s390/mm: add debug user asce support

Verify on exit to user space that always
- the primary ASCE (cr1) is set to kernel ASCE
- the secondary ASCE (cr7) is set to user ASCE

If this is not the case: panic since something went terribly wrong.
Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/Kconfig.debug

@@ -5,3 +5,11 @@ config TRACE_IRQFLAGS_SUPPORT
 
 config EARLY_PRINTK
 	def_bool y
+
+config DEBUG_USER_ASCE
+	bool "Debug User ASCE"
+	help
+	  Check on exit to user space that address space control
+	  elements are setup correctly.
+
+	  If unsure, say N.

arch/s390/configs/debug_defconfig

@@ -826,6 +826,7 @@ CONFIG_FTRACE_SYSCALLS=y
 CONFIG_BLK_DEV_IO_TRACE=y
 CONFIG_BPF_KPROBE_OVERRIDE=y
 CONFIG_HIST_TRIGGERS=y
+CONFIG_DEBUG_USER_ASCE=y
 CONFIG_NOTIFIER_ERROR_INJECTION=m
 CONFIG_NETDEV_NOTIFIER_ERROR_INJECT=m
 CONFIG_FAULT_INJECTION=y

arch/s390/include/asm/uaccess.h

@@ -18,6 +18,8 @@
 #include <asm/extable.h>
 #include <asm/facility.h>
 
+void debug_user_asce(void);
+
 static inline int __range_ok(unsigned long addr, unsigned long size)
 {
 	return 1;

arch/s390/kernel/entry.S

@@ -90,6 +90,12 @@ _LPP_OFFSET	= __LC_LPP
 #endif
 	.endm
 
+	.macro	DEBUG_USER_ASCE
+#ifdef CONFIG_DEBUG_USER_ASCE
+	brasl	%r14,debug_user_asce
+#endif
+	.endm
+
 	.macro	CHECK_VMAP_STACK savearea,oklabel
 #ifdef CONFIG_VMAP_STACK
 	lgr	%r14,%r15
@@ -428,6 +434,7 @@ ENTRY(system_call)
 	jnz	.Lsysc_work
 	TSTMSK	__TI_flags(%r12),_TIF_WORK
 	jnz	.Lsysc_work			# check for work
+	DEBUG_USER_ASCE
 	lctlg	%c1,%c1,__LC_USER_ASCE
 	BPEXIT	__TI_flags(%r12),_TIF_ISOLATE_BP
 	TSTMSK	__LC_CPU_FLAGS, _CIF_FPU
@@ -793,6 +800,7 @@ ENTRY(io_int_handler)
 	mvc	__LC_RETURN_PSW(16),__PT_PSW(%r11)
 	tm	__PT_PSW+1(%r11),0x01	# returning to user ?
 	jno	.Lio_exit_kernel
+	DEBUG_USER_ASCE
 	lctlg	%c1,%c1,__LC_USER_ASCE
 	BPEXIT	__TI_flags(%r12),_TIF_ISOLATE_BP
 	stpt	__LC_EXIT_TIMER

arch/s390/lib/uaccess.c

@@ -16,6 +16,22 @@
 #include <asm/mmu_context.h>
 #include <asm/facility.h>
 
+#ifdef CONFIG_DEBUG_USER_ASCE
+void debug_user_asce(void)
+{
+	unsigned long cr1, cr7;
+
+	__ctl_store(cr1, 1, 1);
+	__ctl_store(cr7, 7, 7);
+	if (cr1 == S390_lowcore.kernel_asce && cr7 == S390_lowcore.user_asce)
+		return;
+	panic("incorrect ASCE on kernel exit\n"
+	      "cr1:    %016lx cr7:  %016lx\n"
+	      "kernel: %016llx user: %016llx\n",
+	      cr1, cr7, S390_lowcore.kernel_asce, S390_lowcore.user_asce);
+}
+#endif /*CONFIG_DEBUG_USER_ASCE */
+
 #ifndef CONFIG_HAVE_MARCH_Z10_FEATURES
 static DEFINE_STATIC_KEY_FALSE(have_mvcos);