[POWERPC] Harden validate_sp against stack corruption

If something has overflowed or corrupted the stack and causes an oops,
and we try to print a stack trace, that will call validate_sp, which
can itself cause an oops if the cpu field of the thread_info struct at
the bottom of the stack has been corrupted (if CONFIG_IRQSTACKS is
set).  This makes debugging harder.

To avoid the second oops, this adds a check to make sure that the cpu
number is reasonable before using it to check whether the stack is on
the softirq or hardirq stack.
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/process.c

@@ -818,28 +818,45 @@ int sys_execve(unsigned long a0, unsigned long a1, unsigned long a2,
 	return error;
 }
 
-int validate_sp(unsigned long sp, struct task_struct *p,
+#ifdef CONFIG_IRQSTACKS
+static inline int valid_irq_stack(unsigned long sp, struct task_struct *p,
 				  unsigned long nbytes)
 {
-	unsigned long stack_page = (unsigned long)task_stack_page(p);
+	unsigned long stack_page;
+	unsigned long cpu = task_cpu(p);
 
+	/*
+	 * Avoid crashing if the stack has overflowed and corrupted
+	 * task_cpu(p), which is in the thread_info struct.
+	 */
+	if (cpu < NR_CPUS && cpu_possible(cpu)) {
+		stack_page = (unsigned long) hardirq_ctx[cpu];
 		if (sp >= stack_page + sizeof(struct thread_struct)
 		    && sp <= stack_page + THREAD_SIZE - nbytes)
 			return 1;
 
-#ifdef CONFIG_IRQSTACKS
-	stack_page = (unsigned long) hardirq_ctx[task_cpu(p)];
+		stack_page = (unsigned long) softirq_ctx[cpu];
 		if (sp >= stack_page + sizeof(struct thread_struct)
 		    && sp <= stack_page + THREAD_SIZE - nbytes)
 			return 1;
+	}
+	return 0;
+}
+
+#else
+#define valid_irq_stack(sp, p, nb)	0
+#endif /* CONFIG_IRQSTACKS */
+
+int validate_sp(unsigned long sp, struct task_struct *p,
+		       unsigned long nbytes)
+{
+	unsigned long stack_page = (unsigned long)task_stack_page(p);
 
-	stack_page = (unsigned long) softirq_ctx[task_cpu(p)];
 	if (sp >= stack_page + sizeof(struct thread_struct)
 	    && sp <= stack_page + THREAD_SIZE - nbytes)
 		return 1;
-#endif
 
-	return 0;
+	return valid_irq_stack(sp, p, nbytes);
 }
 
 #ifdef CONFIG_PPC64