Re: [PATCH net] gso: fix gso fraglist segmentation after pull from frag_list

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Felix Fietkau wrote:
> On 25.09.24 21:09, Willem de Bruijn wrote:
> > Felix Fietkau wrote:
> >> On 22.09.24 17:03, Willem de Bruijn wrote:
> >> > From: Willem de Bruijn <willemb@google.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@google.com>
> >> > Cc: stable@vger.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?
> > 
> > Thanks. That was not intentional. I think you're right. Am a bit
> > concerned that all this testing did not catch it. Perhaps because
> > CHECKSUM_PARTIAL looped to ingress on the same machine is simply
> > interpreted as CHECKSUM_UNNECESSARY. Need to look into that.
> > 
> > If respinning this, I should also change the Fixes to
> > 
> > Fixes: 9fd1ff5d2ac7 ("udp: Support UDP fraglist GRO/GSO.")
> > 
> > Analogous to the eventual TCP fix to
> > 
> > Fixes: bee88cd5bd83 ("net: add support for segmenting TCP fraglist GSO packets")
> 
> In the mean time, I've been working on the TCP side. I managed to
> reproduce the issue on one of my devices by routing traffic from
> Ethernet to Wifi using your BPF test program.
> 
> The following patch makes it work for me for TCP v4. Still need to
> test and fix v6.
> 
> - Felix
> 
> ---
> --- a/net/ipv4/tcp_offload.c
> +++ b/net/ipv4/tcp_offload.c
> @@ -101,8 +101,20 @@ static struct sk_buff *tcp4_gso_segment(struct sk_buff *skb,
>   	if (!pskb_may_pull(skb, sizeof(struct tcphdr)))
>   		return ERR_PTR(-EINVAL);
>   
> -	if (skb_shinfo(skb)->gso_type & SKB_GSO_FRAGLIST)
> -		return __tcp4_gso_segment_list(skb, features);
> +	if (skb_shinfo(skb)->gso_type & SKB_GSO_FRAGLIST) {
> +		struct tcphdr *th = tcp_hdr(skb);
> +		const struct iphdr *iph;
> +
> +		if (skb_pagelen(skb) - th->doff * 4 == skb_shinfo(skb)->gso_size)
> +			return __tcp4_gso_segment_list(skb, features);
> +
> +		iph = ip_hdr(skb);
> +		skb_shinfo(skb)->gso_type &= ~SKB_GSO_FRAGLIST;

I left this out, because skb_segment does not care about this bit.

> +		skb->csum_start = (unsigned char *)th - skb->head;
> +		skb->csum_offset = offsetof(struct tcphdr, check);
> +		skb->ip_summed = CHECKSUM_PARTIAL;
> +		th->check = ~tcp_v4_check(skb->len, iph->saddr, iph->daddr, 0);
> +	}
>   
>   	if (unlikely(skb->ip_summed != CHECKSUM_PARTIAL)) {
>   		const struct iphdr *iph = ip_hdr(skb);
>