linux-kernel - Re: [PATCH] vduse: Fix msg list race in vduse_dev_read

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Message-ID: <CAJaqyWepzL6uzwDtEd7MJLxp2Ywy8RODDFUpDXA1iksCEGW3nw@mail.gmail.com>
Date: Fri, 30 Jan 2026 12:51:42 +0100
From: Eugenio Perez Martin <eperezma@...hat.com>
To: Zhang Tianci <zhangtianci.1997@...edance.com>
Cc: mst@...hat.com, jasowang@...hat.com, xuanzhuo@...ux.alibaba.com, 
	marco.crivellari@...e.com, anders.roxell@...aro.org, 
	virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	Xie Yongji <xieyongji@...edance.com>
Subject: Re: [PATCH] vduse: Fix msg list race in vduse_dev_read_iter

On Fri, Jan 30, 2026 at 9:15 AM Zhang Tianci
<zhangtianci.1997@...edance.com> wrote:
>
> 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.
>

Missed Fixes: tag and Cc: stable@...r.kernel.org. Or maybe we can
consider this a change in the behavior? I don't think any VDUSE
instance should trust it will never receive that message that it
received partially once but still...


> Signed-off-by: Zhang Tianci <zhangtianci.1997@...edance.com>
> Reviewed-by: Xie Yongji <xieyongji@...edance.com>
> ---
>  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.

But in the timeout case, msg->completed is false so list_del is never
called, isn't it?

Is there even any event that may cause more than one packet in either
queue? Maybe we can simplify a lot of this if we don't have that
assumption.

> +        */
> +       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
>