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

Florian Westphal [off-list ref] wrote:

Thadeu Lima de Souza Cascardo [off-list ref] wrote:

quoted

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 ]

quoted

I have hit this as well, this fixes it for me on an older kernel. Can you try it
on latest kernel?

quoted

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().

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.

Cascardo, can you cook a patch?

Thanks!