_addr_t phys)
 
 #define pte_pfn(pte)		(__pte_to_phys(pte) >> PAGE_SHIFT)
 #define pfn_pte(pfn,prot)	\
-	__pte(__phys_to_pte_val((phys_addr_t)(pfn) << PAGE_SHIFT) | pgprot_val(prot))
+	__pte(__phys_to_pte_val((phys_addr_t)(pfn) << PAGE_SHIFT) | \
+		((pgprot_val(prot) & ~(PTE_TYPE_MASK & ~PTE_VALID)) | \
+		(PTE_TYPE_PAGE & ~PTE_VALID)))
 
 #define pte_none(pte)		(!pte_val(pte))
 #define pte_page(pte)		(pfn_to_page(pte_pfn(pte)))
diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 1662c5a8f445..a4dbd81d42bf 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -738,6 +738,10 @@ static inline pgprotval_t check_pgprot(pgprot_t pgprot)
 static inline pte_t pfn_pte(unsigned long page_nr, pgprot_t pgprot)
 {
 	phys_addr_t pfn = (phys_addr_t)page_nr << PAGE_SHIFT;
+
+	/* Filter out _PAGE_PSE to ensure PTEs never carry the huge page bit */
+	pgprot = __pgprot(pgprot_val(pgprot) & ~_PAGE_PSE);
+
 	/* This bit combination is used to mark shadow stacks */
 	WARN_ON_ONCE((pgprot_val(pgprot) & (_PAGE_DIRTY | _PAGE_RW)) ==
 			_PAGE_DIRTY);
-- 
2.22.0