s390/mm: use invalid asce instead of kernel asce

Create a region 3 page table which contains only invalid entries, and
use that via "s390_invalid_asce" instead of the kernel ASCE whenever
there is either
- no user address space available, e.g. during early startup
- as an intermediate ASCE when address spaces are switched

This makes sure that user space accesses in such situations are
guaranteed to fail.
Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/include/asm/mmu_context.h

@@ -79,7 +79,7 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 	S390_lowcore.user_asce = next->context.asce;
 	cpumask_set_cpu(cpu, &next->context.cpu_attach_mask);
 	/* Clear previous user-ASCE from CR7 */
-	__ctl_load(S390_lowcore.kernel_asce, 7, 7);
+	__ctl_load(s390_invalid_asce, 7, 7);
 	if (prev != next)
 		cpumask_clear_cpu(cpu, &prev->context.cpu_attach_mask);
 }

arch/s390/include/asm/pgtable.h

@@ -23,6 +23,7 @@
 
 extern pgd_t swapper_pg_dir[];
 extern void paging_init(void);
+extern unsigned long s390_invalid_asce;
 
 enum {
 	PG_DIRECT_MAP_4K = 0,

arch/s390/kernel/smp.c

@@ -259,7 +259,7 @@ static void pcpu_prepare_secondary(struct pcpu *pcpu, int cpu)
 	lc->spinlock_index = 0;
 	lc->percpu_offset = __per_cpu_offset[cpu];
 	lc->kernel_asce = S390_lowcore.kernel_asce;
-	lc->user_asce = S390_lowcore.kernel_asce;
+	lc->user_asce = s390_invalid_asce;
 	lc->machine_flags = S390_lowcore.machine_flags;
 	lc->user_timer = lc->system_timer =
 		lc->steal_timer = lc->avg_steal_timer = 0;

arch/s390/kernel/vmlinux.lds.S

@@ -10,7 +10,8 @@
  * Put .bss..swapper_pg_dir as the first thing in .bss. This will
  * make sure it has 16k alignment.
  */
-#define BSS_FIRST_SECTIONS *(.bss..swapper_pg_dir)
+#define BSS_FIRST_SECTIONS *(.bss..swapper_pg_dir) \
+			   *(.bss..invalid_pg_dir)
 
 /* Handle ro_after_init data on our own. */
 #define RO_AFTER_INIT_DATA

arch/s390/mm/init.c

@@ -49,6 +49,9 @@
 #include <linux/virtio_config.h>
 
 pgd_t swapper_pg_dir[PTRS_PER_PGD] __section(".bss..swapper_pg_dir");
+static pgd_t invalid_pg_dir[PTRS_PER_PGD] __section(".bss..invalid_pg_dir");
+
+unsigned long s390_invalid_asce;
 
 unsigned long empty_zero_page, zero_page_mask;
 EXPORT_SYMBOL(empty_zero_page);
@@ -92,6 +95,9 @@ void __init paging_init(void)
 	unsigned long pgd_type, asce_bits;
 	psw_t psw;
 
+	s390_invalid_asce  = (unsigned long)invalid_pg_dir;
+	s390_invalid_asce |= _ASCE_TYPE_REGION3 | _ASCE_TABLE_LENGTH;
+	crst_table_init((unsigned long *)invalid_pg_dir, _REGION3_ENTRY_EMPTY);
 	init_mm.pgd = swapper_pg_dir;
 	if (VMALLOC_END > _REGION2_SIZE) {
 		asce_bits = _ASCE_TYPE_REGION2 | _ASCE_TABLE_LENGTH;
@@ -102,14 +108,14 @@ void __init paging_init(void)
 	}
 	init_mm.context.asce = (__pa(init_mm.pgd) & PAGE_MASK) | asce_bits;
 	S390_lowcore.kernel_asce = init_mm.context.asce;
-	S390_lowcore.user_asce = S390_lowcore.kernel_asce;
+	S390_lowcore.user_asce = s390_invalid_asce;
 	crst_table_init((unsigned long *) init_mm.pgd, pgd_type);
 	vmem_map_init();
 	kasan_copy_shadow_mapping();
 
 	/* enable virtual mapping in kernel mode */
 	__ctl_load(S390_lowcore.kernel_asce, 1, 1);
-	__ctl_load(S390_lowcore.kernel_asce, 7, 7);
+	__ctl_load(S390_lowcore.user_asce, 7, 7);
 	__ctl_load(S390_lowcore.kernel_asce, 13, 13);
 	psw.mask = __extract_psw();
 	psw_bits(psw).dat = 1;