arm64/mm: move runtime pgds to rodata

Now that deliberate writes to swapper_pg_dir are made via the fixmap, we
can defend against errant writes by moving it into the rodata section.
Since tramp_pg_dir and reserved_ttbr0 must be at a fixed offset from
swapper_pg_dir, and are not modified at runtime, these are also moved
into the rodata section. Likewise, idmap_pg_dir is not modified at
runtime, and is moved into rodata.

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

arch/arm64/kernel/head.S

@@ -287,18 +287,20 @@ __create_page_tables:
 	mov	x28, lr
 
 	/*
-	 * Invalidate the idmap and swapper page tables to avoid potential
-	 * dirty cache lines being evicted.
+	 * Invalidate the init page tables to avoid potential dirty cache lines
+	 * being evicted. Other page tables are allocated in rodata as part of
+	 * the kernel image, and thus are clean to the PoC per the boot
+	 * protocol.
 	 */
-	adrp	x0, idmap_pg_dir
+	adrp	x0, init_pg_dir
 	adrp	x1, init_pg_end
 	sub	x1, x1, x0
 	bl	__inval_dcache_area
 
 	/*
-	 * Clear the idmap and swapper page tables.
+	 * Clear the init page tables.
 	 */
-	adrp	x0, idmap_pg_dir
+	adrp	x0, init_pg_dir
 	adrp	x1, init_pg_end
 	sub	x1, x1, x0
 1:	stp	xzr, xzr, [x0], #16

arch/arm64/kernel/vmlinux.lds.S

@@ -138,6 +138,23 @@ SECTIONS
 	EXCEPTION_TABLE(8)		/* __init_begin will be marked RO NX */
 	NOTES
 
+	. = ALIGN(PAGE_SIZE);
+	idmap_pg_dir = .;
+	. += IDMAP_DIR_SIZE;
+
+#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
+	tramp_pg_dir = .;
+	. += PAGE_SIZE;
+#endif
+
+#ifdef CONFIG_ARM64_SW_TTBR0_PAN
+	reserved_ttbr0 = .;
+	. += RESERVED_TTBR0_SIZE;
+#endif
+	swapper_pg_dir = .;
+	. += PAGE_SIZE;
+	swapper_pg_end = .;
+
 	. = ALIGN(SEGMENT_ALIGN);
 	__init_begin = .;
 	__inittext_begin = .;
@@ -216,22 +233,6 @@ SECTIONS
 	BSS_SECTION(0, 0, 0)
 
 	. = ALIGN(PAGE_SIZE);
-	idmap_pg_dir = .;
-	. += IDMAP_DIR_SIZE;
-
-#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
-	tramp_pg_dir = .;
-	. += PAGE_SIZE;
-#endif
-
-#ifdef CONFIG_ARM64_SW_TTBR0_PAN
-	reserved_ttbr0 = .;
-	. += RESERVED_TTBR0_SIZE;
-#endif
-	swapper_pg_dir = .;
-	. += PAGE_SIZE;
-	swapper_pg_end = .;
-
 	init_pg_dir = .;
 	. += INIT_DIR_SIZE;
 	init_pg_end = .;