netdev - Re: [PATCH net] gso: fix gso fraglist segmentation after pull from frag

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Message-ID: <7144e848-9919-44a5-a313-f9b2076532bf@nbd.name>
Date: Wed, 25 Sep 2024 10:10:36 +0200
From: Felix Fietkau <nbd@....name>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>, netdev@...r.kernel.org
Cc: davem@...emloft.net, kuba@...nel.org, edumazet@...gle.com,
 pabeni@...hat.com, stable@...r.kernel.org, maze@...gle.com,
 shiming.cheng@...iatek.com, daniel@...earbox.net, lena.wang@...iatek.com,
 herbert@...dor.apana.org.au, Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net] gso: fix gso fraglist segmentation after pull from
 frag_list

On 22.09.24 17:03, Willem de Bruijn wrote:
> From: Willem de Bruijn <willemb@...gle.com>
> 
> Detect gso fraglist skbs with corrupted geometry (see below) and
> pass these to skb_segment instead of skb_segment_list, as the first
> can segment them correctly.
> 
> Valid SKB_GSO_FRAGLIST skbs
> - consist of two or more segments
> - the head_skb holds the protocol headers plus first gso_size
> - one or more frag_list skbs hold exactly one segment
> - all but the last must be gso_size
> 
> Optional datapath hooks such as NAT and BPF (bpf_skb_pull_data) can
> modify these skbs, breaking these invariants.
> 
> In extreme cases they pull all data into skb linear. For UDP, this
> causes a NULL ptr deref in __udpv4_gso_segment_list_csum at
> udp_hdr(seg->next)->dest.
> 
> Detect invalid geometry due to pull, by checking head_skb size.
> Don't just drop, as this may blackhole a destination. Convert to be
> able to pass to regular skb_segment.
> 
> Link: https://lore.kernel.org/netdev/20240428142913.18666-1-shiming.cheng@mediatek.com/
> Fixes: 3a1296a38d0c ("net: Support GRO/GSO fraglist chaining.")
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> Cc: stable@...r.kernel.org
> 
> ---
> diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> index d842303587af..e457fa9143a6 100644
> --- a/net/ipv4/udp_offload.c
> +++ b/net/ipv4/udp_offload.c
> @@ -296,8 +296,16 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
>   		return NULL;
>   	}
>   
> -	if (skb_shinfo(gso_skb)->gso_type & SKB_GSO_FRAGLIST)
> -		return __udp_gso_segment_list(gso_skb, features, is_ipv6);
> +	if (skb_shinfo(gso_skb)->gso_type & SKB_GSO_FRAGLIST) {
> +		 /* Detect modified geometry and pass these to skb_segment. */
> +		if (skb_pagelen(gso_skb) - sizeof(*uh) == skb_shinfo(gso_skb)->gso_size)
> +			return __udp_gso_segment_list(gso_skb, features, is_ipv6);
> +
> +		 /* Setup csum, as fraglist skips this in udp4_gro_receive. */
> +		gso_skb->csum_start = skb_transport_header(gso_skb) - gso_skb->head;
> +		gso_skb->csum_offset = offsetof(struct udphdr, check);
> +		gso_skb->ip_summed = CHECKSUM_PARTIAL;

I also noticed this uh->check update done by udp4_gro_complete only in 
case of non-fraglist GRO:

     if (uh->check)
         uh->check = ~udp_v4_check(skb->len - nhoff, iph->saddr,
                       iph->daddr, 0);

I didn't see any equivalent in your patch. Is it missing or left out 
intentionally?

- Felix