[SRU][J][PATCH 1/1] ipv6: sit: reload inner IPv6 header after GSO offloads

[Cengiz Can]

From: Kyle Zeng <kylebot at openai.com>

ipip6_tunnel_xmit() caches the inner IPv6 header pointer at function
entry and continues using it after iptunnel_handle_offloads().

For GSO skbs, iptunnel_handle_offloads() calls skb_header_unclone().
When the skb header is cloned, skb_header_unclone() can call
pskb_expand_head(), which may move the skb head. The pskb_expand_head()
contract requires pointers into the skb header to be reloaded after the
call.

If the later skb_realloc_headroom() branch is not taken, SIT uses the
stale iph6 pointer to read the inner hop limit and DS field. That can
read from a freed skb head after the old head's remaining clone is
released.

Reload iph6 after the offload helper succeeds and before subsequent
reads from the inner IPv6 header. Keep the existing reload after
skb_realloc_headroom(), since that branch can also replace the skb.

Fixes: 14909664e4e1 ("sit: Setup and TX path for sit/UDP foo-over-udp encapsulation")
Signed-off-by: Kyle Zeng <kylebot at openai.com>
Reviewed-by: Eric Dumazet <edumazet at google.com>
Reported-by: syzbot+6eb9ca986d80f6f88cf9 at syzkaller.appspotmail.com
Link: https://patch.msgid.link/20260605073448.6524-1-kylebot@openai.com
Signed-off-by: Jakub Kicinski <kuba at kernel.org>
(cherry picked from commit f0e42f0c4337b1f220de1ddd63f47197c7dee4de)
CVE-2026-53228
Assisted-by: kybele:claude-opus-4.8
Signed-off-by: Cengiz Can <cengiz.can at canonical.com>
---
 net/ipv6/sit.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/ipv6/sit.c b/net/ipv6/sit.c
index 3bc02ab9ceac..bc5db4a9dbfa 100644
--- a/net/ipv6/sit.c
+++ b/net/ipv6/sit.c
@@ -971,6 +971,7 @@ static netdev_tx_t ipip6_tunnel_xmit(struct sk_buff *skb,
 		ip_rt_put(rt);
 		goto tx_error;
 	}
+	iph6 = ipv6_hdr(skb);
 
 	if (df) {
 		mtu = dst_mtu(&rt->dst) - t_hlen;
-- 
2.43.0