Merge tag 'aio-poll-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux

Pull aio poll fixes from Eric Biggers:
 "Fix three bugs in aio poll, and one issue with POLLFREE more broadly:

   - aio poll didn't handle POLLFREE, causing a use-after-free.

   - aio poll could block while the file is ready.

   - aio poll called eventfd_signal() when it isn't allowed.

   - POLLFREE didn't handle multiple exclusive waiters correctly.

  This has been tested with the libaio test suite, as well as with test
  programs I wrote that reproduce the first two bugs. I am sending this
  pull request myself as no one seems to be maintaining this code"

* tag 'aio-poll-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux:
  aio: Fix incorrect usage of eventfd_signal_allowed()
  aio: fix use-after-free due to missing POLLFREE handling
  aio: keep poll requests on waitqueue until completed
  signalfd: use wake_up_pollfree()
  binder: use wake_up_pollfree()
  wait: add wake_up_pollfree()

Author: torvalds

drivers/android/binder.c

@@ -4422,23 +4422,20 @@ static int binder_thread_release(struct binder_proc *proc,
 	__release(&t->lock);
 
 	/*
-	 * If this thread used poll, make sure we remove the waitqueue
-	 * from any epoll data structures holding it with POLLFREE.
-	 * waitqueue_active() is safe to use here because we're holding
-	 * the inner lock.
+	 * If this thread used poll, make sure we remove the waitqueue from any
+	 * poll data structures holding it.
 	 */
-	if ((thread->looper & BINDER_LOOPER_STATE_POLL) &&
-	    waitqueue_active(&thread->wait)) {
-		wake_up_poll(&thread->wait, EPOLLHUP | POLLFREE);
-	}
+	if (thread->looper & BINDER_LOOPER_STATE_POLL)
+		wake_up_pollfree(&thread->wait);
 
 	binder_inner_proc_unlock(thread->proc);
 
 	/*
-	 * This is needed to avoid races between wake_up_poll() above and
-	 * and ep_remove_waitqueue() called for other reasons (eg the epoll file
-	 * descriptor being closed); ep_remove_waitqueue() holds an RCU read
-	 * lock, so we can be sure it's done after calling synchronize_rcu().
+	 * This is needed to avoid races between wake_up_pollfree() above and
+	 * someone else removing the last entry from the queue for other reasons
+	 * (e.g. ep_remove_wait_queue() being called due to an epoll file
+	 * descriptor being closed).  Such other users hold an RCU read lock, so
+	 * we can be sure they're done after we call synchronize_rcu().
 	 */
 	if (thread->looper & BINDER_LOOPER_STATE_POLL)
 		synchronize_rcu();

fs/aio.c

@@ -181,8 +181,9 @@ struct poll_iocb {
 	struct file		*file;
 	struct wait_queue_head	*head;
 	__poll_t		events;
-	bool			done;
 	bool			cancelled;
+	bool			work_scheduled;
+	bool			work_need_resched;
 	struct wait_queue_entry	wait;
 	struct work_struct	work;
 };
@@ -1619,6 +1620,51 @@ static void aio_poll_put_work(struct work_struct *work)
 	iocb_put(iocb);
 }
 
+/*
+ * Safely lock the waitqueue which the request is on, synchronizing with the
+ * case where the ->poll() provider decides to free its waitqueue early.
+ *
+ * Returns true on success, meaning that req->head->lock was locked, req->wait
+ * is on req->head, and an RCU read lock was taken.  Returns false if the
+ * request was already removed from its waitqueue (which might no longer exist).
+ */
+static bool poll_iocb_lock_wq(struct poll_iocb *req)
+{
+	wait_queue_head_t *head;
+
+	/*
+	 * While we hold the waitqueue lock and the waitqueue is nonempty,
+	 * wake_up_pollfree() will wait for us.  However, taking the waitqueue
+	 * lock in the first place can race with the waitqueue being freed.
+	 *
+	 * We solve this as eventpoll does: by taking advantage of the fact that
+	 * all users of wake_up_pollfree() will RCU-delay the actual free.  If
+	 * we enter rcu_read_lock() and see that the pointer to the queue is
+	 * non-NULL, we can then lock it without the memory being freed out from
+	 * under us, then check whether the request is still on the queue.
+	 *
+	 * Keep holding rcu_read_lock() as long as we hold the queue lock, in
+	 * case the caller deletes the entry from the queue, leaving it empty.
+	 * In that case, only RCU prevents the queue memory from being freed.
+	 */
+	rcu_read_lock();
+	head = smp_load_acquire(&req->head);
+	if (head) {
+		spin_lock(&head->lock);
+		if (!list_empty(&req->wait.entry))
+			return true;
+		spin_unlock(&head->lock);
+	}
+	rcu_read_unlock();
+	return false;
+}
+
+static void poll_iocb_unlock_wq(struct poll_iocb *req)
+{
+	spin_unlock(&req->head->lock);
+	rcu_read_unlock();
+}
+
 static void aio_poll_complete_work(struct work_struct *work)
 {
 	struct poll_iocb *req = container_of(work, struct poll_iocb, work);
@@ -1638,14 +1684,27 @@ static void aio_poll_complete_work(struct work_struct *work)
 	 * avoid further branches in the fast path.
 	 */
 	spin_lock_irq(&ctx->ctx_lock);
-	if (!mask && !READ_ONCE(req->cancelled)) {
-		add_wait_queue(req->head, &req->wait);
-		spin_unlock_irq(&ctx->ctx_lock);
-		return;
-	}
+	if (poll_iocb_lock_wq(req)) {
+		if (!mask && !READ_ONCE(req->cancelled)) {
+			/*
+			 * The request isn't actually ready to be completed yet.
+			 * Reschedule completion if another wakeup came in.
+			 */
+			if (req->work_need_resched) {
+				schedule_work(&req->work);
+				req->work_need_resched = false;
+			} else {
+				req->work_scheduled = false;
+			}
+			poll_iocb_unlock_wq(req);
+			spin_unlock_irq(&ctx->ctx_lock);
+			return;
+		}
+		list_del_init(&req->wait.entry);
+		poll_iocb_unlock_wq(req);
+	} /* else, POLLFREE has freed the waitqueue, so we must complete */
 	list_del_init(&iocb->ki_list);
 	iocb->ki_res.res = mangle_poll(mask);
-	req->done = true;
 	spin_unlock_irq(&ctx->ctx_lock);
 
 	iocb_put(iocb);
@@ -1657,13 +1716,14 @@ static int aio_poll_cancel(struct kiocb *iocb)
 	struct aio_kiocb *aiocb = container_of(iocb, struct aio_kiocb, rw);
 	struct poll_iocb *req = &aiocb->poll;
 
-	spin_lock(&req->head->lock);
-	WRITE_ONCE(req->cancelled, true);
-	if (!list_empty(&req->wait.entry)) {
-		list_del_init(&req->wait.entry);
-		schedule_work(&aiocb->poll.work);
-	}
-	spin_unlock(&req->head->lock);
+	if (poll_iocb_lock_wq(req)) {
+		WRITE_ONCE(req->cancelled, true);
+		if (!req->work_scheduled) {
+			schedule_work(&aiocb->poll.work);
+			req->work_scheduled = true;
+		}
+		poll_iocb_unlock_wq(req);
+	} /* else, the request was force-cancelled by POLLFREE already */
 
 	return 0;
 }
@@ -1680,21 +1740,27 @@ static int aio_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync,
 	if (mask && !(mask & req->events))
 		return 0;
 
-	list_del_init(&req->wait.entry);
-
-	if (mask && spin_trylock_irqsave(&iocb->ki_ctx->ctx_lock, flags)) {
+	/*
+	 * Complete the request inline if possible.  This requires that three
+	 * conditions be met:
+	 *   1. An event mask must have been passed.  If a plain wakeup was done
+	 *	instead, then mask == 0 and we have to call vfs_poll() to get
+	 *	the events, so inline completion isn't possible.
+	 *   2. The completion work must not have already been scheduled.
+	 *   3. ctx_lock must not be busy.  We have to use trylock because we
+	 *	already hold the waitqueue lock, so this inverts the normal
+	 *	locking order.  Use irqsave/irqrestore because not all
+	 *	filesystems (e.g. fuse) call this function with IRQs disabled,
+	 *	yet IRQs have to be disabled before ctx_lock is obtained.
+	 */
+	if (mask && !req->work_scheduled &&
+	    spin_trylock_irqsave(&iocb->ki_ctx->ctx_lock, flags)) {
 		struct kioctx *ctx = iocb->ki_ctx;
 
-		/*
-		 * Try to complete the iocb inline if we can. Use
-		 * irqsave/irqrestore because not all filesystems (e.g. fuse)
-		 * call this function with IRQs disabled and because IRQs
-		 * have to be disabled before ctx_lock is obtained.
-		 */
+		list_del_init(&req->wait.entry);
 		list_del(&iocb->ki_list);
 		iocb->ki_res.res = mangle_poll(mask);
-		req->done = true;
-		if (iocb->ki_eventfd && eventfd_signal_allowed()) {
+		if (iocb->ki_eventfd && !eventfd_signal_allowed()) {
 			iocb = NULL;
 			INIT_WORK(&req->work, aio_poll_put_work);
 			schedule_work(&req->work);
@@ -1703,14 +1769,51 @@ static int aio_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync,
 		if (iocb)
 			iocb_put(iocb);
 	} else {
-		schedule_work(&req->work);
+		/*
+		 * Schedule the completion work if needed.  If it was already
+		 * scheduled, record that another wakeup came in.
+		 *
+		 * Don't remove the request from the waitqueue here, as it might
+		 * not actually be complete yet (we won't know until vfs_poll()
+		 * is called), and we must not miss any wakeups.  POLLFREE is an
+		 * exception to this; see below.
+		 */
+		if (req->work_scheduled) {
+			req->work_need_resched = true;
+		} else {
+			schedule_work(&req->work);
+			req->work_scheduled = true;
+		}
+
+		/*
+		 * If the waitqueue is being freed early but we can't complete
+		 * the request inline, we have to tear down the request as best
+		 * we can.  That means immediately removing the request from its
+		 * waitqueue and preventing all further accesses to the
+		 * waitqueue via the request.  We also need to schedule the
+		 * completion work (done above).  Also mark the request as
+		 * cancelled, to potentially skip an unneeded call to ->poll().
+		 */
+		if (mask & POLLFREE) {
+			WRITE_ONCE(req->cancelled, true);
+			list_del_init(&req->wait.entry);
+
+			/*
+			 * Careful: this *must* be the last step, since as soon
+			 * as req->head is NULL'ed out, the request can be
+			 * completed and freed, since aio_poll_complete_work()
+			 * will no longer need to take the waitqueue lock.
+			 */
+			smp_store_release(&req->head, NULL);
+		}
 	}
 	return 1;
 }
 
 struct aio_poll_table {
 	struct poll_table_struct	pt;
 	struct aio_kiocb		*iocb;
+	bool				queued;
 	int				error;
 };
 
@@ -1721,11 +1824,12 @@ aio_poll_queue_proc(struct file *file, struct wait_queue_head *head,
 	struct aio_poll_table *pt = container_of(p, struct aio_poll_table, pt);
 
 	/* multiple wait queues per file are not supported */
-	if (unlikely(pt->iocb->poll.head)) {
+	if (unlikely(pt->queued)) {
 		pt->error = -EINVAL;
 		return;
 	}
 
+	pt->queued = true;
 	pt->error = 0;
 	pt->iocb->poll.head = head;
 	add_wait_queue(head, &pt->iocb->poll.wait);
@@ -1750,12 +1854,14 @@ static int aio_poll(struct aio_kiocb *aiocb, const struct iocb *iocb)
 	req->events = demangle_poll(iocb->aio_buf) | EPOLLERR | EPOLLHUP;
 
 	req->head = NULL;
-	req->done = false;
 	req->cancelled = false;
+	req->work_scheduled = false;
+	req->work_need_resched = false;
 
 	apt.pt._qproc = aio_poll_queue_proc;
 	apt.pt._key = req->events;
 	apt.iocb = aiocb;
+	apt.queued = false;
 	apt.error = -EINVAL; /* same as no support for IOCB_CMD_POLL */
 
 	/* initialized the list so that we can do list_empty checks */
@@ -1764,23 +1870,35 @@ static int aio_poll(struct aio_kiocb *aiocb, const struct iocb *iocb)
 
 	mask = vfs_poll(req->file, &apt.pt) & req->events;
 	spin_lock_irq(&ctx->ctx_lock);
-	if (likely(req->head)) {
-		spin_lock(&req->head->lock);
-		if (unlikely(list_empty(&req->wait.entry))) {
-			if (apt.error)
+	if (likely(apt.queued)) {
+		bool on_queue = poll_iocb_lock_wq(req);
+
+		if (!on_queue || req->work_scheduled) {
+			/*
+			 * aio_poll_wake() already either scheduled the async
+			 * completion work, or completed the request inline.
+			 */
+			if (apt.error) /* unsupported case: multiple queues */
 				cancel = true;
 			apt.error = 0;
 			mask = 0;
 		}
 		if (mask || apt.error) {
+			/* Steal to complete synchronously. */
 			list_del_init(&req->wait.entry);
 		} else if (cancel) {
+			/* Cancel if possible (may be too late though). */
 			WRITE_ONCE(req->cancelled, true);
-		} else if (!req->done) { /* actually waiting for an event */
+		} else if (on_queue) {
+			/*
+			 * Actually waiting for an event, so add the request to
+			 * active_reqs so that it can be cancelled if needed.
+			 */
 			list_add_tail(&aiocb->ki_list, &ctx->active_reqs);
 			aiocb->ki_cancel = aio_poll_cancel;
 		}
-		spin_unlock(&req->head->lock);
+		if (on_queue)
+			poll_iocb_unlock_wq(req);
 	}
 	if (mask) { /* no async, we'd stolen it */
 		aiocb->ki_res.res = mangle_poll(mask);

fs/signalfd.c

@@ -35,17 +35,7 @@
 
 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_TYPESAFE_BY_RCU, we can safely return.
-	 */
-	if (likely(!waitqueue_active(wqh)))
-		return;
-
-	/* wait_queue_entry_t->func(POLLFREE) should do remove_wait_queue() */
-	wake_up_poll(wqh, EPOLLHUP | POLLFREE);
+	wake_up_pollfree(&sighand->signalfd_wqh);
 }
 
 struct signalfd_ctx {

include/linux/wait.h

@@ -217,6 +217,7 @@ void __wake_up_sync_key(struct wait_queue_head *wq_head, unsigned int mode, void
 void __wake_up_locked_sync_key(struct wait_queue_head *wq_head, unsigned int mode, void *key);
 void __wake_up_locked(struct wait_queue_head *wq_head, unsigned int mode, int nr);
 void __wake_up_sync(struct wait_queue_head *wq_head, unsigned int mode);
+void __wake_up_pollfree(struct wait_queue_head *wq_head);
 
 #define wake_up(x)			__wake_up(x, TASK_NORMAL, 1, NULL)
 #define wake_up_nr(x, nr)		__wake_up(x, TASK_NORMAL, nr, NULL)
@@ -245,6 +246,31 @@ void __wake_up_sync(struct wait_queue_head *wq_head, unsigned int mode);
 #define wake_up_interruptible_sync_poll_locked(x, m)				\
 	__wake_up_locked_sync_key((x), TASK_INTERRUPTIBLE, poll_to_key(m))
 
+/**
+ * wake_up_pollfree - signal that a polled waitqueue is going away
+ * @wq_head: the wait queue head
+ *
+ * In the very rare cases where a ->poll() implementation uses a waitqueue whose
+ * lifetime is tied to a task rather than to the 'struct file' being polled,
+ * this function must be called before the waitqueue is freed so that
+ * non-blocking polls (e.g. epoll) are notified that the queue is going away.
+ *
+ * The caller must also RCU-delay the freeing of the wait_queue_head, e.g. via
+ * an explicit synchronize_rcu() or call_rcu(), or via SLAB_TYPESAFE_BY_RCU.
+ */
+static inline void wake_up_pollfree(struct wait_queue_head *wq_head)
+{
+	/*
+	 * For performance reasons, we don't always take the queue lock here.
+	 * Therefore, we might race with someone removing the last entry from
+	 * the queue, and proceed while they still hold the queue lock.
+	 * However, rcu_read_lock() is required to be held in such cases, so we
+	 * can safely proceed with an RCU-delayed free.
+	 */
+	if (waitqueue_active(wq_head))
+		__wake_up_pollfree(wq_head);
+}
+
 #define ___wait_cond_timeout(condition)						\
 ({										\
 	bool __cond = (condition);						\

include/uapi/asm-generic/poll.h

@@ -29,7 +29,7 @@
 #define POLLRDHUP       0x2000
 #endif
 
-#define POLLFREE	(__force __poll_t)0x4000	/* currently only for epoll */
+#define POLLFREE	(__force __poll_t)0x4000
 
 #define POLL_BUSY_LOOP	(__force __poll_t)0x8000
 

kernel/sched/wait.c

@@ -238,6 +238,13 @@ void __wake_up_sync(struct wait_queue_head *wq_head, unsigned int mode)
 }
 EXPORT_SYMBOL_GPL(__wake_up_sync);	/* For internal use only */
 
+void __wake_up_pollfree(struct wait_queue_head *wq_head)
+{
+	__wake_up(wq_head, TASK_NORMAL, 0, poll_to_key(EPOLLHUP | POLLFREE));
+	/* POLLFREE must have cleared the queue. */
+	WARN_ON_ONCE(waitqueue_active(wq_head));
+}
+
 /*
  * Note: we use "set_current_state()" _after_ the wait-queue add,
  * because we need a memory barrier there on SMP, so that any