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Tue, 04 Aug 2026 02:51:07 -0700 (PDT) From: David Lee To: pablo@netfilter.org, fw@strlen.de, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com Cc: Kyle Zeng , Dominik 'Disconnect3d' Czarnota , phil@nwl.cc, horms@kernel.org, netfilter-devel@vger.kernel.org, coreteam@netfilter.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, David Lee Subject: [PATCH nf] netfilter: nf_reject_ipv4: initialize IPCB at inet ingress Date: Tue, 4 Aug 2026 09:51:05 +0000 Message-ID: <20260804095105.715387-1-david.lee@trailofbits.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Kyle Zeng The inet ingress hook runs before ip_rcv_core(), so IPCB has not been initialized when nft_reject_inet passes an IPv4 packet to nf_send_unreach(). skb->cb can therefore retain data from an earlier protocol layer. 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. Clear IPCB for NF_INET_INGRESS and restore its input interface, matching the initialization normally performed by ip_rcv_core(). Fixes: 117ca1f8920c ("netfilter: nft_reject_inet: allow to use reject from inet ingress") Assisted-by: Codex:gpt-5.6-sol Codex:gpt-5.5-cyber Signed-off-by: Kyle Zeng Co-developed-by: David Lee Signed-off-by: David Lee --- Bug found and triaged by OpenAI Security Research and validated by Trail of Bits. Trail of Bits has a reproducer for this bug that triggers a KASAN stack-out-of-bounds write in __ip_options_echo() and can share if needed. net/ipv4/netfilter/nf_reject_ipv4.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/net/ipv4/netfilter/nf_reject_ipv4.c b/net/ipv4/netfilter/nf_reject_ipv4.c index 4626dc468..4933f6513 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); -- 2.53.0