Re: [PATCH net] net: prevent pulling SKB_GSO_FRAGLIST skb

[Willem de Bruijn]

shiming.cheng@ wrote:
> From: Shiming Cheng <shiming.cheng@xxxxxxxxxxxx>
> 
> BPF or TC callers may pull in a length longer than skb_headlen()
> for a SKB_GSO_FRAGLIST skb. The data in fraglist will be pulled
> into the linear space. However it destroys the skb's structure
> and may result in an invalid segmentation or kernel exception.
> 
> So we should add protection to stop the operation and return
> error to remind callers.
> 
> Fixes: 3a1296a38d0c ("net: Support GRO/GSO fraglist chaining.")
> Signed-off-by: Shiming Cheng <shiming.cheng@xxxxxxxxxxxx>
> Signed-off-by: Lena Wang <lena.wang@xxxxxxxxxxxx>
>
> ---
>  net/core/skbuff.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index f68f2679b086..2d35e009e814 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -6100,6 +6100,12 @@ EXPORT_SYMBOL(skb_vlan_untag);
>  
>  int skb_ensure_writable(struct sk_buff *skb, unsigned int write_len)
>  {
> +	if (skb_is_gso(skb) &&
> +	    (skb_shinfo(skb)->gso_type & SKB_GSO_FRAGLIST) &&
> +	     write_len > skb_headlen(skb)) {
> +		return -ENOMEM;
> +	}
> +

Most callers of skb_ensure_writable pull less than headlen.
It might be good to start with the write_len check. Before
looking at gso type.

>  	if (!pskb_may_pull(skb, write_len))
>  		return -ENOMEM;
>  
> -- 
> 2.18.0
>