[PATCH net v2 1/1] seg6: reset IP6CB after IPv6 decapsulation

From: Zhiling Zou <[email protected]>

decap_and_validate() pulls the outer SRv6 headers and makes the inner
packet the skb network header. The IPv6 control block still contains
values collected while parsing the outer packet, including nhoff and
extension-header flags.

End.DX6 and End.DT6 route the inner IPv6 packet directly to the IPv6
input path. An unprivileged user can reach End.DT6 from a user and net
namespace by installing a local SID and injecting an outer packet with
Hop-by-Hop and Destination Options headers followed by an SRH and a
minimal inner IPv6 packet.

The outer extension headers leave a large nhoff in IP6CB. After
decapsulation, ip6_protocol_deliver_rcu() uses that stale offset on the
inner packet and reads beyond the skb head. KASAN reports:

  BUG: KASAN: slab-out-of-bounds in ip6_protocol_deliver_rcu
  ip6_protocol_deliver_rcu+0x1118/0x1450
  ip6_input_finish+0x11b/0x240
  seg6_local_input_core+0xed/0x2e0
  lwtunnel_input+0x1e9/0x4e0
  ipv6_rthdr_rcv+0x525f/0x6c50
  ip6_protocol_deliver_rcu+0xcb7/0x1450

Before clearing IP6CB for an inner IPv6 packet, save its incoming
interface index and L3 slave state. Restore both after the clear and set
nhoff to the inner IPv6 base-header nexthdr field.

Use IP6CB(skb)->iif rather than skb->skb_iif because VRF processing can
replace skb_iif with the L3 master while IP6CB keeps the receiving
interface. Preserve IP6SKB_L3SLAVE for the same reason.

Fixes: d7a669dd2f8b ("ipv6: sr: add helper functions for seg6local")
Cc: [email protected]
Reported-by: Vega <[email protected]>
Signed-off-by: Zhiling Zou <[email protected]>
---
changes in v2:
- Preserve the ingress index from IP6CB instead of using skb->skb_iif.
- Preserve IP6SKB_L3SLAVE across the IPv6 control-block reset.
- Place the IPv6 reset next to the merged IPv4 reset.
- Add reachability and the relevant KASAN call chain to the commit log.
- v1 Link: https://lore.kernel.org/all/6a236dcc43d141a9105e8ee1b84bcccc628d83b0.1786894895.git.zhilinz@nebusec.ai/

 net/ipv6/seg6_local.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/net/ipv6/seg6_local.c b/net/ipv6/seg6_local.c
index 95ea0b62729ac..7b52122201858 100644
--- a/net/ipv6/seg6_local.c
+++ b/net/ipv6/seg6_local.c
@@ -261,6 +261,15 @@ static bool decap_and_validate(struct sk_buff *skb, int proto)
 
 		memset(IPCB(skb), 0, sizeof(*IPCB(skb)));
 		IPCB(skb)->iif = iif;
+	} else if (proto == IPPROTO_IPV6) {
+		bool l3slave = ipv6_l3mdev_skb(IP6CB(skb)->flags);
+		int iif = IP6CB(skb)->iif;
+
+		memset(IP6CB(skb), 0, sizeof(*IP6CB(skb)));
+		IP6CB(skb)->iif = iif;
+		IP6CB(skb)->nhoff = offsetof(struct ipv6hdr, nexthdr);
+		if (l3slave)
+			IP6CB(skb)->flags |= IP6SKB_L3SLAVE;
 	}
 
 	return true;
-- 
2.43.0