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

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

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")