linux-kernel - Re: [PATCH 1/1] vhost: Protect the virtqueue from being cleared whilst still in use

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Date:   Tue, 8 Mar 2022 08:08:25 +0000
From:   Lee Jones <lee.jones@...aro.org>
To:     Jason Wang <jasowang@...hat.com>
Cc:     mst <mst@...hat.com>, linux-kernel <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        netdev <netdev@...r.kernel.org>, stable@...r.kernel.org,
        syzbot+adc3cb32385586bec859@...kaller.appspotmail.com
Subject: Re: [PATCH 1/1] vhost: Protect the virtqueue from being cleared
 whilst still in use

On Tue, 08 Mar 2022, Jason Wang wrote:

> On Tue, Mar 8, 2022 at 3:18 AM Lee Jones <lee.jones@...aro.org> wrote:
> >
> > vhost_vsock_handle_tx_kick() already holds the mutex during its call
> > to vhost_get_vq_desc().  All we have to do here is take the same lock
> > during virtqueue clean-up and we mitigate the reported issues.
> >
> > Also WARN() as a precautionary measure.  The purpose of this is to
> > capture possible future race conditions which may pop up over time.
> >
> > Link: https://syzkaller.appspot.com/bug?extid=279432d30d825e63ba00
> >
> > Cc: <stable@...r.kernel.org>
> > Reported-by: syzbot+adc3cb32385586bec859@...kaller.appspotmail.com
> > Signed-off-by: Lee Jones <lee.jones@...aro.org>
> > ---
> >  drivers/vhost/vhost.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 59edb5a1ffe28..ef7e371e3e649 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -693,6 +693,15 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
> >         int i;
> >
> >         for (i = 0; i < dev->nvqs; ++i) {
> > +               /* No workers should run here by design. However, races have
> > +                * previously occurred where drivers have been unable to flush
> > +                * all work properly prior to clean-up.  Without a successful
> > +                * flush the guest will malfunction, but avoiding host memory
> > +                * corruption in those cases does seem preferable.
> > +                */
> > +               WARN_ON(mutex_is_locked(&dev->vqs[i]->mutex));
> > +
> 
> I don't get how this can help, the mutex could be grabbed in the
> middle of the above and below line.

The worst that happens in this slim scenario is we miss a warning.
The mutexes below will still function as expected and prevent possible
memory corruption.

> > +               mutex_lock(&dev->vqs[i]->mutex);
> >                 if (dev->vqs[i]->error_ctx)
> >                         eventfd_ctx_put(dev->vqs[i]->error_ctx);
> >                 if (dev->vqs[i]->kick)
> > @@ -700,6 +709,7 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
> >                 if (dev->vqs[i]->call_ctx.ctx)
> >                         eventfd_ctx_put(dev->vqs[i]->call_ctx.ctx);
> >                 vhost_vq_reset(dev, dev->vqs[i]);
> > +               mutex_unlock(&dev->vqs[i]->mutex);
> >         }
> 
> I'm not sure it's correct to assume some behaviour of a buggy device.
> For the device mutex, we use that to protect more than just err/call
> and vq.

When I authored this, I did so as *the* fix.  However, since the cause
of today's crash has now been patched, this has become a belt and
braces solution.  Michael's addition of the WARN() also has the
benefit of providing us with an early warning system for future
breakages.  Personally, I think it's kinda neat.

-- 
Lee Jones [李琼斯]
Principal Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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