arm64/mm: use fixmap to modify swapper_pg_dir

Once swapper_pg_dir is in the rodata section, it will not be possible to
modify it directly, but we will need to modify it in some cases.

To enable this, we can use the fixmap when deliberately modifying
swapper_pg_dir. As the pgd is only transiently mapped, this provides
some resilience against illicit modification of the pgd, e.g. for
Kernel Space Mirror Attack (KSMA).

Signed-off-by: Jun Yao <yaojun8558363@gmail.com>
Reviewed-by: James Morse <james.morse@arm.com>
[Mark: simplify ifdeffery, commit message]
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/mm/mmu.c

@@ -67,6 +67,24 @@ static pte_t bm_pte[PTRS_PER_PTE] __page_aligned_bss;
 static pmd_t bm_pmd[PTRS_PER_PMD] __page_aligned_bss __maybe_unused;
 static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss __maybe_unused;
 
+static DEFINE_SPINLOCK(swapper_pgdir_lock);
+
+void set_swapper_pgd(pgd_t *pgdp, pgd_t pgd)
+{
+	pgd_t *fixmap_pgdp;
+
+	spin_lock(&swapper_pgdir_lock);
+	fixmap_pgdp = pgd_set_fixmap(__pa(pgdp));
+	WRITE_ONCE(*fixmap_pgdp, pgd);
+	/*
+	 * We need dsb(ishst) here to ensure the page-table-walker sees
+	 * our new entry before set_p?d() returns. The fixmap's
+	 * flush_tlb_kernel_range() via clear_fixmap() does this for us.
+	 */
+	pgd_clear_fixmap();
+	spin_unlock(&swapper_pgdir_lock);
+}
+
 pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
 			      unsigned long size, pgprot_t vma_prot)
 {
@@ -629,8 +647,12 @@ static void __init map_kernel(pgd_t *pgdp)
  */
 void __init paging_init(void)
 {
-	map_kernel(swapper_pg_dir);
-	map_mem(swapper_pg_dir);
+	pgd_t *pgdp = pgd_set_fixmap(__pa_symbol(swapper_pg_dir));
+
+	map_kernel(pgdp);
+	map_mem(pgdp);
+
+	pgd_clear_fixmap();
 
 	cpu_replace_ttbr1(lm_alias(swapper_pg_dir));
 	init_mm.pgd = swapper_pg_dir;