CVE-2026-43080

In the Linux kernel, the following vulnerability has been resolved:

l2tp: Drop large packets with UDP encap

syzbot reported a WARN on my patch series [1]. The actual issue is an

overflow of 16-bit UDP length field, and it exists in the upstream code.

My series added a debug WARN with an overflow check that exposed the

issue, that's why syzbot tripped on my patches, rather than on upstream

code.

syzbot's repro:

r0 = socket$pppl2tp(0x18, 0x1, 0x1)

r1 = socket$inet6_udp(0xa, 0x2, 0x0)

connect$inet6(r1, &(0x7f00000000c0)={0xa, 0x0, 0x0, @loopback, 0xfffffffc}, 0x1c)

connect$pppl2tp(r0, &(0x7f0000000240)=@pppol2tpin6={0x18, 0x1, {0x0, r1, 0x4, 0x0, 0x0, 0x0, {0xa, 0x4e22, 0xffff, @ipv4={'\x00', '\xff\xff', @empty}}}}, 0x32)

writev(r0, &(0x7f0000000080)=[{&(0x7f0000000000)="ee", 0x34000}], 0x1)

It basically sends an oversized (0x34000 bytes) PPPoL2TP packet with UDP

encapsulation, and l2tp_xmit_core doesn't check for overflows when it

assigns the UDP length field. The value gets trimmed to 16 bites.

Add an overflow check that drops oversized packets and avoids sending

packets with trimmed UDP length to the wire.

syzbot's stack trace (with my patch applied):

len >= 65536u

WARNING: ./include/linux/udp.h:38 at udp_set_len_short include/linux/udp.h:38 [inline], CPU#1: syz.0.17/5957

WARNING: ./include/linux/udp.h:38 at l2tp_xmit_core net/l2tp/l2tp_core.c:1293 [inline], CPU#1: syz.0.17/5957

WARNING: ./include/linux/udp.h:38 at l2tp_xmit_skb+0x1204/0x18d0 net/l2tp/l2tp_core.c:1327, CPU#1: syz.0.17/5957

Modules linked in:

CPU: 1 UID: 0 PID: 5957 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)

Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014

RIP: 0010:udp_set_len_short include/linux/udp.h:38 [inline]

RIP: 0010:l2tp_xmit_core net/l2tp/l2tp_core.c:1293 [inline]

RIP: 0010:l2tp_xmit_skb+0x1204/0x18d0 net/l2tp/l2tp_core.c:1327

Code: 0f 0b 90 e9 21 f9 ff ff e8 e9 05 ec f6 90 0f 0b 90 e9 8d f9 ff ff e8 db 05 ec f6 90 0f 0b 90 e9 cc f9 ff ff e8 cd 05 ec f6 90 <0f> 0b 90 e9 de fa ff ff 44 89 f1 80 e1 07 80 c1 03 38 c1 0f 8c 4f

RSP: 0018:ffffc90003d67878 EFLAGS: 00010293

RAX: ffffffff8ad985e3 RBX: ffff8881a6400090 RCX: ffff8881697f0000

RDX: 0000000000000000 RSI: 0000000000034010 RDI: 000000000000ffff

RBP: dffffc0000000000 R08: 0000000000000003 R09: 0000000000000004

R10: dffffc0000000000 R11: fffff520007acf00 R12: ffff8881baf20900

R13: 0000000000034010 R14: ffff8881a640008e R15: ffff8881760f7000

FS: 000055557e81f500(0000) GS:ffff8882a9467000(0000) knlGS:0000000000000000

CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033

CR2: 0000200000033000 CR3: 00000001612f4000 CR4: 00000000000006f0

Call Trace:

<TASK>

pppol2tp_sendmsg+0x40a/0x5f0 net/l2tp/l2tp_ppp.c:302

sock_sendmsg_nosec net/socket.c:727 [inline]

__sock_sendmsg net/socket.c:742 [inline]

sock_write_iter+0x503/0x550 net/socket.c:1195

do_iter_readv_writev+0x619/0x8c0 fs/read_write.c:-1

vfs_writev+0x33c/0x990 fs/read_write.c:1059

do_writev+0x154/0x2e0 fs/read_write.c:1105

do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]

do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94

entry_SYSCALL_64_after_hwframe+0x77/0x7f

RIP: 0033:0x7f636479c629

Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48

RSP: 002b:00007ffffd4241c8 EFLAGS: 00000246 ORIG_RAX: 0000000000000014

RAX: ffffffffffffffda RBX: 00007f6364a15fa0 RCX: 00007f636479c629

RDX: 0000000000000001 RSI: 0000200000000080 RDI: 0000000000000003

RBP: 00007f6364832b39 R08: 0000000000000000 R09: 0000000000000000

R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000

R13: 00007f6364a15fac R14: 00007f6364a15fa0 R15: 00007f6364a15fa0

</TASK>

[1]: https://lore.kernel.org/all/[email protected]/