ARM: 7912/1: check stack pointer in get_wchan

get_wchan() is lockless. Task may wakeup at any time and change its own stack,
thus each next stack frame may be overwritten and filled with random stuff.

/proc/$pid/stack interface had been disabled for non-current tasks, see [1]
But 'wchan' still allows to trigger stack frame unwinding on volatile stack.

This patch fixes oops in unwind_frame() by adding stack pointer validation on
each step (as x86 code do), unwind_frame() already checks frame pointer.

Also I've found another report of this oops on stackoverflow (irony).

Link: http://www.spinics.net/lists/arm-kernel/msg110589.html [1]
Link: http://stackoverflow.com/questions/18479894/unwind-frame-cause-a-kernel-paging-error

Cc: <stable@vger.kernel.org>
Signed-off-by: Konstantin Khlebnikov <k.khlebnikov@samsung.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/process.c

@@ -404,6 +404,7 @@ EXPORT_SYMBOL(dump_fpu);
 unsigned long get_wchan(struct task_struct *p)
 {
 	struct stackframe frame;
+	unsigned long stack_page;
 	int count = 0;
 	if (!p || p == current || p->state == TASK_RUNNING)
 		return 0;
@@ -412,9 +413,11 @@ unsigned long get_wchan(struct task_struct *p)
 	frame.sp = thread_saved_sp(p);
 	frame.lr = 0;			/* recovered from the stack */
 	frame.pc = thread_saved_pc(p);
+	stack_page = (unsigned long)task_stack_page(p);
 	do {
-		int ret = unwind_frame(&frame);
-		if (ret < 0)
+		if (frame.sp < stack_page ||
+		    frame.sp >= stack_page + THREAD_SIZE ||
+		    unwind_frame(&frame) < 0)
 			return 0;
 		if (!in_sched_functions(frame.pc))
 			return frame.pc;