linux-kernel - Re: [RFC 1/2] virtio_net: timeout control virtqueue commands

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAJaqyWfRmhXM8eV+=wMVKgrc8SJ98OTtBCtNDJj8ZRUM5Y9Nmg@mail.gmail.com>
Date: Wed, 15 Oct 2025 09:45:01 +0200
From: Eugenio Perez Martin <eperezma@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Maxime Coquelin <mcoqueli@...hat.com>, Yongji Xie <xieyongji@...edance.com>, 
	virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Dragos Tatulea DE <dtatulea@...dia.com>, jasowang@...hat.com
Subject: Re: [RFC 1/2] virtio_net: timeout control virtqueue commands

On Wed, Oct 15, 2025 at 9:05 AM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Wed, Oct 15, 2025 at 08:52:50AM +0200, Eugenio Perez Martin wrote:
> > On Wed, Oct 15, 2025 at 8:33 AM Michael S. Tsirkin <mst@...hat.com> wrote:
> > >
> > > On Wed, Oct 15, 2025 at 08:08:31AM +0200, Eugenio Perez Martin wrote:
> > > > On Tue, Oct 14, 2025 at 11:25 AM Michael S. Tsirkin <mst@...hat.com> wrote:
> > > > >
> > > > > On Tue, Oct 14, 2025 at 11:14:40AM +0200, Maxime Coquelin wrote:
> > > > > > On Tue, Oct 14, 2025 at 10:29 AM Michael S. Tsirkin <mst@...hat.com> wrote:
> > > > > > >
> > > > > > > On Tue, Oct 07, 2025 at 03:06:21PM +0200, Eugenio Pérez wrote:
> > > > > > > > An userland device implemented through VDUSE could take rtnl forever if
> > > > > > > > the virtio-net driver is running on top of virtio_vdpa.  Let's break the
> > > > > > > > device if it does not return the buffer in a longer-than-assumible
> > > > > > > > timeout.
> > > > > > >
> > > > > > > So now I can't debug qemu with gdb because guest dies :(
> > > > > > > Let's not break valid use-cases please.
> > > > > > >
> > > > > > >
> > > > > > > Instead, solve it in vduse, probably by handling cvq within
> > > > > > > kernel.
> > > > > >
> > > > > > Would a shadow control virtqueue implementation in the VDUSE driver work?
> > > > > > It would ack systematically messages sent by the Virtio-net driver,
> > > > > > and so assume the userspace application will Ack them.
> > > > > >
> > > > > > When the userspace application handles the message, if the handling fails,
> > > > > > it somehow marks the device as broken?
> > > > > >
> > > > > > Thanks,
> > > > > > Maxime
> > > > >
> > > > > Yes but it's a bit more convoluted  than just acking them.
> > > > > Once you use the buffer you can get another one and so on
> > > > > with no limit.
> > > > > One fix is to actually maintain device state in the
> > > > > kernel, update it, and then notify userspace.
> > > > >
> > > >
> > > > I thought of implementing this approach at first, but it has two drawbacks.
> > > >
> > > > The first one: it's racy. Let's say the driver updates the MAC filter,
> > > > VDUSE timeout occurs, the guest receives the fail, and then the device
> > > > replies with an OK. There is no way for the device or VDUSE to update
> > > > the driver.
> > >
> > > There's no timeout. Kernel can guarantee executing all requests.
> > >
> >
> > I don't follow this. How should the VDUSE kernel module act if the
> > VDUSE userland device does not use the CVQ buffer then?
>
> First I am not sure a VQ is the best interface for talking to userspace.
> But assuming yes - just avoid sending more data, send it later after
> userspace used the buffer.
>

Let me take a step back, I think I didn't describe the scenario well enough.

We have a VDUSE device, and then the same host is interacting with the
device through the virtio_net driver over virtio_vdpa.

Then, the virtio_net driver sends a control command though its CVQ, so
it *takes the RTNL*. That command reaches the VDUSE CVQ.

It does not matter if the VDUSE device in the userland processes the
commands through a CVQ, reading the vduse character device, or another
system. The question is: what to do if the VDUSE device does not
process that command in a timely manner? Should we just let the RTNL
be taken forever?