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

From: Jason Wang <jasowang@redhat.com>
To: Lee Jones <lee.jones@linaro.org>
Cc: mst <mst@redhat.com>, linux-kernel <linux-kernel@vger.kernel.org>,
	kvm <kvm@vger.kernel.org>,
	virtualization <virtualization@lists.linux-foundation.org>,
	netdev <netdev@vger.kernel.org>,
	stable@vger.kernel.org,
	syzbot+adc3cb32385586bec859@syzkaller.appspotmail.com
Subject: Re: [PATCH 1/1] vhost: Protect the virtqueue from being cleared whilst still in use
Date: Tue, 8 Mar 2022 14:15:56 +0800	[thread overview]
Message-ID: <CACGkMEsjmCNQPjxPjXL0WUfbMg8ARnumEp4yjUxqznMKR1nKSQ@mail.gmail.com> (raw)
In-Reply-To: <20220307191757.3177139-1-lee.jones@linaro.org>

On Tue, Mar 8, 2022 at 3:18 AM Lee Jones <lee.jones@linaro.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@vger.kernel.org>
> Reported-by: syzbot+adc3cb32385586bec859@syzkaller.appspotmail.com
> Signed-off-by: Lee Jones <lee.jones@linaro.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.

> +               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.

Thanks

>         vhost_dev_free_iovecs(dev);
>         if (dev->log_ctx)
> --
> 2.35.1.616.g0bdcbb4464-goog
>