x86/stacktrace: Prevent infinite loop in arch_stack_walk_user()

arch_stack_walk_user() checks `if (fp == frame.next_fp)` to prevent a
infinite loop by self reference but it's not enogh for circular reference.

Once a lack of return address is found, there is no point to continue the
loop, so break out.

Fixes: 02b67518 ("tracing: add support for userspace stacktraces in tracing/iter_ctrl")
Signed-off-by: Eiichi Tsukata <devel@etsukata.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lkml.kernel.org/r/20190711023501.963-1-devel@etsukata.com

arch/x86/kernel/stacktrace.c

@@ -129,11 +129,9 @@ void arch_stack_walk_user(stack_trace_consume_fn consume_entry, void *cookie,
 			break;
 		if ((unsigned long)fp < regs->sp)
 			break;
-		if (frame.ret_addr) {
+		if (!frame.ret_addr)
+			break;
 		if (!consume_entry(cookie, frame.ret_addr, false))
-				return;
-		}
-		if (fp == frame.next_fp)
 			break;
 		fp = frame.next_fp;
 	}