tracing/kprobes: Check whether get_kretprobe() returns NULL in kretprobe_dispatcher()

There is a small chance that get_kretprobe(ri) returns NULL in
kretprobe_dispatcher() when another CPU unregisters the kretprobe
right after __kretprobe_trampoline_handler().

To avoid this issue, kretprobe_dispatcher() checks the get_kretprobe()
return value again. And if it is NULL, it returns soon because that
kretprobe is under unregistering process.

This issue has been introduced when the kretprobe is decoupled
from the struct kretprobe_instance by commit d741bf41d7
("kprobes: Remove kretprobe hash"). Before that commit, the
struct kretprob_instance::rp directly points the kretprobe
and it is never be NULL.

Link: https://lkml.kernel.org/r/165366693881.797669.16926184644089588731.stgit@devnote2

Reported-by: Yonghong Song <yhs@fb.com>
Fixes: d741bf41d7 ("kprobes: Remove kretprobe hash")
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: bpf <bpf@vger.kernel.org>
Cc: Kernel Team <kernel-team@fb.com>
Cc: stable@vger.kernel.org
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

kernel/trace/trace_kprobe.c

@@ -1718,8 +1718,17 @@ static int
 kretprobe_dispatcher(struct kretprobe_instance *ri, struct pt_regs *regs)
 {
 	struct kretprobe *rp = get_kretprobe(ri);
-	struct trace_kprobe *tk = container_of(rp, struct trace_kprobe, rp);
+	struct trace_kprobe *tk;
 
+	/*
+	 * There is a small chance that get_kretprobe(ri) returns NULL when
+	 * the kretprobe is unregister on another CPU between kretprobe's
+	 * trampoline_handler and this function.
+	 */
+	if (unlikely(!rp))
+		return 0;
+
+	tk = container_of(rp, struct trace_kprobe, rp);
 	raw_cpu_inc(*tk->nhit);
 
 	if (trace_probe_test_flag(&tk->tp, TP_FLAG_TRACE))