Re: [PATCH] net: preserve IP control block during GSO segmentation

[Thadeu Lima de Souza Cascardo <cascardo@redhat.com>]

On Fri, Jan 08, 2016 at 03:00:41PM +0300, Konstantin Khlebnikov wrote:
> Skb_gso_segment() uses skb control block during segmentation.
> This patch adds 32-bytes room for previous control block which
> will be copied into all resulting segments.
> 
> This patch fixes kernel crash during fragmenting forwarded packets.
> Fragmentation requires valid IP CB in skb for clearing ip options.
> Also patch removes custom save/restore in ovs code, now it's redundant.
> 
> Signed-off-by: Konstantin Khlebnikov <koct9i@gmail.com>
> Link: http://lkml.kernel.org/r/CALYGNiP-0MZ-FExV2HutTvE9U-QQtkKSoE--KN=JQE5STYsjAA@mail.gmail.com
> ---
>  include/linux/skbuff.h     |    3 ++-
>  net/core/dev.c             |    5 +++++
>  net/ipv4/ip_output.c       |    1 +
>  net/openvswitch/datapath.c |    4 +---
>  net/xfrm/xfrm_output.c     |    2 ++
>  5 files changed, 11 insertions(+), 4 deletions(-)
> 

> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 4355129fff91..9147f9f34cbe 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -3446,7 +3446,8 @@ struct skb_gso_cb {
>  	int	encap_level;
>  	__u16	csum_start;
>  };
> -#define SKB_GSO_CB(skb) ((struct skb_gso_cb *)(skb)->cb)
> +#define SKB_SGO_CB_OFFSET	32
> +#define SKB_GSO_CB(skb) ((struct skb_gso_cb *)((skb)->cb + SKB_SGO_CB_OFFSET))
>  
>  static inline int skb_tnl_header_len(const struct sk_buff *inner_skb)
>  {
> diff --git a/net/core/dev.c b/net/core/dev.c
> index ae00b894e675..7f00f2439770 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -2542,6 +2542,8 @@ static inline bool skb_needs_check(struct sk_buff *skb, bool tx_path)
>   *
>   *	It may return NULL if the skb requires no segmentation.  This is
>   *	only possible when GSO is used for verifying header integrity.
> + *
> + *	Segmentation preserves SKB_SGO_CB_OFFSET bytes of previous skb cb.
>   */
>  struct sk_buff *__skb_gso_segment(struct sk_buff *skb,
>  				  netdev_features_t features, bool tx_path)
> @@ -2556,6 +2558,9 @@ struct sk_buff *__skb_gso_segment(struct sk_buff *skb,
>  			return ERR_PTR(err);
>  	}
>  
> +	BUILD_BUG_ON(SKB_SGO_CB_OFFSET +
> +		     sizeof(*SKB_GSO_CB(skb)) > sizeof(skb->cb));
> +
>  	SKB_GSO_CB(skb)->mac_offset = skb_headroom(skb);
>  	SKB_GSO_CB(skb)->encap_level = 0;
>  
> diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> index 4233cbe47052..59ed4b89b67a 100644
> --- a/net/ipv4/ip_output.c
> +++ b/net/ipv4/ip_output.c
> @@ -240,6 +240,7 @@ static int ip_finish_output_gso(struct net *net, struct sock *sk,
>  	 * from host network stack.
>  	 */
>  	features = netif_skb_features(skb);
> +	BUILD_BUG_ON(sizeof(*IPCB(skb)) > SKB_SGO_CB_OFFSET);
>  	segs = skb_gso_segment(skb, features & ~NETIF_F_GSO_MASK);
>  	if (IS_ERR_OR_NULL(segs)) {
>  		kfree_skb(skb);
> diff --git a/net/openvswitch/datapath.c b/net/openvswitch/datapath.c
> index 91a8b004dc51..b1b380ee667d 100644
> --- a/net/openvswitch/datapath.c
> +++ b/net/openvswitch/datapath.c
> @@ -336,12 +336,10 @@ static int queue_gso_packets(struct datapath *dp, struct sk_buff *skb,
>  	unsigned short gso_type = skb_shinfo(skb)->gso_type;
>  	struct sw_flow_key later_key;
>  	struct sk_buff *segs, *nskb;
> -	struct ovs_skb_cb ovs_cb;
>  	int err;
>  
> -	ovs_cb = *OVS_CB(skb);
> +	BUILD_BUG_ON(sizeof(*OVS_CB(skb)) > SKB_SGO_CB_OFFSET);
>  	segs = __skb_gso_segment(skb, NETIF_F_SG, false);
> -	*OVS_CB(skb) = ovs_cb;
>  	if (IS_ERR(segs))
>  		return PTR_ERR(segs);
>  	if (segs == NULL)

You are missing this hunk.

--- a/net/openvswitch/datapath.c
+++ b/net/openvswitch/datapath.c
@@ -359,7 +359,6 @@ static int queue_gso_packets(struct datapath *dp, struct
sk_buff *skb,
        /* Queue all of the segments. */
        skb = segs;
        do {
-               *OVS_CB(skb) = ovs_cb;
                if (gso_type & SKB_GSO_UDP && skb != segs)
                        key = &later_key;


> diff --git a/net/xfrm/xfrm_output.c b/net/xfrm/xfrm_output.c
> index cc3676eb6239..ff4a91fcab9f 100644
> --- a/net/xfrm/xfrm_output.c
> +++ b/net/xfrm/xfrm_output.c
> @@ -167,6 +167,8 @@ static int xfrm_output_gso(struct net *net, struct sock *sk, struct sk_buff *skb
>  {
>  	struct sk_buff *segs;
>  
> +	BUILD_BUG_ON(sizeof(*IPCB(skb)) > SKB_SGO_CB_OFFSET);
> +	BUILD_BUG_ON(sizeof(*IP6CB(skb)) > SKB_SGO_CB_OFFSET);
>  	segs = skb_gso_segment(skb, 0);
>  	kfree_skb(skb);
>  	if (IS_ERR(segs))
>