[PATCH nf v5 1/1] netfilter: nf_dup: reject TEE and IPv4/IPv6/netdev nft dup in userns

[Zihan Xi <zihanx@nebusec.ai>]

The packet-duplication frontends can permit repeated packet duplication and
resource consumption when configured in a network namespace owned by a
non-initial user namespace. Users with CAP_NET_ADMIN in that namespace can
configure the affected rules.

No use case for TEE or nft dup in such namespaces has been identified.
Reject TEE and IPv4/IPv6/netdev nft dup rule or expression setup with
-EPERM there, without adding checks to packet-processing fast paths.

This is a namespace policy restriction, not a general fix for asynchronous
duplication behavior. Rules remain installable in init_user_ns-owned
network namespaces, and their existing packet-processing behavior is
unchanged.

Fixes: cd58bcd9787e ("netfilter: xt_TEE: have cloned packet travel through Xtables too")
Fixes: 502061f81d3e ("netfilter: nf_tables: add packet duplication to the netdev family")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Assisted-by: LLM
Co-developed-by: Luxing Yin <root@tr0jan.top>
Signed-off-by: Luxing Yin <root@tr0jan.top>
Signed-off-by: Zihan Xi <zihanx@nebusec.ai>
---
changes in v5:
  - describe the patch as a non-initial-userns setup restriction, not a
    general recursion fix; state that init_user_ns behavior is unchanged
  - remove path-specific trigger details and run-specific metrics from the
    public commit and cover prose
  - drop d877f07112f1 from Fixes: because it added a later frontend, not the
    shared duplication behavior addressed by this restriction
  - record the v5 build and setup-time validation, distinguishing them from
    earlier unpatched-baseline and root-OOM runs
  - v4 Link:
    https://lore.kernel.org/all/cover.1790408011.git.zihanx@nebusec.ai/
changes in v4:
  - add Fixes tags for TEE, IPv4/IPv6 nft dup, and netdev nft dup
    frontends
  - clarify that fcd53c51d037 only added a synchronous guard
  - clarify that this restricts non-initial user namespaces and does not
    fix asynchronous recursion in init_user_ns-owned network namespaces
  - state explicitly that the netdev loop was not runtime-reproduced
  - v3 Link:
    https://lore.kernel.org/all/cover.1790042930.git.zihanx@nebusec.ai/
changes in v3:
  - reroll the fix against the latest nf.git main after no follow-up on v2
  - add the Co-developed-by trailer and matching Signed-off-by
  - v2 Link:
    https://lore.kernel.org/all/cover.1788425393.git.zihanx@nebusec.ai/
changes in v2:
  - drop the persistent struct sk_buff::nf_duplicated field and nf_copy()
    change from v1
  - disable IPv4/IPv6 duplication in non-initial user namespaces
  - v1 Link:
    https://lore.kernel.org/all/cover.1787903722.git.zihanx@nebusec.ai/
---
 net/ipv4/netfilter/nft_dup_ipv4.c | 4 ++++
 net/ipv6/netfilter/nft_dup_ipv6.c | 4 ++++
 net/netfilter/nft_dup_netdev.c    | 4 ++++
 net/netfilter/xt_TEE.c            | 4 ++++
 4 files changed, 16 insertions(+)

diff --git a/net/ipv4/netfilter/nft_dup_ipv4.c b/net/ipv4/netfilter/nft_dup_ipv4.c
index d53a65ddbd7b..db62e30456e4 100644
--- a/net/ipv4/netfilter/nft_dup_ipv4.c
+++ b/net/ipv4/netfilter/nft_dup_ipv4.c
@@ -7,6 +7,7 @@
 #include <linux/init.h>
 #include <linux/module.h>
 #include <linux/netlink.h>
+#include <linux/user_namespace.h>
 #include <linux/netfilter.h>
 #include <linux/netfilter/nf_tables.h>
 #include <net/netfilter/nf_tables.h>
@@ -37,6 +38,9 @@ static int nft_dup_ipv4_init(const struct nft_ctx *ctx,
 	struct nft_dup_ipv4 *priv = nft_expr_priv(expr);
 	int err;
 
+	if (ctx->net->user_ns != &init_user_ns)
+		return -EPERM;
+
 	if (tb[NFTA_DUP_SREG_ADDR] == NULL)
 		return -EINVAL;
 
diff --git a/net/ipv6/netfilter/nft_dup_ipv6.c b/net/ipv6/netfilter/nft_dup_ipv6.c
index 95ec27b3971c..6219ef57633e 100644
--- a/net/ipv6/netfilter/nft_dup_ipv6.c
+++ b/net/ipv6/netfilter/nft_dup_ipv6.c
@@ -7,6 +7,7 @@
 #include <linux/init.h>
 #include <linux/module.h>
 #include <linux/netlink.h>
+#include <linux/user_namespace.h>
 #include <linux/netfilter.h>
 #include <linux/netfilter/nf_tables.h>
 #include <net/netfilter/nf_tables.h>
@@ -35,6 +36,9 @@ static int nft_dup_ipv6_init(const struct nft_ctx *ctx,
 	struct nft_dup_ipv6 *priv = nft_expr_priv(expr);
 	int err;
 
+	if (ctx->net->user_ns != &init_user_ns)
+		return -EPERM;
+
 	if (tb[NFTA_DUP_SREG_ADDR] == NULL)
 		return -EINVAL;
 
diff --git a/net/netfilter/nft_dup_netdev.c b/net/netfilter/nft_dup_netdev.c
index 06866799e946..8d9dfd18475c 100644
--- a/net/netfilter/nft_dup_netdev.c
+++ b/net/netfilter/nft_dup_netdev.c
@@ -7,6 +7,7 @@
 #include <linux/init.h>
 #include <linux/module.h>
 #include <linux/netlink.h>
+#include <linux/user_namespace.h>
 #include <linux/netfilter.h>
 #include <linux/netfilter/nf_tables.h>
 #include <net/netfilter/nf_tables.h>
@@ -37,6 +38,9 @@ static int nft_dup_netdev_init(const struct nft_ctx *ctx,
 {
 	struct nft_dup_netdev *priv = nft_expr_priv(expr);
 
+	if (ctx->net->user_ns != &init_user_ns)
+		return -EPERM;
+
 	if (tb[NFTA_DUP_SREG_DEV] == NULL)
 		return -EINVAL;
 
diff --git a/net/netfilter/xt_TEE.c b/net/netfilter/xt_TEE.c
index 5d34ceb893ed..51c643987526 100644
--- a/net/netfilter/xt_TEE.c
+++ b/net/netfilter/xt_TEE.c
@@ -10,6 +10,7 @@
 #include <linux/module.h>
 #include <linux/skbuff.h>
 #include <linux/route.h>
+#include <linux/user_namespace.h>
 #include <linux/netfilter/x_tables.h>
 #include <net/net_namespace.h>
 #include <net/netns/generic.h>
@@ -95,6 +96,9 @@ static int tee_tg_check(const struct xt_tgchk_param *par)
 	struct xt_tee_tginfo *info = par->targinfo;
 	struct xt_tee_priv *priv;
 
+	if (par->net->user_ns != &init_user_ns)
+		return -EPERM;
+
 	/* 0.0.0.0 and :: not allowed */
 	if (memcmp(&info->gw, &tee_zero_address,
 		   sizeof(tee_zero_address)) == 0)
-- 
2.43.0