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

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Date:   Wed, 2 Mar 2022 15:11:21 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Lee Jones <lee.jones@...aro.org>, 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 08:35:08AM -0500, Michael S. Tsirkin wrote:
>On Wed, Mar 02, 2022 at 10:34:46AM +0100, Stefano Garzarella wrote:
>> 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
>
>
>Right. I want to queue this but I would like to get a warning
>so we can detect issues like [2] before they cause more issues.

I agree, what about moving the warning that we already have higher up, 
right at the beginning of the function?

I mean something like this:

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 59edb5a1ffe2..1721ff3f18c0 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -692,6 +692,8 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
  {
         int i;
  
+       WARN_ON(!llist_empty(&dev->work_list));
+
         for (i = 0; i < dev->nvqs; ++i) {
                 if (dev->vqs[i]->error_ctx)
                         eventfd_ctx_put(dev->vqs[i]->error_ctx);
@@ -712,7 +714,6 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
         dev->iotlb = NULL;
         vhost_clear_msg(dev);
         wake_up_interruptible_poll(&dev->wait, EPOLLIN | EPOLLRDNORM);
-       WARN_ON(!llist_empty(&dev->work_list));
         if (dev->worker) {
                 kthread_stop(dev->worker);
                 dev->worker = NULL;


And maybe we can also check vq->private and warn in the loop, because 
the work_list may be empty if the device is doing nothing.

Thanks,
Stefano