# Re: \[PATCH net V2\] vhost-vsock: fix use after free

- From: Michael S. Tsirkin \<mst@redhat.com\>
- Date: 2018-09-27 23:50:30 
- List: netdev
- Page: https://ratatoskr.run/netdev/2018/09/5393778
- Thread: https://ratatoskr.run/kvm/2018/09/5393106/t.md

```text
On Fri, Sep 28, 2018 at 07:37:37AM +0800, Jason Wang wrote:
> 
> 
> On 2018年09月28日 01:04, Michael S. Tsirkin wrote:
> > On Thu, Sep 27, 2018 at 08:22:04PM +0800, Jason Wang wrote:
> > > The access of vsock is not protected by vhost_vsock_lock. This may
> > > lead to use after free since vhost_vsock_dev_release() may free the
> > > pointer at the same time.
> > > 
> > > Fix this by holding the lock during the access.
> > > 
> > > Reported-by:syzbot+e3e074963495f92a89ed@syzkaller.appspotmail.com
> > > Fixes: 16320f363ae1 ("vhost-vsock: add pkt cancel capability")
> > > Fixes: 433fc58e6bf2 ("VSOCK: Introduce vhost_vsock.ko")
> > > Cc: Stefan Hajnoczi<stefanha@redhat.com>
> > > Signed-off-by: Jason Wang<redacted>
> > Wow is that really the best we can do?
> 
> For net/stable, probably yes.
> 
> >   A global lock on a data path
> > operation?
> 
> It's already there,

&vhost_vsock_lock? were is it takes on data path?

> and the patch only increase the critical section.
> 
> >   Granted use after free is nasty but Stefan said he sees
> > a way to fix it using a per socket refcount. He's on vacation
> > until Oct 4 though ...
> > 
> 
> Stefan has acked the pacth, so I think it's ok? We can do optimization for
> -next on top.
> 
> Thanks


Well on high SMP serializing can drop performance as much as x100 so I'm
not sure it's appropriate - seems to fix a bug but can introduce a
regression. Let's see how does a proper fix look first?

-- 
MST
```