Subject: Re: [PATCH net-next] l2tp: Drop large packets with UDP encap

From: Simon Horman <[email-masked]>
Date: Wed, 8 Apr 2026 17:48:25 +0100

On Fri, Apr 03, 2026 at 08:49:49PM +0300, Alice Mikityanska wrote:
> From: Alice Mikityanska <[email-masked]>
> 
> 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:
> 
> # {"procs":1,"slowdown":1,"sandbox":"","sandbox_arg":0,"close_fds":false,"callcomments":true}
> 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-masked]/
> 
> Reported-by: [email-masked]
> Closes: https://lore.kernel.org/netdev/[email-masked]/

Hi Alice,

A Fixes tag needs to go here.
And if it's fixing code present in net - that is, the bug can manifest
there - then it should be targeted at net rather than net-next.

> Signed-off-by: Alice Mikityanska <[email-masked]>
> ---
>  net/l2tp/l2tp_core.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
> index c89ae52764b8..157fc23ce4e1 100644
> --- a/net/l2tp/l2tp_core.c
> +++ b/net/l2tp/l2tp_core.c
> @@ -1290,6 +1290,11 @@ static int l2tp_xmit_core(struct l2tp_session *session, struct sk_buff *skb, uns
>  		uh->source = inet->inet_sport;
>  		uh->dest = inet->inet_dport;
>  		udp_len = uhlen + session->hdr_len + data_len;
> +		if (udp_len > U16_MAX) {
> +			kfree_skb(skb);
> +			ret = NET_XMIT_DROP;
> +			goto out_unlock;
> +		}

As a fix, this looks like the right approach.
But I do think this code could benefit from some goto labels
to handle unwinding error cases.

>  		uh->len = htons(udp_len);
>  
>  		/* Calculate UDP checksum if configured to do so */
> -- 
> 2.53.0
>