| From b8570614ed8baeb0e550bfb78bc68086181c652e Mon Sep 17 00:00:00 2001 |
| From: Sasha Levin <sashal@kernel.org> |
| Date: Fri, 3 Apr 2026 20:49:49 +0300 |
| Subject: l2tp: Drop large packets with UDP encap |
| |
| From: Alice Mikityanska <alice@isovalent.com> |
| |
| [ Upstream commit ebe560ea5f54134279356703e73b7f867c89db13 ] |
| |
| 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/20260226201600.222044-1-alice.kernel@fastmail.im/ |
| |
| Fixes: 3557baabf280 ("[L2TP]: PPP over L2TP driver core") |
| Reported-by: syzbot+ci3edea60a44225dec@syzkaller.appspotmail.com |
| Closes: https://lore.kernel.org/netdev/69a1dfba.050a0220.3a55be.0026.GAE@google.com/ |
| Signed-off-by: Alice Mikityanska <alice@isovalent.com> |
| Link: https://patch.msgid.link/20260403174949.843941-1-alice.kernel@fastmail.im |
| Signed-off-by: Paolo Abeni <pabeni@redhat.com> |
| Signed-off-by: Sasha Levin <sashal@kernel.org> |
| --- |
| 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 83615f5968dd5..b03de90e3d418 100644 |
| --- a/net/l2tp/l2tp_core.c |
| +++ b/net/l2tp/l2tp_core.c |
| @@ -1083,6 +1083,11 @@ 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; |
| + } |
| uh->len = htons(udp_len); |
| |
| /* Calculate UDP checksum if configured to do so */ |
| -- |
| 2.53.0 |
| |
