Subject: [PATCH net 05/10] netfilter: disable payload mangling in userns

From: Florian Westphal <[email-masked]>
Date: Fri, 22 May 2026 12:42:52 +0200

Several parts of network stack rely on iph->ihl validation
done by network stack before PRE_ROUTING.

Disable this feature for user namespaces for now.

tcp option handling is likely safe even for LOCAL_IN, so this
this leaves tcp option mangling via nft_exthdr.c as-is.

I don't think these are the only means to alter packets, but these
appear to be relatively prominent.

This could be relaxed later.  Example:
 - allow userns for ingress hook.
 - allow userns if base is transport header.

 Also, we should revalidate or restrict generally:
 - Don't allow linklayer writes to spill into network header
 - restrict ipv4 and ipv6 to 'known safe' writes, e.g.
   saddr/daddr/check/tos

Reported-by: Qi Tang <[email-masked]>
Reported-by: Tong Liu <[email-masked]>
Tested-by: Qi Tang <[email-masked]>
Link: https://lore.kernel.org/netfilter-devel/[email-masked]/
Signed-off-by: Florian Westphal <[email-masked]>
---
 net/netfilter/nfnetlink_queue.c | 6 ++++--
 net/netfilter/nft_payload.c     | 3 +++
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/net/netfilter/nfnetlink_queue.c b/net/netfilter/nfnetlink_queue.c
index 984a0eb9e149..60ab88d45096 100644
--- a/net/netfilter/nfnetlink_queue.c
+++ b/net/netfilter/nfnetlink_queue.c
@@ -1141,6 +1141,9 @@ nfqnl_mangle(void *data, unsigned int data_len, struct nf_queue_entry *e, int di
 {
 	struct sk_buff *nskb;
 
+	if (e->state.net->user_ns != &init_user_ns)
+		return -EPERM;
+
 	if (diff < 0) {
 		unsigned int min_len = skb_transport_offset(e->skb);
 
@@ -1537,8 +1540,7 @@ static int nfqnl_recv_verdict(struct sk_buff *skb, const struct nfnl_info *info,
 		if (nfqnl_mangle(nla_data(nfqa[NFQA_PAYLOAD]),
 				 payload_len, entry, diff) < 0)
 			verdict = NF_DROP;
-
-		if (ct && diff)
+		else if (ct && diff)
 			nfnl_ct->seq_adjust(entry->skb, ct, ctinfo, diff);
 	}
 
diff --git a/net/netfilter/nft_payload.c b/net/netfilter/nft_payload.c
index 01e13e5255a9..484a5490832e 100644
--- a/net/netfilter/nft_payload.c
+++ b/net/netfilter/nft_payload.c
@@ -917,6 +917,9 @@ static int nft_payload_set_init(const struct nft_ctx *ctx,
 	struct nft_payload_set *priv = nft_expr_priv(expr);
 	int err;
 
+	if (ctx->net->user_ns != &init_user_ns)
+		return -EPERM;
+
 	priv->base        = ntohl(nla_get_be32(tb[NFTA_PAYLOAD_BASE]));
 	priv->len         = ntohl(nla_get_be32(tb[NFTA_PAYLOAD_LEN]));
 
-- 
2.53.0