linux-hardening - [RFC PATCH 09/12] arm64: mm: remap kernel page tables read-only at end of init

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Date:   Wed, 26 Jan 2022 18:30:08 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     linux-arm-kernel@...ts.infradead.org
Cc:     kvmarm@...ts.cs.columbia.edu, linux-hardening@...r.kernel.org,
        Ard Biesheuvel <ardb@...nel.org>,
        Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Fuad Tabba <tabba@...gle.com>,
        Quentin Perret <qperret@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        Catalin Marinas <catalin.marinas@....com>
Subject: [RFC PATCH 09/12] arm64: mm: remap kernel page tables read-only at end of init

Now that all the handling is in place to deal with read-only page tables
at runtime, do a pass over the kernel page tables at boot to remap all
the page table pages read-only that were allocated early.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/arm64/mm/mmu.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 971501535757..b1212f6d48f2 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -559,8 +559,23 @@ static void __init map_mem(pgd_t *pgdp)
 	memblock_clear_nomap(kernel_start, kernel_end - kernel_start);
 }
 
+static void mark_pgtables_ro(const pmd_t *pmdp, int level, int num_entries)
+{
+	while (num_entries--) {
+		if (pmd_valid(*pmdp) && pmd_table(*pmdp)) {
+			pmd_t *next = __va(__pmd_to_phys(*pmdp));
+
+			if (level < 2)
+				mark_pgtables_ro(next, level + 1, PTRS_PER_PMD);
+			set_pgtable_ro(next);
+		}
+		pmdp++;
+	}
+}
+
 void mark_rodata_ro(void)
 {
+	int pgd_level = 4 - CONFIG_PGTABLE_LEVELS;
 	unsigned long section_size;
 
 	/*
@@ -571,6 +586,11 @@ void mark_rodata_ro(void)
 	update_mapping_prot(__pa_symbol(__start_rodata), (unsigned long)__start_rodata,
 			    section_size, PAGE_KERNEL_RO);
 
+#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
+	mark_pgtables_ro((pmd_t *)&tramp_pg_dir, pgd_level, PTRS_PER_PGD);
+#endif
+	mark_pgtables_ro((pmd_t *)&swapper_pg_dir, pgd_level, PTRS_PER_PGD);
+
 	debug_checkwx();
 }
 
-- 
2.30.2