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 22:59, 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.
> 
> Actually, here is something even simpler that should work for both v4
> and v6:

Makes sense. It does come with higher cost of calling skb_checksum.
 
> ---
> --- a/net/ipv4/tcp_offload.c
> +++ b/net/ipv4/tcp_offload.c
> @@ -101,8 +101,14 @@ 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);
> +
> +		if (skb_pagelen(skb) - th->doff * 4 == skb_shinfo(skb)->gso_size)
> +			return __tcp4_gso_segment_list(skb, features);
> +
> +		skb->ip_summed = CHECKSUM_NONE;
> +	}
>   
>   	if (unlikely(skb->ip_summed != CHECKSUM_PARTIAL)) {
>   		const struct iphdr *iph = ip_hdr(skb);
> --- a/net/ipv6/tcpv6_offload.c
> +++ b/net/ipv6/tcpv6_offload.c
> @@ -159,8 +159,14 @@ static struct sk_buff *tcp6_gso_segment(struct sk_buff *skb,
>   	if (!pskb_may_pull(skb, sizeof(*th)))
>   		return ERR_PTR(-EINVAL);
>   
> -	if (skb_shinfo(skb)->gso_type & SKB_GSO_FRAGLIST)
> -		return __tcp6_gso_segment_list(skb, features);
> +	if (skb_shinfo(skb)->gso_type & SKB_GSO_FRAGLIST) {
> +		struct tcphdr *th = tcp_hdr(skb);
> +
> +		if (skb_pagelen(skb) - th->doff * 4 == skb_shinfo(skb)->gso_size)
> +			return __tcp6_gso_segment_list(skb, features);
> +
> +		skb->ip_summed = CHECKSUM_NONE;
> +	}
>   
>   	if (unlikely(skb->ip_summed != CHECKSUM_PARTIAL)) {
>   		const struct ipv6hdr *ipv6h = ipv6_hdr(skb);
> 
>