Virtualization - Mar 2022 - [PATCH 1/1] vhost: Protect the virtqueue from being cleared whilst still in use

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 at redhat.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.

> > 
> > Cc: <stable at vger.kernel.org>
> > Reported-by: syzbot+adc3cb32385586bec859 at syzkaller.appspotmail.com
> > Signed-off-by: Lee Jones <lee.jones at linaro.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
> >

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 at redhat.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