arm64: Move fixmap and kasan page tables to end of kernel image

Move the fixmap and kasan page tables out of the BSS section, and place
them at the end of the image, right before the init_pg_dir section where
some of the other statically allocated page tables live.

These page tables are currently the only data objects in vmlinux that
are meant to be accessed via the kernel image's linear alias, and so
placing them together allows the remainder of the data/bss section to be
remapped read-only or unmapped entirely.

Reviewed-by: Kevin Brodsky <kevin.brodsky@arm.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>

Author: ardbiesheuvel

arch/arm64/include/asm/mmu.h

@@ -13,6 +13,8 @@
 
 #ifndef __ASSEMBLER__
 
+#define __pgtbl_bss __section(".pgdir.bss") __aligned(PAGE_SIZE)
+
 #include <linux/refcount.h>
 #include <asm/cpufeature.h>
 

arch/arm64/kernel/vmlinux.lds.S

@@ -349,9 +349,15 @@ SECTIONS
 	_edata = .;
 
 	/* start of zero-init region */
-	BSS_SECTION(SBSS_ALIGN, 0, 0)
+	BSS_SECTION(SBSS_ALIGN, 0, PAGE_SIZE)
 	__pi___bss_start = __bss_start;
 
+	/* fixmap BSS starts here - preceding data/BSS is omitted from the linear map */
+	.pgdir.bss (NOLOAD) : ALIGN(PAGE_SIZE) {
+		*(.pgdir.bss)
+	}
+	ASSERT(ADDR(.pgdir.bss) == __bss_stop, ".pgdir.bss must follow BSS")
+
 	. = ALIGN(PAGE_SIZE);
 	__pi_init_pg_dir = .;
 	. += INIT_DIR_SIZE;

arch/arm64/mm/fixmap.c

@@ -31,9 +31,9 @@ static_assert(NR_BM_PMD_TABLES == 1);
 
 #define BM_PTE_TABLE_IDX(addr)	__BM_TABLE_IDX(addr, PMD_SHIFT)
 
-static pte_t bm_pte[NR_BM_PTE_TABLES][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 pte_t bm_pte[NR_BM_PTE_TABLES][PTRS_PER_PTE] __pgtbl_bss;
+static pmd_t bm_pmd[PTRS_PER_PMD] __pgtbl_bss __maybe_unused;
+static pud_t bm_pud[PTRS_PER_PUD] __pgtbl_bss __maybe_unused;
 
 static inline pte_t *fixmap_pte(unsigned long addr)
 {

arch/arm64/mm/kasan_init.c

@@ -214,7 +214,7 @@ asmlinkage void __init kasan_early_init(void)
 		 * shadow pud_t[]/p4d_t[], which could end up getting corrupted
 		 * when the linear region is mapped.
 		 */
-		static pte_t tbl[PTRS_PER_PTE] __page_aligned_bss;
+		static pte_t tbl[PTRS_PER_PTE] __pgtbl_bss;
 		pgd_t *pgdp = pgd_offset_k(KASAN_SHADOW_START);
 
 		set_pgd(pgdp, __pgd(__pa_symbol(tbl) | PGD_TYPE_TABLE));