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ipv6: sr: restore network header before routing and forwarding

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ipv6_srh_rcv() runs with skb->data at the Segment Routing Header (SRH)
while skb_network_header() points at the IPv6 header.

When segments_left > 0, ipv6_srh_rcv() previously restored the skb->data
position by pushing sizeof(struct ipv6hdr), assuming the SRH immediately
followed the fixed IPv6 header. If another extension header (such as a
Hop-by-Hop options header) precedes the SRH, skb_network_offset()
remained negative.

This led to two problems:
1. During ip6_route_input(), fib6_rules_early_flow_dissect() invokes
   __skb_flow_dissect() which passes the negative skb_network_offset()
   to flow dissection, breaking BPF and C flow dissector logic.
2. If forwarded via ip6_forward() or redirected via act_mirred, downstream
   handlers (like sch_fragment() or neighbour output) pass the negative
   offset as an unsigned length, triggering OOB memcpy or buffer overflows.

Fix this by pushing -skb_network_offset(skb) before routing, ensuring
skb_network_offset(skb) is 0 for route lookup / flow dissection as well as
downstream forwarding. On the loopback path, pull skb_transport_offset(skb)
to restore skb->data to the SRH before looping back.

Fixes: 1ababeba4a ("ipv6: implement dataplane support for rthdr type 4 (Segment Routing Header)")
Reported-by: TencentOS Corvus AI <corvus@tencent.com>
Reported-by: Jun Yang <junvyyang@tencent.com>
Reported-by: Fourie Zhang <fouriezhang@tencent.com>
Closes: https://lore.kernel.org/netdev/20260817104128.22681-1-juny24602@gmail.com/
Closes: https://lore.kernel.org/netdev/20260827092345.2301937-1-fouriezhang@tencent.com/
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260828141727.2372570-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Eric Dumazet 2026-08-28 14:17:27 +00:00 committed by Jakub Kicinski
parent 81c600c263
commit 975b5b067f
1 changed files with 2 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
net/ipv6/exthdrs.c
View File

@ -445,7 +445,7 @@ static int ipv6_srh_rcv(struct sk_buff *skb, struct inet6_dev *idev)
hdr->segments_left--;
addr = hdr->segments + hdr->segments_left;
skb_push(skb, sizeof(struct ipv6hdr));
skb_push(skb, -skb_network_offset(skb));
if (skb->ip_summed == CHECKSUM_COMPLETE)
seg6_update_csum(skb);
@ -469,7 +469,7 @@ static int ipv6_srh_rcv(struct sk_buff *skb, struct inet6_dev *idev)
}
ipv6_hdr(skb)->hop_limit--;
skb_pull(skb, sizeof(struct ipv6hdr));
skb_pull(skb, skb_transport_offset(skb));
goto looped_back;
}