[PATCH nf v2] netfilter: nf_reject: initialize IPCB at inet ingress

[David Lee <david.lee@trailofbits.com>]

From: Kyle Zeng <kylebot@openai.com>

The inet ingress hook runs before ip_rcv_core() and ip6_rcv_core(), so
neither IPCB nor IP6CB has been initialized when nft_reject_inet passes
a packet to nf_send_unreach() or nf_send_unreach6(). skb->cb can
therefore retain metadata from an earlier protocol layer.

For IPv4, icmp_send() treats IPCB(skb)->opt as parsed IPv4 options.
Stale option offsets can make __ip_options_echo() copy an
attacker-controlled length into its 40-byte reply option buffer and
overflow the stack. On an unpatched v7.2-rc5 kernel, the IPv4
reproducer produced:

  BUG: KASAN: stack-out-of-bounds in __ip_options_echo()
  Write of size 255

For IPv6, icmpv6_send() consumes IP6CB(skb) as parsed IPv6 metadata. It
always uses iif for source selection and, when MIP6 is enabled, uses
dsthao as an extension-header offset. These fields must not be stale.

For NF_INET_INGRESS, clear the address-family-specific control block
and restore its input interface before constructing the ICMP error.
This matches the initialization normally performed by ip_rcv_core()
and ip6_rcv_core().

Fixes: 117ca1f8920c ("netfilter: nft_reject_inet: allow to use reject from inet ingress")
Cc: stable@vger.kernel.org
Assisted-by: Codex:gpt-5.6-sol Codex:gpt-5.5-cyber
Signed-off-by: Kyle Zeng <kylebot@openai.com>
Co-developed-by: David Lee <david.lee@trailofbits.com>
Signed-off-by: David Lee <david.lee@trailofbits.com>
---
Changes in v2:
- Initialize IP6CB in nf_send_unreach6() for inet ingress.
- Preserve the IPv6 input interface before constructing the reply dst.

v1: https://lore.kernel.org/netfilter-devel/20260804095105.715387-1-david.lee@trailofbits.com/

Bug found and triaged by OpenAI Security Research and
validated by Trail of Bits.

Trail of Bits has an IPv4 reproducer for this bug that triggers the
KASAN stack-out-of-bounds write in __ip_options_echo() and can share it
if needed. A separate IPv6 test reaches nf_send_unreach6() from inet
ingress and confirms that an ICMPv6 unreachable is constructed.

 net/ipv4/netfilter/nf_reject_ipv4.c | 6 ++++++
 net/ipv6/netfilter/nf_reject_ipv6.c | 9 +++++++++
 2 files changed, 15 insertions(+)

diff --git a/net/ipv4/netfilter/nf_reject_ipv4.c b/net/ipv4/netfilter/nf_reject_ipv4.c
index 4626dc46808f..4933f65138d6 100644
--- a/net/ipv4/netfilter/nf_reject_ipv4.c
+++ b/net/ipv4/netfilter/nf_reject_ipv4.c
@@ -355,6 +355,12 @@ void nf_send_unreach(struct sk_buff *skb_in, int code, int hook)
 	if (!skb_dst(skb_in) && nf_reject_fill_skb_dst(skb_in) < 0)
 		return;
 
+	/* Inet ingress runs before IPv4 initializes IPCB. */
+	if (hook == NF_INET_INGRESS) {
+		memset(IPCB(skb_in), 0, sizeof(*IPCB(skb_in)));
+		IPCB(skb_in)->iif = skb_in->skb_iif;
+	}
+
 	if (skb_csum_unnecessary(skb_in) ||
 	    !nf_reject_verify_csum(skb_in, dataoff, proto)) {
 		icmp_send(skb_in, ICMP_DEST_UNREACH, code, 0);
diff --git a/net/ipv6/netfilter/nf_reject_ipv6.c b/net/ipv6/netfilter/nf_reject_ipv6.c
index ef5b7e85cffa..c36910b8a95a 100644
--- a/net/ipv6/netfilter/nf_reject_ipv6.c
+++ b/net/ipv6/netfilter/nf_reject_ipv6.c
@@ -9,6 +9,7 @@
 #include <net/ip6_fib.h>
 #include <net/ip6_checksum.h>
 #include <net/netfilter/ipv6/nf_reject.h>
+#include <net/dst_metadata.h>
 #include <linux/netfilter_ipv6.h>
 #include <linux/netfilter_bridge.h>
 
@@ -440,6 +441,14 @@ void nf_send_unreach6(struct net *net, struct sk_buff *skb_in,
 	if (hooknum == NF_INET_LOCAL_OUT && skb_in->dev == NULL)
 		skb_in->dev = net->loopback_dev;
 
+	/* Inet ingress runs before IPv6 initializes IP6CB. */
+	if (hooknum == NF_INET_INGRESS) {
+		memset(IP6CB(skb_in), 0, sizeof(*IP6CB(skb_in)));
+		IP6CB(skb_in)->iif = skb_valid_dst(skb_in) ?
+				     ip6_dst_idev(skb_dst(skb_in))->dev->ifindex :
+				     skb_in->dev->ifindex;
+	}
+
 	if (!skb_dst(skb_in) && nf_reject6_fill_skb_dst(skb_in) < 0)
 		return;
 
-- 
2.53.0