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Subject[PATCH v2 2/2] epoll: ep_unregister_pollwait() can use the freed pwq->whead
signalfd_cleanup() ensures that ->signalfd_wqh is not used, but
this is not enough. eppoll_entry->whead still points to the memory
we are going to free, ep_unregister_pollwait()->remove_wait_queue()
is obviously unsafe.

Change ep_poll_callback(POLLFREE) to set eppoll_entry->whead = NULL,
change ep_unregister_pollwait() to check pwq->whead != NULL under
rcu_read_lock() before remove_wait_queue(). We add the new helper,
ep_remove_wait_queue(), for this.

This works because sighand_cachep is SLAB_DESTROY_BY_RCU and because
->signalfd_wqh is initialized in sighand_ctor(), not in copy_sighand.
ep_unregister_pollwait()->remove_wait_queue() can play with already
freed and potentially reused ->sighand, but this is fine. This memory
must have the valid ->signalfd_wqh until rcu_read_unlock().

Reported-by: Maxime Bizon <mbizon@freebox.fr>
Cc: <stable@kernel.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
fs/eventpoll.c | 30 +++++++++++++++++++++++++++---
fs/signalfd.c | 6 +++++-
2 files changed, 32 insertions(+), 4 deletions(-)
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 34bbfc6..ea54cde 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -320,6 +320,11 @@ static inline int ep_is_linked(struct list_head *p)
return !list_empty(p);
}

+static inline struct eppoll_entry *ep_pwq_from_wait(wait_queue_t *p)
+{
+ return container_of(p, struct eppoll_entry, wait);
+}
+
/* Get the "struct epitem" from a wait queue pointer */
static inline struct epitem *ep_item_from_wait(wait_queue_t *p)
{
@@ -467,6 +472,18 @@ static void ep_poll_safewake(wait_queue_head_t *wq)
put_cpu();
}

+static void ep_remove_wait_queue(struct eppoll_entry *pwq)
+{
+ wait_queue_head_t *whead;
+
+ rcu_read_lock();
+ /* If it is cleared by POLLFREE, it should be rcu-safe */
+ whead = rcu_dereference(pwq->whead);
+ if (whead)
+ remove_wait_queue(whead, &pwq->wait);
+ rcu_read_unlock();
+}
+
/*
* This function unregisters poll callbacks from the associated file
* descriptor. Must be called with "mtx" held (or "epmutex" if called from
@@ -481,7 +498,7 @@ static void ep_unregister_pollwait(struct eventpoll *ep, struct epitem *epi)
pwq = list_first_entry(lsthead, struct eppoll_entry, llink);

list_del(&pwq->llink);
- remove_wait_queue(pwq->whead, &pwq->wait);
+ ep_remove_wait_queue(pwq);
kmem_cache_free(pwq_cache, pwq);
}
}
@@ -842,9 +859,16 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *k
struct epitem *epi = ep_item_from_wait(wait);
struct eventpoll *ep = epi->ep;

- /* the caller holds eppoll_entry->whead->lock */
- if ((unsigned long)key & POLLFREE)
+ if ((unsigned long)key & POLLFREE) {
+ ep_pwq_from_wait(wait)->whead = NULL;
+ /*
+ * whead = NULL above can race with ep_remove_wait_queue()
+ * which can do another remove_wait_queue() after us, so we
+ * can't use __remove_wait_queue(). whead->lock is held by
+ * the caller.
+ */
list_del_init(&wait->task_list);
+ }

spin_lock_irqsave(&ep->lock, flags);

diff --git a/fs/signalfd.c b/fs/signalfd.c
index 79c1eea..7ae2a57 100644
--- a/fs/signalfd.c
+++ b/fs/signalfd.c
@@ -33,7 +33,11 @@
void signalfd_cleanup(struct sighand_struct *sighand)
{
wait_queue_head_t *wqh = &sighand->signalfd_wqh;
-
+ /*
+ * The lockless check can race with remove_wait_queue() in progress,
+ * but in this case its caller should run under rcu_read_lock() and
+ * sighand_cachep is SLAB_DESTROY_BY_RCU, we can safely return.
+ */
if (likely(!waitqueue_active(wqh)))
return;

--
1.5.5.1



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