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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 32 insertions, 4 deletions

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 34bbfc6dd8dc..ea54cdef04dd 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 79c1eea98a3a..7ae2a574cb25 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;