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

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Date:	Tue, 17 Apr 2012 13:22:20 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	netdev@...r.kernel.org, xma@...ibm.com, davem@...emloft.net,
	linux-kernel@...r.kernel.org, ebiederm@...ssion.com
Subject: Re: [PATCH 5/6] vhost_net: fix use after free of vq->ubufs

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<jasowang@...hat.com>
> >
> >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
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