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Subject[PATCH v2] cgroup: fix cgroup_rmdir() vs close(eventfd) race
commit 205a872bd6f9a9a09ef035ef1e90185a8245cc58 ("cgroup: fix lockdep
warning for event_control") solved a deadlock by introducing a new
bug.

Move cgrp->event_list to a temporary list doesn't mean you can traverse
this list locklessly, because at the same time cgroup_event_wake() can
be called and remove the event from the list. The result of this race
is disastrous.

We adopt the way how kvm irqfd code implements race-free event removal,
which is now described in the comments in cgroup_event_wake().

Signed-off-by: Li Zefan <lizefan@huawei.com>
---
kernel/cgroup.c | 50 ++++++++++++++++++++++++++++++++++----------------
1 file changed, 34 insertions(+), 16 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 26c071c..65c8101 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -217,6 +217,10 @@ struct cgroup_event {
*/
struct list_head list;
/*
+ * Need to notify userspace when this event is removed?
+ */
+ bool signal_on_remove;
+ /*
* All fields below needed to unregister event when
* userspace closes eventfd.
*/
@@ -3833,8 +3837,17 @@ static void cgroup_event_remove(struct work_struct *work)
remove);
struct cgroup *cgrp = event->cgrp;

+ remove_wait_queue(event->wqh, &event->wait);
+
event->cft->unregister_event(cgrp, event->cft, event->eventfd);

+ /*
+ * If this event is to be removed due to cgroup removal,
+ * we notify userspace.
+ */
+ if (event->signal_on_remove)
+ eventfd_signal(event->eventfd, 1);
+
eventfd_ctx_put(event->eventfd);
kfree(event);
dput(cgrp->dentry);
@@ -3854,15 +3867,25 @@ static int cgroup_event_wake(wait_queue_t *wait, unsigned mode,
unsigned long flags = (unsigned long)key;

if (flags & POLLHUP) {
- __remove_wait_queue(event->wqh, &event->wait);
- spin_lock(&cgrp->event_list_lock);
- list_del_init(&event->list);
- spin_unlock(&cgrp->event_list_lock);
/*
- * We are in atomic context, but cgroup_event_remove() may
- * sleep, so we have to call it in workqueue.
+ * If the event has been detached at cgroup removal, we
+ * can simply return knowing the other side will cleanup
+ * for us.
+ *
+ * We can't race against event freeing since the other
+ * side will require wqh->lock via remove_wait_queue(),
+ * which we hold.
*/
- schedule_work(&event->remove);
+ spin_lock(&cgrp->event_list_lock);
+ if (!list_empty(&event->list)) {
+ list_del_init(&event->list);
+ /*
+ * We are in atomic context, but cgroup_event_remove()
+ * may sleep, so we have to call it in workqueue.
+ */
+ schedule_work(&event->remove);
+ }
+ spin_unlock(&cgrp->event_list_lock);
}

return 0;
@@ -4428,20 +4451,15 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
/*
* Unregister events and notify userspace.
* Notify userspace about cgroup removing only after rmdir of cgroup
- * directory to avoid race between userspace and kernelspace. Use
- * a temporary list to avoid a deadlock with cgroup_event_wake(). Since
- * cgroup_event_wake() is called with the wait queue head locked,
- * remove_wait_queue() cannot be called while holding event_list_lock.
+ * directory to avoid race between userspace and kernelspace.
*/
spin_lock(&cgrp->event_list_lock);
- list_splice_init(&cgrp->event_list, &tmp_list);
- spin_unlock(&cgrp->event_list_lock);
- list_for_each_entry_safe(event, tmp, &tmp_list, list) {
+ list_for_each_entry_safe(event, tmp, &cgrp->event_list, list) {
+ event->signal_on_remove = true;
list_del_init(&event->list);
- remove_wait_queue(event->wqh, &event->wait);
- eventfd_signal(event->eventfd, 1);
schedule_work(&event->remove);
}
+ spin_unlock(&cgrp->event_list_lock);

return 0;
}
--
1.8.0.2

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