Subject: [RFC PATCH 07/12] arm64: mm: remap PTE level user page tables r/o in the linear region

From: Ard Biesheuvel <[email-masked]>
Date: Wed, 26 Jan 2022 18:30:06 +0100

Now that all PTE manipulations for user space tables go via the fixmap,
we can remap these tables read-only in the linear region so they cannot
be corrupted inadvertently.

Signed-off-by: Ard Biesheuvel <[email-masked]>
---
 arch/arm64/include/asm/pgalloc.h |  5 +++++
 arch/arm64/include/asm/tlb.h     |  2 ++
 arch/arm64/mm/mmu.c              | 23 ++++++++++++++++++++
 3 files changed, 30 insertions(+)

diff --git a/arch/arm64/include/asm/pgalloc.h b/arch/arm64/include/asm/pgalloc.h
index 63f9ae9e96fe..18a5bb0c9ee4 100644
--- a/arch/arm64/include/asm/pgalloc.h
+++ b/arch/arm64/include/asm/pgalloc.h
@@ -18,10 +18,15 @@
 #define __HAVE_ARCH_PUD_FREE
 #define __HAVE_ARCH_PMD_ALLOC_ONE
 #define __HAVE_ARCH_PMD_FREE
+#define __HAVE_ARCH_PTE_ALLOC_ONE
+#define __HAVE_ARCH_PTE_FREE
 #include <asm-generic/pgalloc.h>
 
 #define PGD_SIZE	(PTRS_PER_PGD * sizeof(pgd_t))
 
+pgtable_t pte_alloc_one(struct mm_struct *mm);
+void pte_free(struct mm_struct *mm, struct page *pte_page);
+
 #if CONFIG_PGTABLE_LEVELS > 2
 
 pmd_t *pmd_alloc_one(struct mm_struct *mm, unsigned long addr);
diff --git a/arch/arm64/include/asm/tlb.h b/arch/arm64/include/asm/tlb.h
index 0f54fbb59bba..e69a44160cce 100644
--- a/arch/arm64/include/asm/tlb.h
+++ b/arch/arm64/include/asm/tlb.h
@@ -75,6 +75,8 @@ static inline void tlb_flush(struct mmu_gather *tlb)
 static inline void __pte_free_tlb(struct mmu_gather *tlb, pgtable_t pte,
 				  unsigned long addr)
 {
+	if (page_tables_are_ro())
+		set_pgtable_rw(page_address(pte));
 	pgtable_pte_page_dtor(pte);
 	tlb_remove_table(tlb, pte);
 }
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index e55d91a5f1ed..949846654797 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -1686,3 +1686,26 @@ void pmd_free(struct mm_struct *mm, pmd_t *pmd)
 	free_page((u64)pmd);
 }
 #endif
+
+pgtable_t pte_alloc_one(struct mm_struct *mm)
+{
+	pgtable_t pgt = __pte_alloc_one(mm, GFP_PGTABLE_USER);
+
+	VM_BUG_ON(mm == &init_mm);
+
+	if (!pgt)
+		return NULL;
+	if (page_tables_are_ro())
+		set_pgtable_ro(page_address(pgt));
+	return pgt;
+}
+
+void pte_free(struct mm_struct *mm, struct page *pte_page)
+{
+	VM_BUG_ON(mm == &init_mm);
+
+	if (page_tables_are_ro())
+		set_pgtable_rw(page_address(pte_page));
+	pgtable_pte_page_dtor(pte_page);
+	__free_page(pte_page);
+}
-- 
2.30.2

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