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

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Date:   Wed, 2 Mar 2022 10:34:46 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Lee Jones <lee.jones@...aro.org>
Cc:     mst@...hat.com, jasowang@...hat.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        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 Wed, Mar 02, 2022 at 07:54:21AM +0000, Lee Jones wrote:
>vhost_vsock_handle_tx_kick() already holds the mutex during its call
>to vhost_get_vq_desc().  All we have to do is take the same lock
>during virtqueue clean-up and we mitigate the reported issues.
>
>Link: https://syzkaller.appspot.com/bug?extid=279432d30d825e63ba00

This issue is similar to [1] that should be already fixed upstream by 
[2].

However I think this patch would have prevented some issues, because 
vhost_vq_reset() sets vq->private to NULL, preventing the worker from 
running.

Anyway I think that when we enter in vhost_dev_cleanup() the worker 
should be already stopped, so it shouldn't be necessary to take the 
mutex. But in order to prevent future issues maybe it's better to take 
them, so:

Reviewed-by: Stefano Garzarella <sgarzare@...hat.com>

[1] 
https://syzkaller.appspot.com/bug?id=993d8b5e64393ed9e6a70f9ae4de0119c605a822
[2] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a58da53ffd70294ebea8ecd0eb45fd0d74add9f9

>
>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 | 2 ++
> 1 file changed, 2 insertions(+)
>
>diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>index 59edb5a1ffe28..bbaff6a5e21b8 100644
>--- a/drivers/vhost/vhost.c
>+++ b/drivers/vhost/vhost.c
>@@ -693,6 +693,7 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
> 	int i;
>
> 	for (i = 0; i < dev->nvqs; ++i) {
>+		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 +701,7 @@ 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);
> 	}
> 	vhost_dev_free_iovecs(dev);
> 	if (dev->log_ctx)
>-- 
>2.35.1.574.g5d30c73bfb-goog
>