Re: [PATCH net-next] skbuff: revert "skbuff: remove some unnecessary operation in skb_segment_list()"

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2021/4/14 18:48, Paolo Abeni wrote:
> the commit 1ddc3229ad3c ("skbuff: remove some unnecessary operation
> in skb_segment_list()") introduces an issue very similar to the
> one already fixed by commit 53475c5dd856 ("net: fix use-after-free when
> UDP GRO with shared fraglist").
> 
> If the GSO skb goes though skb_clone() and pskb_expand_head() before
> entering skb_segment_list(), the latter  will unshare the frag_list
> skbs and will release the old list. With the reverted commit in place,
> when skb_segment_list() completes, skb->next points to the just
> released list, and later on the kernel will hit UaF.

In that case, is "nskb->next = list_skb" needed before jumpping to
error when __skb_linearize() fails? As there is "nskb->next = list_skb"
before jumpping to error handling when skb_clone() fails.

The inconsistency above is the reason I sent the reverted patch:)

> 
> Note that since commit e0e3070a9bc9 ("udp: properly complete L4 GRO
> over UDP tunnel packet") the critical scenario can be reproduced also
> receiving UDP over vxlan traffic with:
> 
> NIC (NETIF_F_GRO_FRAGLIST enabled) -> vxlan -> UDP sink
> 
> Attaching a packet socket to the NIC will cause skb_clone() and the
> tunnel decapsulation will call pskb_expand_head().
> 
> Fixes: 1ddc3229ad3c ("skbuff: remove some unnecessary operation in skb_segment_list()")
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> ---
>  net/core/skbuff.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 3ad9e8425ab2..14010c0eec48 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -3773,13 +3773,13 @@ struct sk_buff *skb_segment_list(struct sk_buff *skb,
>  	unsigned int tnl_hlen = skb_tnl_header_len(skb);
>  	unsigned int delta_truesize = 0;
>  	unsigned int delta_len = 0;
> +	struct sk_buff *tail = NULL;
>  	struct sk_buff *nskb, *tmp;
>  	int err;
>  
>  	skb_push(skb, -skb_network_offset(skb) + offset);
>  
>  	skb_shinfo(skb)->frag_list = NULL;
> -	skb->next = list_skb;
>  
>  	do {
>  		nskb = list_skb;
> @@ -3797,8 +3797,17 @@ struct sk_buff *skb_segment_list(struct sk_buff *skb,
>  			}
>  		}
>  
> -		if (unlikely(err))
> +		if (!tail)
> +			skb->next = nskb;
> +		else
> +			tail->next = nskb;
> +
> +		if (unlikely(err)) {
> +			nskb->next = list_skb;
>  			goto err_linearize;
> +		}
> +
> +		tail = nskb;
>  
>  		delta_len += nskb->len;
>  		delta_truesize += nskb->truesize;
> @@ -3825,7 +3834,7 @@ struct sk_buff *skb_segment_list(struct sk_buff *skb,
>  
>  	skb_gso_reset(skb);
>  
> -	skb->prev = nskb;
> +	skb->prev = tail;
>  
>  	if (skb_needs_linearize(skb, features) &&
>  	    __skb_linearize(skb))
>