Subject: Re: [PATCH net] vhost: correctly remove wait queue during poll failure

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 27 Mar 2018 16:58:59 +0300

On Tue, Mar 27, 2018 at 05:43:14PM +0800, Jason Wang wrote:
> 
> 
> On 2018年03月27日 17:28, Darren Kenny wrote:
> > Hi Jason,
> > 
> > On Tue, Mar 27, 2018 at 11:47:22AM +0800, Jason Wang wrote:
> > > We tried to remove vq poll from wait queue, but do not check whether
> > > or not it was in a list before. This will lead double free. Fixing
> > > this by checking poll->wqh to make sure it was in a list.
> > 
> > This text seems at odds with the code below, instead of checking
> > poll-whq, you are removing that check...
> > 
> > Maybe the text needs rewording?
> 
> Yes, I admit it's bad, thanks for pointing out.
> 
> How about:
> 
> "Fixing this by switching to use vhost_poll_stop() which zeros poll->wqh
> after removing poll from waitqueue to make sure it won't be freed twice."
> 
> Thanks

Let's be a bit more specific about the problem maybe?

when vhost's attempt to start polling a descriptor fails, we remove the
poll->wqh entry from wait queue but do not clear it, so the following
cleanup (e.g. on release) will attempt to remove it again.

To fix, switch to vhost_poll_stop() which zeros poll->wqh
after removing poll from waitqueue to make sure it won't be freed twice."

the patch itself is fine though:

Acked-by: Michael S. Tsirkin <[email-masked]>

> > 
> > Thanks,
> > 
> > Darren.
> > 
> > > 
> > > Reported-by: [email-masked]
> > > Fixes: 2b8b328b61c79 ("vhost_net: handle polling errors when setting
> > > backend")
> > > Signed-off-by: Jason Wang <[email-masked]>
> > > ---
> > > drivers/vhost/vhost.c | 3 +--
> > > 1 file changed, 1 insertion(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > index 1b3e8d2d..5d5a9d9 100644
> > > --- a/drivers/vhost/vhost.c
> > > +++ b/drivers/vhost/vhost.c
> > > @@ -212,8 +212,7 @@ int vhost_poll_start(struct vhost_poll *poll,
> > > struct file *file)
> > >     if (mask)
> > >         vhost_poll_wakeup(&poll->wait, 0, 0, poll_to_key(mask));
> > >     if (mask & EPOLLERR) {
> > > -        if (poll->wqh)
> > > -            remove_wait_queue(poll->wqh, &poll->wait);
> > > +        vhost_poll_stop(poll);
> > >         ret = -EINVAL;
> > >     }
> > > 
> > > -- 
> > > 2.7.4
> > > 
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