[PATCH net v2] ipvs: fix infinite loop with ipvlan L3 from unconditional ipvs_property clear

[Chenguang Zhao <chenguang.zhao@linux.dev>]

From: Chenguang Zhao <zhaochenguang@kylinos.cn>

Commit de2c211868b9 ("ipvs: Always clear ipvs_property flag in
skb_scrub_packet()") moved ipvs_reset() before the xnet check, making
the call unconditional. The intent was to fix a bpf_redirect case where
stale ipvs_property on an skb re-entering the RX path caused the SNAT
hook to be skipped. However the change is too broad: when IPVS NAT
sits above an ipvlan L3 interface in the same netns, the following
loop happens:

  LOCAL_OUT -> IPVS DNAT (sets ipvs_property=1) -> dst_output -> ipvlan
    -> skb_scrub_packet() -> ipvs_reset() clears the flag
    -> ipvlan_process_v4_outbound() -> ip_local_out() -> LOCAL_OUT
    -> IPVS sees ipvs_property=0, processes again -> infinite recursion

syzbot reported this as a stack overflow on a KASAN kernel where each
level burns ~3.3 KB of stack and XMIT_RECURSION_LIMIT falls short. On
non-KASAN kernels the dead-loop detector catches it and prints "Dead
loop on virtual device", but traffic is still broken.

Fixes: de2c211868b9 ("ipvs: Always clear ipvs_property flag in skb_scrub_packet()")
Reported-by: syzbot+2b120190d9e54ad8c65d@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=2b120190d9e54ad8c65d
Signed-off-by: Chenguang Zhao <zhaochenguang@kylinos.cn>
---
v2:
 add Julian into v2.

v1:
 - https://lore.kernel.org/all/20260924063312.1194019-1-chenguang.zhao@linux.dev/

 net/core/skbuff.c    | 3 +--
 net/ipv4/ip_input.c  | 1 +
 net/ipv6/ip6_input.c | 1 +
 3 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index cc3b4b70288b..6912ca0d2228 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -6303,11 +6303,10 @@ void skb_scrub_packet(struct sk_buff *skb, bool xnet)
 	skb->offload_fwd_mark = 0;
 	skb->offload_l3_fwd_mark = 0;
 #endif
-	ipvs_reset(skb);
-
 	if (!xnet)
 		return;
 
+	ipvs_reset(skb);
 	skb->mark = 0;
 	skb_clear_tstamp(skb);
 }
diff --git a/net/ipv4/ip_input.c b/net/ipv4/ip_input.c
index 9860178752b8..00f3b328e90a 100644
--- a/net/ipv4/ip_input.c
+++ b/net/ipv4/ip_input.c
@@ -609,6 +609,7 @@ int ip_rcv(struct sk_buff *skb, struct net_device *dev, struct packet_type *pt,
 	if (skb == NULL)
 		return NET_RX_DROP;
 
+	ipvs_reset(skb);
 	return NF_HOOK(NFPROTO_IPV4, NF_INET_PRE_ROUTING,
 		       net, NULL, skb, dev, NULL,
 		       ip_rcv_finish);
diff --git a/net/ipv6/ip6_input.c b/net/ipv6/ip6_input.c
index d332ec60f915..05917095ef6d 100644
--- a/net/ipv6/ip6_input.c
+++ b/net/ipv6/ip6_input.c
@@ -348,6 +348,7 @@ int ipv6_rcv(struct sk_buff *skb, struct net_device *dev, struct packet_type *pt
 	skb = ip6_rcv_core(skb, dev, net);
 	if (skb == NULL)
 		return NET_RX_DROP;
+	ipvs_reset(skb);
 	return NF_HOOK(NFPROTO_IPV6, NF_INET_PRE_ROUTING,
 		       net, NULL, skb, dev, NULL,
 		       ip6_rcv_finish);
-- 
2.25.1