fuse: {io-uring} Fix a possible req cancellation race

task-A (application) might be in request_wait_answer and
try to remove the request when it has FR_PENDING set.

task-B (a fuse-server io-uring task) might handle this
request with FUSE_IO_URING_CMD_COMMIT_AND_FETCH, when
fetching the next request and accessed the req from
the pending list in fuse_uring_ent_assign_req().
That code path was not protected by fiq->lock and so
might race with task-A.

For scaling reasons we better don't use fiq->lock, but
add a handler to remove canceled requests from the queue.

This also removes usage of fiq->lock from
fuse_uring_add_req_to_ring_ent() altogether, as it was
there just to protect against this race and incomplete.

Also added is a comment why FR_PENDING is not cleared.

Fixes: c090c8abae4b ("fuse: Add io-uring sqe commit and fetch support")
Cc: <stable@vger.kernel.org> # v6.14
Reported-by: Joanne Koong <joannelkoong@gmail.com>
Closes: https://lore.kernel.org/all/CAJnrk1ZgHNb78dz-yfNTpxmW7wtT88A=m-zF0ZoLXKLUHRjNTw@mail.gmail.com/
Signed-off-by: Bernd Schubert <bschubert@ddn.com>
Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/dev.c

@@ -407,6 +407,24 @@ static int queue_interrupt(struct fuse_req *req)
 	return 0;
 }
 
+bool fuse_remove_pending_req(struct fuse_req *req, spinlock_t *lock)
+{
+	spin_lock(lock);
+	if (test_bit(FR_PENDING, &req->flags)) {
+		/*
+		 * FR_PENDING does not get cleared as the request will end
+		 * up in destruction anyway.
+		 */
+		list_del(&req->list);
+		spin_unlock(lock);
+		__fuse_put_request(req);
+		req->out.h.error = -EINTR;
+		return true;
+	}
+	spin_unlock(lock);
+	return false;
+}
+
 static void request_wait_answer(struct fuse_req *req)
 {
 	struct fuse_conn *fc = req->fm->fc;
@@ -428,22 +446,20 @@ static void request_wait_answer(struct fuse_req *req)
 	}
 
 	if (!test_bit(FR_FORCE, &req->flags)) {
+		bool removed;
+
 		/* Only fatal signals may interrupt this */
 		err = wait_event_killable(req->waitq,
 					test_bit(FR_FINISHED, &req->flags));
 		if (!err)
 			return;
 
-		spin_lock(&fiq->lock);
-		/* Request is not yet in userspace, bail out */
-		if (test_bit(FR_PENDING, &req->flags)) {
-			list_del(&req->list);
-			spin_unlock(&fiq->lock);
-			__fuse_put_request(req);
-			req->out.h.error = -EINTR;
+		if (test_bit(FR_URING, &req->flags))
+			removed = fuse_uring_remove_pending_req(req);
+		else
+			removed = fuse_remove_pending_req(req, &fiq->lock);
+		if (removed)
 			return;
-		}
-		spin_unlock(&fiq->lock);
 	}
 
 	/*

fs/fuse/dev_uring.c

@@ -726,8 +726,6 @@ static void fuse_uring_add_req_to_ring_ent(struct fuse_ring_ent *ent,
 					   struct fuse_req *req)
 {
 	struct fuse_ring_queue *queue = ent->queue;
-	struct fuse_conn *fc = req->fm->fc;
-	struct fuse_iqueue *fiq = &fc->iq;
 
 	lockdep_assert_held(&queue->lock);
 
@@ -737,9 +735,7 @@ static void fuse_uring_add_req_to_ring_ent(struct fuse_ring_ent *ent,
 			ent->state);
 	}
 
-	spin_lock(&fiq->lock);
 	clear_bit(FR_PENDING, &req->flags);
-	spin_unlock(&fiq->lock);
 	ent->fuse_req = req;
 	ent->state = FRRS_FUSE_REQ;
 	list_move(&ent->list, &queue->ent_w_req_queue);
@@ -1238,6 +1234,8 @@ void fuse_uring_queue_fuse_req(struct fuse_iqueue *fiq, struct fuse_req *req)
 	if (unlikely(queue->stopped))
 		goto err_unlock;
 
+	set_bit(FR_URING, &req->flags);
+	req->ring_queue = queue;
 	ent = list_first_entry_or_null(&queue->ent_avail_queue,
 				       struct fuse_ring_ent, list);
 	if (ent)
@@ -1276,6 +1274,8 @@ bool fuse_uring_queue_bq_req(struct fuse_req *req)
 		return false;
 	}
 
+	set_bit(FR_URING, &req->flags);
+	req->ring_queue = queue;
 	list_add_tail(&req->list, &queue->fuse_req_bg_queue);
 
 	ent = list_first_entry_or_null(&queue->ent_avail_queue,
@@ -1306,6 +1306,13 @@ bool fuse_uring_queue_bq_req(struct fuse_req *req)
 	return true;
 }
 
+bool fuse_uring_remove_pending_req(struct fuse_req *req)
+{
+	struct fuse_ring_queue *queue = req->ring_queue;
+
+	return fuse_remove_pending_req(req, &queue->lock);
+}
+
 static const struct fuse_iqueue_ops fuse_io_uring_ops = {
 	/* should be send over io-uring as enhancement */
 	.send_forget = fuse_dev_queue_forget,

fs/fuse/dev_uring_i.h

@@ -142,6 +142,7 @@ void fuse_uring_abort_end_requests(struct fuse_ring *ring);
 int fuse_uring_cmd(struct io_uring_cmd *cmd, unsigned int issue_flags);
 void fuse_uring_queue_fuse_req(struct fuse_iqueue *fiq, struct fuse_req *req);
 bool fuse_uring_queue_bq_req(struct fuse_req *req);
+bool fuse_uring_remove_pending_req(struct fuse_req *req);
 
 static inline void fuse_uring_abort(struct fuse_conn *fc)
 {
@@ -200,6 +201,11 @@ static inline bool fuse_uring_ready(struct fuse_conn *fc)
 	return false;
 }
 
+static inline bool fuse_uring_remove_pending_req(struct fuse_req *req)
+{
+	return false;
+}
+
 #endif /* CONFIG_FUSE_IO_URING */
 
 #endif /* _FS_FUSE_DEV_URING_I_H */

fs/fuse/fuse_dev_i.h

@@ -61,6 +61,7 @@ int fuse_copy_out_args(struct fuse_copy_state *cs, struct fuse_args *args,
 void fuse_dev_queue_forget(struct fuse_iqueue *fiq,
 			   struct fuse_forget_link *forget);
 void fuse_dev_queue_interrupt(struct fuse_iqueue *fiq, struct fuse_req *req);
+bool fuse_remove_pending_req(struct fuse_req *req, spinlock_t *lock);
 
 #endif
 

fs/fuse/fuse_i.h

@@ -378,6 +378,7 @@ struct fuse_io_priv {
  * FR_FINISHED:		request is finished
  * FR_PRIVATE:		request is on private list
  * FR_ASYNC:		request is asynchronous
+ * FR_URING:		request is handled through fuse-io-uring
  */
 enum fuse_req_flag {
 	FR_ISREPLY,
@@ -392,6 +393,7 @@ enum fuse_req_flag {
 	FR_FINISHED,
 	FR_PRIVATE,
 	FR_ASYNC,
+	FR_URING,
 };
 
 /**
@@ -441,6 +443,7 @@ struct fuse_req {
 
 #ifdef CONFIG_FUSE_IO_URING
 	void *ring_entry;
+	void *ring_queue;
 #endif
 };