[PATCH] s390: show_task oops

The show_task function walks the kernel stack backchain of processes assuming
that the processes are not running.  Since this assumption is not correct
walking the backchain can lead to an addressing exception and therefore to a
kernel hang.  So prevent the kernel hang (you still get incorrect results)
verity that all read accesses are within the bounds of the kernel stack before
performing them.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/s390/kernel/process.c

@@ -58,10 +58,18 @@ asmlinkage void ret_from_fork(void) __asm__("ret_from_fork");
  */
 unsigned long thread_saved_pc(struct task_struct *tsk)
 {
-	struct stack_frame *sf;
+	struct stack_frame *sf, *low, *high;
 
-	sf = (struct stack_frame *) tsk->thread.ksp;
-	sf = (struct stack_frame *) sf->back_chain;
+	if (!tsk || !task_stack_page(tsk))
+		return 0;
+	low = task_stack_page(tsk);
+	high = (struct stack_frame *) task_pt_regs(tsk);
+	sf = (struct stack_frame *) (tsk->thread.ksp & PSW_ADDR_INSN);
+	if (sf <= low || sf > high)
+		return 0;
+	sf = (struct stack_frame *) (sf->back_chain & PSW_ADDR_INSN);
+	if (sf <= low || sf > high)
+		return 0;
 	return sf->gprs[8];
 }