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

[Felix Fietkau <nbd@nbd.name>]

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:

---
--- 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);