On 4/9/2018 4:38 PM, Peter Zijlstra wrote:
On Tue, Mar 06, 2018 at 06:49:10PM +0900, Prashant Bhole wrote:
Sorry for late reply. I tried these changes. It didn't fix the problem. With

He, sorry for completely forgetting about this one :/

these changes, the use-after-free access of task_struct occurs at
_free_event() for the last remaining event.

In your changes, I tried keeping get/put_task_struct() in
perf_alloc_context()/put_ctx() intact and The problem did not occur. Changes
are mentioned below.

Yes, I think you're right in that this is the cleanest solution; it adds
reference counting to the exact pointer we're using.

diff --git a/kernel/events/core.c b/kernel/events/core.c
index c98cce4ceebd..65889d2b5ae2 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4109,6 +4109,8 @@ static void _free_event(struct perf_event *event)

        if (event->ctx)
+       if (event->hw.target)
+               put_task_struct(event->hw.target);

@@ -9593,6 +9595,7 @@ perf_event_alloc(struct perf_event_attr *attr, int
                 * and we cannot use the ctx information because we need the
                 * pmu before we get a ctx.
+               get_task_struct(task);
                event->hw.target = task;

@@ -9708,6 +9711,8 @@ perf_event_alloc(struct perf_event_attr *attr, int
        if (event->ns)
+       if (task)

Should this not too be 'event->hw.target', for consistency and clarity?

Yes, I am sending a patch with this change. Thanks.


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