Re: [PATCH] tracing/perf: Fix double put of trace event when init fails

[Krister Johansen <kjlx@templeofstupid.com>]

On Tue, Aug 16, 2022 at 07:28:17PM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
> 
> If in perf_trace_event_init(), the perf_trace_event_open() fails, then it
> will call perf_trace_event_unreg() which will not only unregister the perf
> trace event, but will also call the put() function of the tp_event.
> 
> The problem here is that the trace_event_try_get_ref() is called by the
> caller of perf_trace_event_init() and if perf_trace_event_init() returns a
> failure, it will then call trace_event_put(). But since the
> perf_trace_event_unreg() already called the trace_event_put() function, it
> triggers a WARN_ON().
> 
>  WARNING: CPU: 1 PID: 30309 at kernel/trace/trace_dynevent.c:46 trace_event_dyn_put_ref+0x15/0x20
> 
> If perf_trace_event_reg() does not call the trace_event_try_get_ref() then
> the perf_trace_event_unreg() should not be calling trace_event_put(). This
> breaks symmetry and causes bugs like these.
> 
> Pull out the trace_event_put() from perf_trace_event_unreg() and call it
> in the locations that perf_trace_event_unreg() is called. This not only
> fixes this bug, but also brings back the proper symmetry of the reg/unreg
> vs get/put logic.
> 
> Link: https://lore.kernel.org/all/cover.1660347763.git.kjlx@templeofstupid.com/
> 
> Reported-by: Krister Johansen <kjlx@templeofstupid.com>
> Reviewed-by: Krister Johansen <kjlx@templeofstupid.com>
> Tested-by: Krister Johansen <kjlx@templeofstupid.com>
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Thanks again, Steven.  Is this one that you would consider tagging for a
backport to stable at the appropriate time? I believe this one showed up
in 5.15, if it's any help.

-K

> ---
>  kernel/trace/trace_event_perf.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/trace/trace_event_perf.c b/kernel/trace/trace_event_perf.c
> index a114549720d6..61e3a2620fa3 100644
> --- a/kernel/trace/trace_event_perf.c
> +++ b/kernel/trace/trace_event_perf.c
> @@ -157,7 +157,7 @@ static void perf_trace_event_unreg(struct perf_event *p_event)
>  	int i;
>  
>  	if (--tp_event->perf_refcount > 0)
> -		goto out;
> +		return;
>  
>  	tp_event->class->reg(tp_event, TRACE_REG_PERF_UNREGISTER, NULL);
>  
> @@ -176,8 +176,6 @@ static void perf_trace_event_unreg(struct perf_event *p_event)
>  			perf_trace_buf[i] = NULL;
>  		}
>  	}
> -out:
> -	trace_event_put_ref(tp_event);
>  }
>  
>  static int perf_trace_event_open(struct perf_event *p_event)
> @@ -241,6 +239,7 @@ void perf_trace_destroy(struct perf_event *p_event)
>  	mutex_lock(&event_mutex);
>  	perf_trace_event_close(p_event);
>  	perf_trace_event_unreg(p_event);
> +	trace_event_put_ref(p_event->tp_event);
>  	mutex_unlock(&event_mutex);
>  }
>  
> @@ -292,6 +291,7 @@ void perf_kprobe_destroy(struct perf_event *p_event)
>  	mutex_lock(&event_mutex);
>  	perf_trace_event_close(p_event);
>  	perf_trace_event_unreg(p_event);
> +	trace_event_put_ref(p_event->tp_event);
>  	mutex_unlock(&event_mutex);
>  
>  	destroy_local_trace_kprobe(p_event->tp_event);
> @@ -347,6 +347,7 @@ void perf_uprobe_destroy(struct perf_event *p_event)
>  	mutex_lock(&event_mutex);
>  	perf_trace_event_close(p_event);
>  	perf_trace_event_unreg(p_event);
> +	trace_event_put_ref(p_event->tp_event);
>  	mutex_unlock(&event_mutex);
>  	destroy_local_trace_uprobe(p_event->tp_event);
>  }
> -- 
> 2.35.1
>