[PATCH v10 15/17] fuse: {io-uring} Prevent mount point hang on fuse-server termination

[Bernd Schubert <bschubert@ddn.com>]

When the fuse-server terminates while the fuse-client or kernel
still has queued URING_CMDs, these commands retain references
to the struct file used by the fuse connection. This prevents
fuse_dev_release() from being invoked, resulting in a hung mount
point.

This patch addresses the issue by making queued URING_CMDs
cancelable, allowing fuse_dev_release() to proceed as expected
and preventing the mount point from hanging.

Signed-off-by: Bernd Schubert <bschubert@ddn.com>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com> # io_uring
---
 fs/fuse/dev_uring.c   | 70 +++++++++++++++++++++++++++++++++++++++++++++++++--
 fs/fuse/dev_uring_i.h |  9 +++++++
 2 files changed, 77 insertions(+), 2 deletions(-)

diff --git a/fs/fuse/dev_uring.c b/fs/fuse/dev_uring.c
index 859e53893eeb5544d57dd961da0e99e7b3d5d9a9..fcce03174ee18153d597e9cd1a2659b1c237e3eb 100644
--- a/fs/fuse/dev_uring.c
+++ b/fs/fuse/dev_uring.c
@@ -150,6 +150,7 @@ void fuse_uring_destruct(struct fuse_conn *fc)
 
 	for (qid = 0; qid < ring->nr_queues; qid++) {
 		struct fuse_ring_queue *queue = ring->queues[qid];
+		struct fuse_ring_ent *ent, *next;
 
 		if (!queue)
 			continue;
@@ -159,6 +160,12 @@ void fuse_uring_destruct(struct fuse_conn *fc)
 		WARN_ON(!list_empty(&queue->ent_commit_queue));
 		WARN_ON(!list_empty(&queue->ent_in_userspace));
 
+		list_for_each_entry_safe(ent, next, &queue->ent_released,
+					 list) {
+			list_del_init(&ent->list);
+			kfree(ent);
+		}
+
 		kfree(queue->fpq.processing);
 		kfree(queue);
 		ring->queues[qid] = NULL;
@@ -242,6 +249,7 @@ static struct fuse_ring_queue *fuse_uring_create_queue(struct fuse_ring *ring,
 	INIT_LIST_HEAD(&queue->ent_in_userspace);
 	INIT_LIST_HEAD(&queue->fuse_req_queue);
 	INIT_LIST_HEAD(&queue->fuse_req_bg_queue);
+	INIT_LIST_HEAD(&queue->ent_released);
 
 	queue->fpq.processing = pq;
 	fuse_pqueue_init(&queue->fpq);
@@ -280,6 +288,7 @@ static void fuse_uring_stop_fuse_req_end(struct fuse_ring_ent *ent)
  */
 static void fuse_uring_entry_teardown(struct fuse_ring_ent *ent)
 {
+	struct fuse_ring_queue *queue = ent->queue;
 	if (ent->cmd) {
 		io_uring_cmd_done(ent->cmd, -ENOTCONN, 0, IO_URING_F_UNLOCKED);
 		ent->cmd = NULL;
@@ -288,8 +297,16 @@ static void fuse_uring_entry_teardown(struct fuse_ring_ent *ent)
 	if (ent->fuse_req)
 		fuse_uring_stop_fuse_req_end(ent);
 
-	list_del_init(&ent->list);
-	kfree(ent);
+	/*
+	 * The entry must not be freed immediately, due to access of direct
+	 * pointer access of entries through IO_URING_F_CANCEL - there is a risk
+	 * of race between daemon termination (which triggers IO_URING_F_CANCEL
+	 * and accesses entries without checking the list state first
+	 */
+	spin_lock(&queue->lock);
+	list_move(&ent->list, &queue->ent_released);
+	ent->state = FRRS_RELEASED;
+	spin_unlock(&queue->lock);
 }
 
 static void fuse_uring_stop_list_entries(struct list_head *head,
@@ -309,6 +326,7 @@ static void fuse_uring_stop_list_entries(struct list_head *head,
 			continue;
 		}
 
+		ent->state = FRRS_TEARDOWN;
 		list_move(&ent->list, &to_teardown);
 	}
 	spin_unlock(&queue->lock);
@@ -423,6 +441,46 @@ void fuse_uring_stop_queues(struct fuse_ring *ring)
 	}
 }
 
+/*
+ * Handle IO_URING_F_CANCEL, typically should come on daemon termination.
+ *
+ * Releasing the last entry should trigger fuse_dev_release() if
+ * the daemon was terminated
+ */
+static void fuse_uring_cancel(struct io_uring_cmd *cmd,
+			      unsigned int issue_flags)
+{
+	struct fuse_ring_ent *ent = uring_cmd_to_ring_ent(cmd);
+	struct fuse_ring_queue *queue;
+	bool need_cmd_done = false;
+
+	/*
+	 * direct access on ent - it must not be destructed as long as
+	 * IO_URING_F_CANCEL might come up
+	 */
+	queue = ent->queue;
+	spin_lock(&queue->lock);
+	if (ent->state == FRRS_AVAILABLE) {
+		ent->state = FRRS_USERSPACE;
+		list_move(&ent->list, &queue->ent_in_userspace);
+		need_cmd_done = true;
+		ent->cmd = NULL;
+	}
+	spin_unlock(&queue->lock);
+
+	if (need_cmd_done) {
+		/* no queue lock to avoid lock order issues */
+		io_uring_cmd_done(cmd, -ENOTCONN, 0, issue_flags);
+	}
+}
+
+static void fuse_uring_prepare_cancel(struct io_uring_cmd *cmd, int issue_flags,
+				      struct fuse_ring_ent *ring_ent)
+{
+	uring_cmd_set_ring_ent(cmd, ring_ent);
+	io_uring_cmd_mark_cancelable(cmd, issue_flags);
+}
+
 /*
  * Checks for errors and stores it into the request
  */
@@ -836,6 +894,7 @@ static int fuse_uring_commit_fetch(struct io_uring_cmd *cmd, int issue_flags,
 	spin_unlock(&queue->lock);
 
 	/* without the queue lock, as other locks are taken */
+	fuse_uring_prepare_cancel(cmd, issue_flags, ent);
 	fuse_uring_commit(ent, issue_flags);
 
 	/*
@@ -885,6 +944,8 @@ static void fuse_uring_do_register(struct fuse_ring_ent *ent,
 	struct fuse_conn *fc = ring->fc;
 	struct fuse_iqueue *fiq = &fc->iq;
 
+	fuse_uring_prepare_cancel(cmd, issue_flags, ent);
+
 	spin_lock(&queue->lock);
 	ent->cmd = cmd;
 	fuse_uring_ent_avail(ent, queue);
@@ -1035,6 +1096,11 @@ int __maybe_unused fuse_uring_cmd(struct io_uring_cmd *cmd,
 		return -EOPNOTSUPP;
 	}
 
+	if ((unlikely(issue_flags & IO_URING_F_CANCEL))) {
+		fuse_uring_cancel(cmd, issue_flags);
+		return 0;
+	}
+
 	/* This extra SQE size holds struct fuse_uring_cmd_req */
 	if (!(issue_flags & IO_URING_F_SQE128))
 		return -EINVAL;
diff --git a/fs/fuse/dev_uring_i.h b/fs/fuse/dev_uring_i.h
index 0182be61778b26a94bda2607289a7b668df6362f..2102b3d0c1aed1105e9c1200c91e1cb497b9a597 100644
--- a/fs/fuse/dev_uring_i.h
+++ b/fs/fuse/dev_uring_i.h
@@ -28,6 +28,12 @@ enum fuse_ring_req_state {
 
 	/* The ring entry is in or on the way to user space */
 	FRRS_USERSPACE,
+
+	/* The ring entry is in teardown */
+	FRRS_TEARDOWN,
+
+	/* The ring entry is released, but not freed yet */
+	FRRS_RELEASED,
 };
 
 /** A fuse ring entry, part of the ring queue */
@@ -79,6 +85,9 @@ struct fuse_ring_queue {
 	/* entries in userspace */
 	struct list_head ent_in_userspace;
 
+	/* entries that are released */
+	struct list_head ent_released;
+
 	/* fuse requests waiting for an entry slot */
 	struct list_head fuse_req_queue;
 

-- 
2.43.0