Subject: [syzbot ci] Re: BIG TCP for UDP tunnels

From: syzbot ci <[email-masked]>
Date: Fri, 27 Feb 2026 10:17:30 -0800

syzbot ci has tested the following series

[v2] BIG TCP for UDP tunnels
https://lore.kernel.org/all/[email-masked]
* [PATCH net-next v2 01/12] net/sched: act_csum: don't mangle UDP tunnel GSO packets
* [PATCH net-next v2 02/12] udp: gso: Simplify handling length in GSO_PARTIAL
* [PATCH net-next v2 03/12] geneve: Fix off-by-one comparing with GRO_LEGACY_MAX_SIZE
* [PATCH net-next v2 04/12] net: Use helpers to get/set UDP len tree-wide
* [PATCH net-next v2 05/12] net: Enable BIG TCP with partial GSO
* [PATCH net-next v2 06/12] udp: Support gro_ipv4_max_size > 65536
* [PATCH net-next v2 07/12] udp: Support BIG TCP GSO packets where they can occur
* [PATCH net-next v2 08/12] udp: Validate UDP length in udp_gro_receive
* [PATCH net-next v2 09/12] udp: Set length in UDP header to 0 for big GSO packets
* [PATCH net-next v2 10/12] vxlan: Enable BIG TCP packets
* [PATCH net-next v2 11/12] geneve: Enable BIG TCP packets
* [PATCH net-next v2 12/12] selftests: net: Add a test for BIG TCP in UDP tunnels

and found the following issue:
WARNING in l2tp_xmit_skb

Full report is available here:
https://ci.syzbot.org/series/8de1e666-84bd-4f74-b980-539c4b651492

***

WARNING in l2tp_xmit_skb

tree:      net-next
URL:       https://kernel.googlesource.com/pub/scm/linux/kernel/git/netdev/net-next.git
base:      fd6dad4e1ae296b67b87291256878a58dad36c93
arch:      amd64
compiler:  Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
config:    https://ci.syzbot.org/builds/f58ab8e4-fdf3-4b35-8063-129b877f1ed8/config
C repro:   https://ci.syzbot.org/findings/a09a9cb9-7ad3-49ee-8662-3581bd42cd46/c_repro
syz repro: https://ci.syzbot.org/findings/a09a9cb9-7ad3-49ee-8662-3581bd42cd46/syz_repro

------------[ cut here ]------------
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>

***

If these findings have caused you to resend the series or submit a
separate fix, please add the following tag to your commit message:
  Tested-by: [email-masked]

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This report is generated by a bot. It may contain errors.
syzbot ci engineers can be reached at [email-masked].