[PATCH net] ipv6: ioam6: do not modify shared tuninfo eh.nexthdr

[Hangbin Liu]

From: Hangbin Liu <liuhangbin@xxxxxxxxxx>

ioam6_do_inline() and ioam6_do_encap() stored the next header into
the shared ilwt->tuninfo template and then memcpy()ed it into the
skb. The lwtstate is used from multiple CPUs without locking, so
another packet can overwrite eh.nexthdr between the store and the
memcpy.

The packet can then leave with an HBH nexthdr that does not match its
payload (e.g. TCP labelled as UDP, or a local packet labelled
IPPROTO_IPV6). Set nexthdr on the per-packet Hop-by-Hop copy after
memcpy instead, leave the shared template read-only.

Fixes: 3edede08ff37 ("ipv6: ioam: Support for IOAM injection with lwtunnels")
Reported-by: Sashiko <netdev-bot+sashiko@xxxxxxxxxx>
Closes: https://lore.kernel.org/netdev/179336082844.3134.1265197633714491521@xxxxxxxxxx
Signed-off-by: Hangbin Liu <liuhangbin@xxxxxxxxxx>
---
The issue is reported by Sashiko. It was confirmed with concurrent TCP
and UDP traffic through a shared ioam6 inline route on a multi-CPU host.

Without the fix, a packet can leave with an HBH nexthdr that does not
match its payload (e.g. hbh->nexthdr=17 with a TCP segment after the HBH,
or hbh->nexthdr=6 with a UDP segment). With the fix, the same load reports
no HBH/L4 mismatch: nexthdr is written only on the per-packet Hop-by-Hop
header after memcpy, never on the shared tuninfo template.
---
 net/ipv6/ioam6_iptunnel.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv6/ioam6_iptunnel.c b/net/ipv6/ioam6_iptunnel.c
index cfb2c41634a0..b23f8a0cbaa9 100644
--- a/net/ipv6/ioam6_iptunnel.c
+++ b/net/ipv6/ioam6_iptunnel.c
@@ -276,12 +276,12 @@ static int ioam6_do_inline(struct net *net, struct sk_buff *skb,
 
 	hdr = ipv6_hdr(skb);
 	memmove(hdr, oldhdr, sizeof(*oldhdr));
-	tuninfo->eh.nexthdr = hdr->nexthdr;
 
 	skb_set_transport_header(skb, sizeof(*hdr));
 	skb_postpush_rcsum(skb, hdr, sizeof(*hdr) + hdrlen);
 
 	memcpy(skb_transport_header(skb), (u8 *)tuninfo, hdrlen);
+	((struct ipv6_opt_hdr *)skb_transport_header(skb))->nexthdr = hdr->nexthdr;
 
 	hdr->nexthdr = NEXTHDR_HOP;
 	hdr->payload_len = cpu_to_be16(skb->len - sizeof(*hdr));
@@ -314,8 +314,8 @@ static int ioam6_do_encap(struct net *net, struct sk_buff *skb,
 	skb_mac_header_rebuild(skb);
 	skb_set_transport_header(skb, sizeof(*hdr));
 
-	tuninfo->eh.nexthdr = NEXTHDR_IPV6;
 	memcpy(skb_transport_header(skb), (u8 *)tuninfo, hdrlen);
+	((struct ipv6_opt_hdr *)skb_transport_header(skb))->nexthdr = NEXTHDR_IPV6;
 
 	hdr = ipv6_hdr(skb);
 	memcpy(hdr, inner_hdr, sizeof(*hdr));

---
base-commit: c3e67bd82b9b9b1a5118f09a429ccefe2aab0741
change-id: 20261103-ipv6_ioam_lock-65102ca54908

Best regards,
--  
Hangbin Liu <liuhangbin@xxxxxxxxxx>