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

Lee Jones
2022-03-08T08:10:51Z

On Mon, 07 Mar 2022, Greg KH wrote:

> On Mon, Mar 07, 2022 at 07:17:57PM +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 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));
>
> So you are trading one syzbot triggered issue for another one in the
> future? :)
>
> If this ever can happen, handle it, but don't log it with a WARN_ON() as
> that will trigger the panic-on-warn boxes, as well as syzbot. Unless
> you want that to happen?

No, Syzbot doesn't report warnings, only BUGs and memory corruption.

> And what happens if the mutex is locked _RIGHT_ after you checked it?
> You still have a race...

No, we miss a warning that one time. Memory is still protected.

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