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

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Message-ID: <4F8D4A28.7030802@redhat.com>
Date:	Tue, 17 Apr 2012 18:47:04 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	"Michael S. Tsirkin" <mst@...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 04/17/2012 06:22 PM, Michael S. Tsirkin wrote:
> 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;

Yes, It's more is simpler.
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