[PATCH] vduse: Fix msg list race in vduse_dev_read_iter

[Zhang Tianci]

Move the message to recv_list before dropping msg_lock and copying the
request to userspace, avoiding a transient unlinked state that can race
with the msg_sync timeout path. Roll back to send_list on copy failures.

Signed-off-by: Zhang Tianci <zhangtianci.1997@xxxxxxxxxxxxx>
Reviewed-by: Xie Yongji <xieyongji@xxxxxxxxxxxxx>
---
 drivers/vdpa/vdpa_user/vduse_dev.c | 30 ++++++++++++++++++++++--------
 1 file changed, 22 insertions(+), 8 deletions(-)

diff --git a/drivers/vdpa/vdpa_user/vduse_dev.c b/drivers/vdpa/vdpa_user/vduse_dev.c
index ae357d014564c..b6a558341c06c 100644
--- a/drivers/vdpa/vdpa_user/vduse_dev.c
+++ b/drivers/vdpa/vdpa_user/vduse_dev.c
@@ -325,6 +325,7 @@ static ssize_t vduse_dev_read_iter(struct kiocb *iocb, struct iov_iter *to)
 	struct file *file = iocb->ki_filp;
 	struct vduse_dev *dev = file->private_data;
 	struct vduse_dev_msg *msg;
+	struct vduse_dev_request req;
 	int size = sizeof(struct vduse_dev_request);
 	ssize_t ret;
 
@@ -339,7 +340,7 @@ static ssize_t vduse_dev_read_iter(struct kiocb *iocb, struct iov_iter *to)
 
 		ret = -EAGAIN;
 		if (file->f_flags & O_NONBLOCK)
-			goto unlock;
+			break;
 
 		spin_unlock(&dev->msg_lock);
 		ret = wait_event_interruptible_exclusive(dev->waitq,
@@ -349,17 +350,30 @@ static ssize_t vduse_dev_read_iter(struct kiocb *iocb, struct iov_iter *to)
 
 		spin_lock(&dev->msg_lock);
 	}
+	if (!msg) {
+		spin_unlock(&dev->msg_lock);
+		return ret;
+	}
+
+	memcpy(&req, &msg->req, sizeof(req));
+	/*
+	 * Move @msg to recv_list before dropping msg_lock.
+	 * This avoids a window where @msg is detached from any list and
+	 * vduse_dev_msg_sync() timeout path may operate on an unlinked node.
+	 */
+	vduse_enqueue_msg(&dev->recv_list, msg);
 	spin_unlock(&dev->msg_lock);
-	ret = copy_to_iter(&msg->req, size, to);
-	spin_lock(&dev->msg_lock);
+
+	ret = copy_to_iter(&req, size, to);
 	if (ret != size) {
+		spin_lock(&dev->msg_lock);
+		/* Roll back: move msg back to send_list if still pending. */
+		msg = vduse_find_msg(&dev->recv_list, req.request_id);
+		if (msg)
+			vduse_enqueue_msg(&dev->send_list, msg);
+		spin_unlock(&dev->msg_lock);
 		ret = -EFAULT;
-		vduse_enqueue_msg(&dev->send_list, msg);
-		goto unlock;
 	}
-	vduse_enqueue_msg(&dev->recv_list, msg);
-unlock:
-	spin_unlock(&dev->msg_lock);
 
 	return ret;
 }
-- 
2.39.5