[syzbot] [net?] divide error in __tcp_select_window (4)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: e2c20036a887 Merge branch 'devlink-net-mlx5-implement-swp_..
git tree: net-next
console output: https://syzkaller.appspot.com/x/log.txt?x=1164c484580000
kernel config: https://syzkaller.appspot.com/x/.config?x=a881ccda32df4e75
dashboard link: https://syzkaller.appspot.com/bug?extid=3a92d359bc2ec6255a33
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=13f8fa12580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=113a5a12580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/07279e689a07/disk-e2c20036.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/b13e2e59c1ed/vmlinux-e2c20036.xz
kernel image: https://storage.googleapis.com/syzbot-assets/f6f519394597/bzImage-e2c20036.xz

The issue was bisected to:

commit ae155060247be8dcae3802a95bd1bdf93ab3215d
Author: Paolo Abeni <pab...@redhat.com>
Date: Tue Nov 18 07:20:24 2025 +0000

mptcp: fix duplicate reset on fastclose

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=11f698b4580000
final oops: https://syzkaller.appspot.com/x/report.txt?x=13f698b4580000
console output: https://syzkaller.appspot.com/x/log.txt?x=15f698b4580000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+3a92d3...@syzkaller.appspotmail.com
Fixes: ae155060247b ("mptcp: fix duplicate reset on fastclose")

RBP: 00007ffff9acee10 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
R13: 00007f66e9be5fa0 R14: 00007f66e9be5fa0 R15: 0000000000000006
</TASK>
Oops: divide error: 0000 [#1] SMP KASAN PTI
CPU: 0 UID: 0 PID: 6068 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
RIP: 0010:__tcp_select_window+0x824/0x1320 net/ipv4/tcp_output.c:3336
Code: ff ff ff 44 89 f1 d3 e0 89 c1 f7 d1 41 01 cc 41 21 c4 e9 a9 00 00 00 e8 ca 49 01 f8 e9 9c 00 00 00 e8 c0 49 01 f8 44 89 e0 99 <f7> 7c 24 1c 41 29 d4 48 bb 00 00 00 00 00 fc ff df e9 80 00 00 00
RSP: 0018:ffffc90003017640 EFLAGS: 00010293
RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff88807b469e40
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffc90003017730 R08: ffff888033268143 R09: 1ffff1100664d028
R10: dffffc0000000000 R11: ffffed100664d029 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
FS: 000055557faa0500(0000) GS:ffff888126135000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f64a1912ff8 CR3: 0000000072122000 CR4: 00000000003526f0
Call Trace:
<TASK>
tcp_select_window net/ipv4/tcp_output.c:281 [inline]
__tcp_transmit_skb+0xbc7/0x3aa0 net/ipv4/tcp_output.c:1568
tcp_transmit_skb net/ipv4/tcp_output.c:1649 [inline]
tcp_send_active_reset+0x2d1/0x5b0 net/ipv4/tcp_output.c:3836
mptcp_do_fastclose+0x27e/0x380 net/mptcp/protocol.c:2793
mptcp_disconnect+0x238/0x710 net/mptcp/protocol.c:3253
mptcp_sendmsg_fastopen+0x2f8/0x580 net/mptcp/protocol.c:1776
mptcp_sendmsg+0x1774/0x1980 net/mptcp/protocol.c:1855
sock_sendmsg_nosec net/socket.c:727 [inline]
__sock_sendmsg+0xe5/0x270 net/socket.c:742
__sys_sendto+0x3bd/0x520 net/socket.c:2244
__do_sys_sendto net/socket.c:2251 [inline]
__se_sys_sendto net/socket.c:2247 [inline]
__x64_sys_sendto+0xde/0x100 net/socket.c:2247
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f66e998f749
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 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 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffff9acedb8 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 00007f66e9be5fa0 RCX: 00007f66e998f749
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000003
RBP: 00007ffff9acee10 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
R13: 00007f66e9be5fa0 R14: 00007f66e9be5fa0 R15: 0000000000000006
</TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:__tcp_select_window+0x824/0x1320 net/ipv4/tcp_output.c:3336
Code: ff ff ff 44 89 f1 d3 e0 89 c1 f7 d1 41 01 cc 41 21 c4 e9 a9 00 00 00 e8 ca 49 01 f8 e9 9c 00 00 00 e8 c0 49 01 f8 44 89 e0 99 <f7> 7c 24 1c 41 29 d4 48 bb 00 00 00 00 00 fc ff df e9 80 00 00 00
RSP: 0018:ffffc90003017640 EFLAGS: 00010293
RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff88807b469e40
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffc90003017730 R08: ffff888033268143 R09: 1ffff1100664d028
R10: dffffc0000000000 R11: ffffed100664d029 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
FS: 000055557faa0500(0000) GS:ffff888126135000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f64a1912ff8 CR3: 0000000072122000 CR4: 00000000003526f0
----------------
Code disassembly (best guess), 2 bytes skipped:
0: ff 44 89 f1 incl -0xf(%rcx,%rcx,4)
4: d3 e0 shl %cl,%eax
6: 89 c1 mov %eax,%ecx
8: f7 d1 not %ecx
a: 41 01 cc add %ecx,%r12d
d: 41 21 c4 and %eax,%r12d
10: e9 a9 00 00 00 jmp 0xbe
15: e8 ca 49 01 f8 call 0xf80149e4
1a: e9 9c 00 00 00 jmp 0xbb
1f: e8 c0 49 01 f8 call 0xf80149e4
24: 44 89 e0 mov %r12d,%eax
27: 99 cltd
* 28: f7 7c 24 1c idivl 0x1c(%rsp) <-- trapping instruction
2c: 41 29 d4 sub %edx,%r12d
2f: 48 bb 00 00 00 00 00 movabs $0xdffffc0000000000,%rbx
36: fc ff df
39: e9 80 00 00 00 jmp 0xbe


---
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