Re: use-after-free in __perf_install_in_context

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Dec 10, 2015 at 08:57:40PM +0100, Peter Zijlstra wrote:
> I'm not sure I can explain your problem with this, but I figure its
> worth a try.

Did it make a difference?

Also, does the syzkaller thing do cpu-hotplug during its runs?

> ---
>  kernel/events/core.c | 15 +++++----------
>  1 file changed, 5 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index c3d61b92d805..d5293325d8c5 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3136,15 +3136,16 @@ static int event_enable_on_exec(struct perf_event *event,
>   * Enable all of a task's events that have been marked enable-on-exec.
>   * This expects task == current.
>   */
> -static void perf_event_enable_on_exec(struct perf_event_context *ctx)
> +static void perf_event_enable_on_exec(int ctxn)
>  {
> -	struct perf_event_context *clone_ctx = NULL;
> +	struct perf_event_context *ctx, *clone_ctx = NULL;
>  	struct perf_event *event;
>  	unsigned long flags;
>  	int enabled = 0;
>  	int ret;
>  
>  	local_irq_save(flags);
> +	ctx = current->perf_event_ctxp[ctxn];
>  	if (!ctx || !ctx->nr_events)
>  		goto out;
>  
> @@ -3187,17 +3188,11 @@ static void perf_event_enable_on_exec(struct perf_event_context *ctx)
>  
>  void perf_event_exec(void)
>  {
> -	struct perf_event_context *ctx;
>  	int ctxn;
>  
>  	rcu_read_lock();
> -	for_each_task_context_nr(ctxn) {
> -		ctx = current->perf_event_ctxp[ctxn];
> -		if (!ctx)
> -			continue;
> -
> -		perf_event_enable_on_exec(ctx);
> -	}
> +	for_each_task_context_nr(ctxn)
> +		perf_event_enable_on_exec(ctxn);
>  	rcu_read_unlock();
>  }
>