* Re: [BUG] netfilter: IPv6 conntrack fragment reassembly truncates header offset
2026-09-29 9:01 ` [BUG] netfilter: IPv6 conntrack fragment reassembly truncates header offset Pablo Neira Ayuso
@ 2026-09-29 9:14 ` 성병찬
0 siblings, 0 replies; 2+ messages in thread
From: 성병찬 @ 2026-09-29 9:14 UTC (permalink / raw)
To: Pablo Neira Ayuso; +Cc: fw, phil, netfilter-devel, netdev, linux-kernel
[-- Attachment #1: Type: text/plain, Size: 1330 bytes --]
Thanks for the information. I confirmed that Jérémy Jean's existing
patch is identical to the fix I tested. Please treat my patch as
superseded by that patch.
I have attached my self-contained reproducer as repro-v2.c. It only
uses IPv6 loopback (::1) and does not send traffic outside the test
system.
Build:
cc -std=c11 -O2 -Wall -Wextra -Werror \
-o repro-v2 repro-v2.c
The test requires root or CAP_NET_RAW. I ran it in a QEMU guest booted
with:
nf_conntrack.enable_hooks=1
Run:
./repro-v2 1
Expected result on the unmodified kernel:
iteration=1 control_received=yes
iteration=1 boundary_received=no
RESULT: differential observed (control delivered, boundary not delivered)
The reproducer exits with status 0 when the bug is reproduced.
Expected result with Jérémy Jean's patch applied:
iteration=1 control_received=yes
iteration=1 boundary_received=yes
RESULT: control passed but expected boundary drop was not observed
In this case it exits with status 2 because the bug is no longer
reproduced.
I reproduced the unmodified result twice and tested the fixed result
twice on Linux v7.2.8.
The SHA-256 of the attached source is:
511dd29a2c41d0c734bb1369a6ab538a85f2498d92c137802c6775240ca17903
You may add:
Tested-by: 성병찬 <tjdqudcks0424@naver.com>
Regards,
Sung Byeongchan
[-- Attachment #2: repro-v2.c --]
[-- Type: application/octet-stream, Size: 15352 bytes --]
#define _GNU_SOURCE
#include <arpa/inet.h>
#include <errno.h>
#include <netinet/in.h>
#include <poll.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <time.h>
#include <unistd.h>
/*
* NFA-02 differential reproducer, version 2.
*
* Packets are injected as complete IPv6 packets through
* AF_INET6/SOCK_RAW/IPV6_HDRINCL to ::1. No link-layer header is supplied by
* userspace. Linux rawv6_send_hdrinc() sends these skbs through
* NF_INET_LOCAL_OUT before loopback delivery, so netfilter reassembly happens
* before the receive-side IPv6 extension-header parser initializes its skb
* offsets.
*
* All offsets below are relative to IPv6 byte zero.
*
* control:
* IPv6 [0,40), Hop-by-Hop [40,248), Destination Options [248,256),
* Fragment [256,264), then 8 bytes of fragment data.
* The predecessor Next Header byte is exactly offset 248.
*
* boundary:
* IPv6 [0,40), Hop-by-Hop [40,256), Destination Options [256,264),
* Fragment [264,272), then 8 bytes of fragment data.
* The predecessor Next Header byte is exactly offset 256.
*
* Each UDP datagram is split into two fragments. Fragment zero contains the
* complete 8-byte UDP header and has M=1. Fragment one has byte offset 8,
* M=0, and contains the complete 8-byte UDP payload.
*/
enum {
IPV6_LEN = 40,
DEST_LEN = 8,
FRAG_LEN = 8,
UDP_LEN = 8,
APP_PAYLOAD_LEN = 8,
UDP_DATAGRAM_LEN = UDP_LEN + APP_PAYLOAD_LEN,
CONTROL_HBH_LEN = 208,
BOUNDARY_HBH_LEN = 216,
CONTROL_PREDECESSOR_NHOFF = 248,
BOUNDARY_PREDECESSOR_NHOFF = 256,
CONTROL_FRAGMENT_OFFSET = 256,
BOUNDARY_FRAGMENT_OFFSET = 264,
MAX_PACKET_LEN = 512,
DEST_PORT = 45555,
MAX_ITERATIONS = 10,
RECEIVE_TIMEOUT_MS = 1500,
V6_NH_HOP = 0,
V6_NH_UDP = 17,
V6_NH_FRAGMENT = 44,
V6_NH_DEST = 60,
FRAGMENT_M_FLAG = 1,
UNKNOWN_SKIP_OPTION = 0x1e,
PADN_OPTION = 1,
};
static void write_be16(unsigned char *p, uint16_t value)
{
p[0] = (unsigned char)(value >> 8);
p[1] = (unsigned char)value;
}
static void write_be32(unsigned char *p, uint32_t value)
{
p[0] = (unsigned char)(value >> 24);
p[1] = (unsigned char)(value >> 16);
p[2] = (unsigned char)(value >> 8);
p[3] = (unsigned char)value;
}
static uint16_t read_be16(const unsigned char *p)
{
return (uint16_t)(((uint16_t)p[0] << 8) | p[1]);
}
static uint32_t checksum_add(uint32_t sum, const unsigned char *data,
size_t len)
{
while (len >= 2) {
sum += ((uint32_t)data[0] << 8) | data[1];
data += 2;
len -= 2;
}
if (len != 0)
sum += (uint32_t)data[0] << 8;
return sum;
}
static uint16_t checksum_finish(uint32_t sum)
{
uint16_t result;
while (sum >> 16)
sum = (sum & 0xffffU) + (sum >> 16);
result = (uint16_t)~sum;
return result == 0 ? 0xffffU : result;
}
static uint16_t udp6_checksum(const unsigned char *udp,
const unsigned char *payload)
{
unsigned char loopback[16] = { 0 };
uint32_t sum = 0;
loopback[15] = 1;
sum = checksum_add(sum, loopback, sizeof(loopback));
sum = checksum_add(sum, loopback, sizeof(loopback));
/* 32-bit UDP length contributes 0x0000 and 0x0010. */
sum += UDP_DATAGRAM_LEN;
/* Three zero bytes and Next Header 17 contribute 0x0000 and 0x0011. */
sum += V6_NH_UDP;
sum = checksum_add(sum, udp, UDP_LEN);
sum = checksum_add(sum, payload, APP_PAYLOAD_LEN);
return checksum_finish(sum);
}
static int build_udp(unsigned char udp[UDP_LEN], uint16_t source_port,
const unsigned char payload[APP_PAYLOAD_LEN])
{
uint16_t checksum;
memset(udp, 0, UDP_LEN);
write_be16(udp, source_port);
write_be16(udp + 2, DEST_PORT);
write_be16(udp + 4, UDP_DATAGRAM_LEN);
checksum = udp6_checksum(udp, payload);
write_be16(udp + 6, checksum);
return 0;
}
static int validate_udp(const unsigned char udp[UDP_LEN], uint16_t source_port,
const unsigned char payload[APP_PAYLOAD_LEN])
{
unsigned char check[UDP_LEN];
uint16_t expected;
uint16_t actual;
if (read_be16(udp) != source_port ||
read_be16(udp + 2) != DEST_PORT ||
read_be16(udp + 4) != UDP_DATAGRAM_LEN)
return -1;
actual = read_be16(udp + 6);
if (actual == 0)
return -1;
memcpy(check, udp, sizeof(check));
check[6] = 0;
check[7] = 0;
expected = udp6_checksum(check, payload);
return actual == expected ? 0 : -1;
}
static int fill_hop_by_hop(unsigned char *hbh, size_t hbh_len)
{
size_t unknown_total;
size_t unknown_data;
size_t pad_offset;
if (hbh_len != CONTROL_HBH_LEN && hbh_len != BOUNDARY_HBH_LEN)
return -1;
memset(hbh, 0, hbh_len);
hbh[0] = V6_NH_DEST;
hbh[1] = (unsigned char)(hbh_len / 8 - 1);
/*
* One unknown option with action bits 00 (skip) plus one six-byte PadN.
* control: 200-byte unknown TLV + 6-byte PadN = 206 option bytes.
* boundary: 208-byte unknown TLV + 6-byte PadN = 214 option bytes.
*/
unknown_total = hbh_len - 8;
unknown_data = unknown_total - 2;
if (unknown_data > UINT8_MAX)
return -1;
hbh[2] = UNKNOWN_SKIP_OPTION;
hbh[3] = (unsigned char)unknown_data;
pad_offset = 2 + unknown_total;
hbh[pad_offset] = PADN_OPTION;
hbh[pad_offset + 1] = 4;
return 0;
}
static void fill_destination_options(unsigned char dest[DEST_LEN])
{
memset(dest, 0, DEST_LEN);
dest[0] = V6_NH_FRAGMENT;
dest[1] = 0;
dest[2] = PADN_OPTION;
dest[3] = 4;
}
static int build_fragment(unsigned char packet[MAX_PACKET_LEN],
size_t *packet_len, size_t hbh_len,
uint32_t identification, uint16_t fragment_field,
const unsigned char fragment_data[8])
{
unsigned char *ip6 = packet;
unsigned char *hbh = ip6 + IPV6_LEN;
unsigned char *dest = hbh + hbh_len;
unsigned char *frag = dest + DEST_LEN;
unsigned char *data = frag + FRAG_LEN;
size_t payload_len = hbh_len + DEST_LEN + FRAG_LEN + 8;
size_t total_len = IPV6_LEN + payload_len;
if (total_len > MAX_PACKET_LEN || payload_len > UINT16_MAX)
return -1;
memset(packet, 0, MAX_PACKET_LEN);
/* IPv6: version 6, explicit payload length, HBH, hop limit 64, ::1 -> ::1. */
ip6[0] = 0x60;
write_be16(ip6 + 4, (uint16_t)payload_len);
ip6[6] = V6_NH_HOP;
ip6[7] = 64;
ip6[8 + 15] = 1;
ip6[24 + 15] = 1;
if (fill_hop_by_hop(hbh, hbh_len) < 0)
return -1;
fill_destination_options(dest);
frag[0] = V6_NH_UDP;
frag[1] = 0;
write_be16(frag + 2, fragment_field);
write_be32(frag + 4, identification);
memcpy(data, fragment_data, 8);
*packet_len = total_len;
return 0;
}
static int validate_fragment(const unsigned char *packet, size_t packet_len,
size_t hbh_len, uint16_t fragment_field,
const unsigned char fragment_data[8])
{
const unsigned char *hbh = packet + IPV6_LEN;
const unsigned char *dest = hbh + hbh_len;
const unsigned char *frag = dest + DEST_LEN;
const unsigned char *data = frag + FRAG_LEN;
size_t expected_len = IPV6_LEN + hbh_len + DEST_LEN + FRAG_LEN + 8;
if (packet_len != expected_len || packet[0] != 0x60 ||
read_be16(packet + 4) != packet_len - IPV6_LEN ||
packet[6] != V6_NH_HOP || packet[7] != 64)
return -1;
if (packet[8 + 15] != 1 || packet[24 + 15] != 1)
return -1;
if (hbh[0] != V6_NH_DEST || hbh[1] != hbh_len / 8 - 1)
return -1;
if (dest[0] != V6_NH_FRAGMENT || dest[1] != 0 ||
dest[2] != PADN_OPTION || dest[3] != 4)
return -1;
if (frag[0] != V6_NH_UDP || frag[1] != 0 ||
read_be16(frag + 2) != fragment_field)
return -1;
return memcmp(data, fragment_data, 8) == 0 ? 0 : -1;
}
static int send_one_packet(int fd, const unsigned char *packet,
size_t packet_len)
{
struct sockaddr_in6 destination;
ssize_t sent;
memset(&destination, 0, sizeof(destination));
destination.sin6_family = AF_INET6;
destination.sin6_addr = in6addr_loopback;
sent = sendto(fd, packet, packet_len, 0,
(const struct sockaddr *)&destination, sizeof(destination));
if (sent < 0) {
perror("sendto(AF_INET6/SOCK_RAW/IPV6_HDRINCL)");
return -1;
}
if ((size_t)sent != packet_len) {
fprintf(stderr, "short raw IPv6 send: %zd of %zu bytes\n",
sent, packet_len);
return -1;
}
return 0;
}
static int send_case(int fd, const char *name, size_t hbh_len,
uint32_t identification, uint16_t source_port,
const unsigned char payload[APP_PAYLOAD_LEN])
{
unsigned char first[MAX_PACKET_LEN];
unsigned char second[MAX_PACKET_LEN];
unsigned char udp[UDP_LEN];
size_t first_len;
size_t second_len;
if (build_udp(udp, source_port, payload) < 0 ||
validate_udp(udp, source_port, payload) < 0) {
fprintf(stderr, "%s: UDP header/checksum self-check failed\n", name);
return -1;
}
if (build_fragment(first, &first_len, hbh_len, identification,
FRAGMENT_M_FLAG, udp) < 0 ||
validate_fragment(first, first_len, hbh_len, FRAGMENT_M_FLAG,
udp) < 0) {
fprintf(stderr, "%s: first-fragment self-check failed\n", name);
return -1;
}
/* Wire value 8 means byte offset 8 because the low three bits are flags. */
if (build_fragment(second, &second_len, hbh_len, identification,
8, payload) < 0 ||
validate_fragment(second, second_len, hbh_len, 8, payload) < 0) {
fprintf(stderr, "%s: second-fragment self-check failed\n", name);
return -1;
}
printf("%s: first_len=%zu second_len=%zu payload_len=%u "
"udp_checksum=0x%04x fragment_fields=0x0001,0x0008\n",
name, first_len, second_len, read_be16(first + 4),
read_be16(udp + 6));
if (send_one_packet(fd, first, first_len) < 0)
return -1;
if (send_one_packet(fd, second, second_len) < 0)
return -1;
return 0;
}
static int monotonic_ms(uint64_t *value)
{
struct timespec ts;
if (clock_gettime(CLOCK_MONOTONIC, &ts) < 0) {
perror("clock_gettime");
return -1;
}
*value = (uint64_t)ts.tv_sec * 1000U +
(uint64_t)ts.tv_nsec / 1000000U;
return 0;
}
static int drain_receiver(int fd)
{
unsigned char buffer[256];
for (;;) {
ssize_t received = recv(fd, buffer, sizeof(buffer), 0);
if (received >= 0)
continue;
if (errno == EAGAIN || errno == EWOULDBLOCK)
return 0;
perror("recv(drain)");
return -1;
}
}
/* Return 1 for a matching datagram, 0 for timeout, and -1 for syscall error. */
static int wait_for_payload(int fd,
const unsigned char expected[APP_PAYLOAD_LEN])
{
uint64_t start;
uint64_t now;
struct pollfd pfd = { .fd = fd, .events = POLLIN };
if (monotonic_ms(&start) < 0)
return -1;
for (;;) {
unsigned char buffer[256];
ssize_t received;
int remaining;
int ready;
if (monotonic_ms(&now) < 0)
return -1;
if (now - start >= RECEIVE_TIMEOUT_MS)
return 0;
remaining = RECEIVE_TIMEOUT_MS - (int)(now - start);
pfd.revents = 0;
ready = poll(&pfd, 1, remaining);
if (ready < 0) {
perror("poll");
return -1;
}
if (ready == 0)
return 0;
if ((pfd.revents & POLLIN) == 0) {
fprintf(stderr, "unexpected poll revents: 0x%x\n", pfd.revents);
return -1;
}
received = recv(fd, buffer, sizeof(buffer), 0);
if (received < 0) {
perror("recv");
return -1;
}
if (received == APP_PAYLOAD_LEN &&
memcmp(buffer, expected, APP_PAYLOAD_LEN) == 0)
return 1;
fprintf(stderr, "ignored unrelated UDP datagram of %zd bytes\n",
received);
}
}
static int parse_iterations(const char *text, int *iterations)
{
char *end = NULL;
long value;
errno = 0;
value = strtol(text, &end, 10);
if (errno != 0 || end == text || *end != '\0' ||
value < 1 || value > MAX_ITERATIONS)
return -1;
*iterations = (int)value;
return 0;
}
static int close_checked(int fd, const char *name)
{
if (close(fd) < 0) {
fprintf(stderr, "close(%s): %s\n", name, strerror(errno));
return -1;
}
return 0;
}
int main(int argc, char **argv)
{
static const unsigned char control_payload[APP_PAYLOAD_LEN] =
{ 'N', 'F', 'A', '2', 'V', '2', 'C', '!' };
static const unsigned char boundary_payload[APP_PAYLOAD_LEN] =
{ 'N', 'F', 'A', '2', 'V', '2', 'B', '!' };
static const char loopback_device[] = "lo";
struct sockaddr_in6 receive_address;
int hdrincl = 1;
int iterations = 1;
int receiver = -1;
int raw = -1;
int result = EXIT_SUCCESS;
int i;
if (argc > 2 || (argc == 2 && parse_iterations(argv[1], &iterations) < 0)) {
fprintf(stderr, "usage: %s [iterations:1-%d]\n", argv[0],
MAX_ITERATIONS);
return EXIT_FAILURE;
}
receiver = socket(AF_INET6, SOCK_DGRAM | SOCK_CLOEXEC | SOCK_NONBLOCK,
IPPROTO_UDP);
if (receiver < 0) {
perror("socket(AF_INET6, SOCK_DGRAM)");
return EXIT_FAILURE;
}
memset(&receive_address, 0, sizeof(receive_address));
receive_address.sin6_family = AF_INET6;
receive_address.sin6_port = htons(DEST_PORT);
receive_address.sin6_addr = in6addr_loopback;
if (bind(receiver, (const struct sockaddr *)&receive_address,
sizeof(receive_address)) < 0) {
perror("bind(UDP ::1)");
result = EXIT_FAILURE;
goto out;
}
raw = socket(AF_INET6, SOCK_RAW | SOCK_CLOEXEC, IPPROTO_RAW);
if (raw < 0) {
perror("socket(AF_INET6, SOCK_RAW, IPPROTO_RAW); CAP_NET_RAW required");
result = EXIT_FAILURE;
goto out;
}
if (setsockopt(raw, IPPROTO_IPV6, IPV6_HDRINCL, &hdrincl,
sizeof(hdrincl)) < 0) {
perror("setsockopt(IPV6_HDRINCL)");
result = EXIT_FAILURE;
goto out;
}
if (setsockopt(raw, SOL_SOCKET, SO_BINDTODEVICE, loopback_device,
sizeof(loopback_device)) < 0) {
perror("setsockopt(SO_BINDTODEVICE=lo)");
result = EXIT_FAILURE;
goto out;
}
printf("injection=AF_INET6/SOCK_RAW/IPPROTO_RAW/IPV6_HDRINCL "
"device=lo destination=[::1]:%d iterations=%d\n",
DEST_PORT, iterations);
printf("control: hbh=208 predecessor_nhoff=%d fragment_offset=%d\n",
CONTROL_PREDECESSOR_NHOFF, CONTROL_FRAGMENT_OFFSET);
printf("boundary: hbh=216 predecessor_nhoff=%d fragment_offset=%d "
"stored_u8=0\n",
BOUNDARY_PREDECESSOR_NHOFF, BOUNDARY_FRAGMENT_OFFSET);
for (i = 0; i < iterations; i++) {
uint32_t base_id = 0x4e460400U + (uint32_t)i * 2U;
int observed;
if (drain_receiver(receiver) < 0) {
result = EXIT_FAILURE;
goto out;
}
if (send_case(raw, "control", CONTROL_HBH_LEN, base_id,
(uint16_t)(40000 + i * 2), control_payload) < 0) {
result = EXIT_FAILURE;
goto out;
}
observed = wait_for_payload(receiver, control_payload);
if (observed < 0) {
result = EXIT_FAILURE;
goto out;
}
printf("iteration=%d control_received=%s\n", i + 1,
observed ? "yes" : "no");
if (!observed) {
printf("iteration=%d boundary_received=not-tested\n", i + 1);
printf("RESULT: control failed; boundary was not sent and is not interpretable\n");
result = 3;
goto out;
}
if (drain_receiver(receiver) < 0) {
result = EXIT_FAILURE;
goto out;
}
if (send_case(raw, "boundary", BOUNDARY_HBH_LEN, base_id + 1,
(uint16_t)(40001 + i * 2), boundary_payload) < 0) {
result = EXIT_FAILURE;
goto out;
}
observed = wait_for_payload(receiver, boundary_payload);
if (observed < 0) {
result = EXIT_FAILURE;
goto out;
}
printf("iteration=%d boundary_received=%s\n", i + 1,
observed ? "yes" : "no");
if (observed)
result = 2;
}
if (result == EXIT_SUCCESS)
printf("RESULT: differential observed (control delivered, boundary not delivered)\n");
else if (result == 2)
printf("RESULT: control passed but expected boundary drop was not observed\n");
out:
if (raw >= 0 && close_checked(raw, "raw IPv6") < 0)
result = EXIT_FAILURE;
if (receiver >= 0 && close_checked(receiver, "UDP receiver") < 0)
result = EXIT_FAILURE;
return result;
}
^ permalink raw reply [flat|nested] 2+ messages in thread