Subject: Re: [PATCH 5/6] vhost_net: fix use after free of vq->ubufs

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 17 Apr 2012 13:22:20 +0300

On Tue, Apr 17, 2012 at 11:19:42AM +0800, Jason Wang wrote:
> On 04/16/2012 09:28 PM, Michael S. Tsirkin wrote:
> >On Mon, Apr 16, 2012 at 02:08:25PM +0800, Jason Wang wrote:
> >>When zerocopy socket is used, ubufs pointer were used in handle_tx()
> >>without any validation. This would cause NULL pointer deference after
> >>it has been freed in vhost_net_set_backend(). Fix this by check the
> >>pointer before using it.
> >>
> >>Signed-off-by: Jason Wang<[email-masked]>
> >
> >OK so it's NULL dereference and not user after free :)
> >Also could you clarify how does this happen pls?
> >Don't we always initialize ubufs when vhost_sock_zcopy is set?
> 
> The problem happens when we want to disable backend. At this time
> ubufs were assigned to NULL and it may be dereferenced by
> handle_tx():

Heh, I see. How about
- zcopy = vhost_sock_zcopy(sock);
+ zcopy = vq->ubufs;

-- 
MST