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SubjectRe: [PATCH 1/1] vhost: Protect the virtqueue from being cleared whilst still in use
On Tue, 08 Mar 2022, Greg KH wrote:

> On Tue, Mar 08, 2022 at 08:10:06AM +0000, Lee Jones wrote:
> > 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.
>
> Has it changed? Last I looked, it did trigger on WARN_* calls, which
> has resulted in a huge number of kernel fixes because of that.

Everything is customisable in syzkaller, so maybe there are specific
builds which panic_on_warn enabled, but none that I'm involved with
do.

Here follows a topical example. The report above in the Link: tag
comes with a crashlog [0]. In there you can see the WARN() at the
bottom of vhost_dev_cleanup() trigger many times due to a populated
(non-flushed) worker list, before finally tripping the BUG() which
triggers the report:

[0] https://syzkaller.appspot.com/text?tag=CrashLog&x=16a61fce700000

> > > 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.
>
> Then don't warn on something that doesn't matter. This line can be
> dropped as there's nothing anyone can do about it, right?

You'll have to take that point up with Michael.

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