linux-kernel - Re: [BUG] skb corruption and kernel panic at forwarding with fragmentation

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Message-ID: <CALYGNiN-cGxcEwiN-aPH7W1Pa7bLc2UY5LY98mXr+yZK+q5_BA@mail.gmail.com>
Date:	Thu, 7 Jan 2016 14:38:21 +0300
From:	Konstantin Khlebnikov <koct9i@...il.com>
To:	Florian Westphal <fw@...len.de>
Cc:	Thadeu Lima de Souza Cascardo <cascardo@...hat.com>,
	Cong Wang <xiyou.wangcong@...il.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] skb corruption and kernel panic at forwarding with fragmentation

On Thu, Jan 7, 2016 at 2:00 PM, Konstantin Khlebnikov <koct9i@...il.com> wrote:
> On Thu, Jan 7, 2016 at 2:49 AM, Florian Westphal <fw@...len.de> wrote:
>> Florian Westphal <fw@...len.de> wrote:
>>> Thadeu Lima de Souza Cascardo <cascardo@...hat.com> wrote:
>>> > On Wed, Jan 06, 2016 at 11:11:41PM +0300, Konstantin Khlebnikov wrote:
>>
>> [ skb_gso_segment uses skb->cb[], causes oops if ip_fragment is invoked
>>   on segmented skbs ]
>>
>>> > I have hit this as well, this fixes it for me on an older kernel. Can you try it
>>> > on latest kernel?
>>>
>>> > diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
>>> > index d8a1745..f44bc91 100644
>>> > --- a/net/ipv4/ip_output.c
>>> > +++ b/net/ipv4/ip_output.c
>>> > @@ -216,6 +216,7 @@ static int ip_finish_output_gso(struct sk_buff *skb)
>>> >     netdev_features_t features;
>>> >     struct sk_buff *segs;
>>> >     int ret = 0;
>>> > +   struct inet_skb_parm ipcb;
>>> >
>>> >     if (skb_gso_network_seglen(skb) <= ip_skb_dst_mtu(skb))
>>> >             return ip_finish_output2(skb);
>>> > @@ -227,6 +228,10 @@ static int ip_finish_output_gso(struct sk_buff *skb)
>>> >      * 2) skb arrived via virtio-net, we thus get TSO/GSO skbs directly
>>> >      * from host network stack.
>>> >      */
>>> > +   /* We need to save IPCB here because skb_gso_segment will use
>>> > +    * SKB_GSO_CB.
>>> > +    */
>>> > +   ipcb = *IPCB(skb);
>>> >     features = netif_skb_features(skb);
>>> >     segs = skb_gso_segment(skb, features & ~NETIF_F_GSO_MASK);
>>> >     if (IS_ERR_OR_NULL(segs)) {
>>> > @@ -241,6 +246,7 @@ static int ip_finish_output_gso(struct sk_buff *skb)
>>> >             int err;
>>> >
>>> >             segs->next = NULL;
>>> > +           *IPCB(segs) = ipcb;
>>> >             err = ip_fragment(segs, ip_finish_output2);
>>> >
>>> >             if (err && ret == 0)
>>>
>>> I'm worried that this doesn't solve all cases. f.e. xfrm may also
>>> call skb_gso_segment(), and it will call into ipv4/ipv6 netfilter
>>> postrouting + ipv4 output functions...
>>>
>>> nfqnl_enqueue_packet() is also affected.
>>
>> ... but it seems that those three are the only affected callers
>> of skb_gso_segment (tbf is ok since skb isn't owned by anyone,
>> ovs does save/restore already).
>>
>> I think this patch is the right way, we just need similar
>> save/restore in nfqnl_enqueue_packet and xfrm_output_gso().
>
> Which CB could be here? at this point skb isn't owned by netlink yet.
>
>>
>> The latter two can be used by either ipv4 or ipv6 so it might
>> be preferable to just save/restore sizeof(struct skb_gso_cb);
>> or a union of inet_skb_parm+inet6_skb_parm.
>
> Or just shift GSO CB and add couple checks like
> BUILD_BUG_ON(sizeof(SKB_GSO_CB(skb)->room) < sizeof(*IPCB(skb)));

Somethin like this (in attachment)

Also I've found strange thing: reason of expanding skb->cb from 40 to
48 bypes in 2006
3e3850e989c5d2eb1aab6f0fd9257759f0f4cbc6 was that struct inet6_skb_parm does
not fit. But it's is only 24 bytes. Does some arches add pad after
each _u16 field?

Download attachment "net-preserve-lower-bytes-of-skb-cb-during-gso-segmentation" of type "application/octet-stream" (3490 bytes)