s390: enable HAVE_ARCH_STACKLEAK

Add support for the stackleak feature. Whenever the kernel returns to user
space the kernel stack is filled with a poison value.

Enabling this feature is quite expensive: e.g. after instrumenting the
getpid() system call function to have a 4kb stack the result is an
increased runtime of the system call by a factor of 3.

Reviewed-by: Vasily Gorbik <[e-mail]>
Signed-off-by: Heiko Carstens <[e-mail]>
Signed-off-by: Vasily Gorbik <[e-mail]>

arch/s390/Kconfig

···
 	select HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
 	select HAVE_ARCH_SECCOMP_FILTER
 	select HAVE_ARCH_SOFT_DIRTY
+	select HAVE_ARCH_STACKLEAK
 	select HAVE_ARCH_TRACEHOOK
 	select HAVE_ARCH_TRANSPARENT_HUGEPAGE
 	select HAVE_ARCH_VMAP_STACK

arch/s390/kernel/entry.S

···
 	.endm
 #endif
 
+	.macro STACKLEAK_ERASE
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+	brasl	%r14,stackleak_erase_on_task_stack
+#endif
+	.endm
+
 	GEN_BR_THUNK %r14
 
 	.section .kprobes.text, "ax"
···
 	MBEAR	%r2
 	lgr	%r3,%r14
 	brasl	%r14,__do_syscall
+	STACKLEAK_ERASE
 	lctlg	%c1,%c1,__LC_USER_ASCE
 	mvc	__LC_RETURN_PSW(16),STACK_FRAME_OVERHEAD+__PT_PSW(%r15)
 	BPON
···
 ENTRY(ret_from_fork)
 	lgr	%r3,%r11
 	brasl	%r14,__ret_from_fork
+	STACKLEAK_ERASE
 	lctlg	%c1,%c1,__LC_USER_ASCE
 	mvc	__LC_RETURN_PSW(16),STACK_FRAME_OVERHEAD+__PT_PSW(%r15)
 	BPON
···
 	brasl	%r14,__do_pgm_check
 	tmhh	%r8,0x0001		# returning to user space?
 	jno	.Lpgm_exit_kernel
+	STACKLEAK_ERASE
 	lctlg	%c1,%c1,__LC_USER_ASCE
 	BPON
 	stpt	__LC_EXIT_TIMER
···
 	mvc	__LC_RETURN_PSW(16),__PT_PSW(%r11)
 	tmhh	%r8,0x0001		# returning to user ?
 	jno	2f
+	STACKLEAK_ERASE
 	lctlg	%c1,%c1,__LC_USER_ASCE
 	BPON
 	stpt	__LC_EXIT_TIMER