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SubjectRe: [RFC PATCH] perf: Paper over the hw.target problems
On Mon, Jun 24, 2019 at 02:19:02PM +0200, Peter Zijlstra wrote:
>
> close() clone()
>
> copy_process()
> perf_event_init_task()
> perf_event_init_context()
> mutex_lock(parent_ctx->mutex)
> inherit_task_group()
> inherit_group()
> inherit_event()
> mutex_lock(event->child_mutex)
> // expose event on child list
> list_add_tail()
> mutex_unlock(event->child_mutex)
> mutex_unlock(parent_ctx->mutex)
>
> ...
> goto bad_fork_*
>
> bad_fork_cleanup_perf:
> perf_event_free_task()
>
> perf_release()
> perf_event_release_kernel()
> list_for_each_entry()
> mutex_lock(ctx->mutex)
> mutex_lock(event->child_mutex)
> // event is from the failing inherit
> // on the other CPU
> perf_remove_from_context()
> list_move()
> mutex_unlock(event->child_mutex)
> mutex_unlock(ctx->mutex)
>
> mutex_lock(ctx->mutex)
> list_for_each_entry_safe()
> // event already stolen
> mutex_unlock(ctx->mutex)
>
> delayed_free_task()
> free_task()
>
> list_for_each_entry_safe()
> list_del()
> free_event()
> _free_event()
> // and so event->hw.target
> // is the already freed failed clone()
> if (event->hw.target)
> put_task_struct(event->hw.target)
> // WHOOPSIE, already quite dead
>
>
> Which puts the lie to the the comment on perf_event_free_task();
> 'unexposed, unused context' my ass.
>
> Which is a 'fun' confluence of fail; copy_process() doing an
> unconditional free_task() and not respecting refcounts, and perf having
> creative locking. In particular:
>
> 82d94856fa22 ("perf/core: Fix lock inversion between perf,trace,cpuhp")
>
> seems to have overlooked this 'fun' parade.

The below seems to cure things; it uses the fact that detached events
still have a reference count on their context.

So perf_event_free_task() can detect when (child) events have gotten
stolen and wait for it.

The below (which includes a debug printk) confirms the reproducer
triggered the problem by printing a 2 (where a 1 is expected).

Thoughts?

---
kernel/events/core.c | 34 ++++++++++++++++++++++++++++++----
1 file changed, 30 insertions(+), 4 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 10c1dba9068c..e19d036125d1 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4463,12 +4463,16 @@ static void _free_event(struct perf_event *event)
if (event->destroy)
event->destroy(event);

- if (event->ctx)
- put_ctx(event->ctx);
-
+ /*
+ * Must be after ->destroy(), due to uprobe_perf_close() using
+ * hw.target, but before put_ctx() because of perf_event_free_task().
+ */
if (event->hw.target)
put_task_struct(event->hw.target);

+ if (event->ctx)
+ put_ctx(event->ctx);
+
exclusive_event_destroy(event);
module_put(event->pmu->module);

@@ -4648,10 +4652,20 @@ int perf_event_release_kernel(struct perf_event *event)
mutex_unlock(&event->child_mutex);

list_for_each_entry_safe(child, tmp, &free_list, child_list) {
+ void *var = &child->ctx->refcount;
+
list_del(&child->child_list);
free_event(child);
+
+ /*
+ * Wake any perf_event_free_task() waiting for this event to be
+ * freed.
+ */
+ smp_mb(); /* pairs with wait_var_event() */
+ wake_up_var(var);
}

+
no_ctx:
put_event(event); /* Must be the 'last' reference */
return 0;
@@ -11546,7 +11560,19 @@ void perf_event_free_task(struct task_struct *task)
perf_free_event(event, ctx);

mutex_unlock(&ctx->mutex);
- put_ctx(ctx);
+
+ /*
+ * perf_event_release_kernel() could've stolen some of our
+ * child events and still have them on its free_list. In that
+ * case it can do free_event() after the failing copy_process()
+ * has already freed the task, and get a UaF on
+ * event->hw.target.
+ *
+ * Wait for all events to drop their context reference.
+ */
+ printk("%d\n", refcount_read(&ctx->refcount));
+ wait_var_event(&ctx->refcount, refcount_read(&ctx->refcount) == 1);
+ put_ctx(ctx); /* must be last */
}
}

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