x86: avoid dereferencing beyond stack + THREAD_SIZE

It's possible for get_wchan() to dereference past task->stack + THREAD_SIZE
while iterating through instruction pointers if fp equals the upper boundary,
causing a kernel panic.
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/process_64.c

@@ -754,12 +754,12 @@ unsigned long get_wchan(struct task_struct *p)
 	if (!p || p == current || p->state == TASK_RUNNING)
 		return 0;
 	stack = (unsigned long)task_stack_page(p);
-	if (p->thread.sp < stack || p->thread.sp > stack+THREAD_SIZE)
+	if (p->thread.sp < stack || p->thread.sp >= stack+THREAD_SIZE)
 		return 0;
 	fp = *(u64 *)(p->thread.sp);
 	do {
 		if (fp < (unsigned long)stack ||
-		    fp > (unsigned long)stack+THREAD_SIZE)
+		    fp >= (unsigned long)stack+THREAD_SIZE)
 			return 0;
 		ip = *(u64 *)(fp+8);
 		if (!in_sched_functions(ip))