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

[syzbot <syzbot+3a92d359bc2ec6255a33@syzkaller.appspotmail.com>]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
divide error in __tcp_select_window

RSP: 002b:00007f86321dd038 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 00007f86315e5fa0 RCX: 00007f863138f749
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000003
RBP: 00007f86321dd090 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
R13: 00007f86315e6038 R14: 00007f86315e5fa0 R15: 00007ffd93c62a18
 </TASK>
Oops: divide error: 0000 [#1] SMP KASAN PTI
CPU: 1 UID: 0 PID: 6439 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 da 10 01 f8 e9 9c 00 00 00 e8 d0 10 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:ffffc900030a7640 EFLAGS: 00010293
RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff8880258abc80
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffc900030a7730 R08: ffff8880310e8dc3 R09: 1ffff1100621d1b8
R10: dffffc0000000000 R11: ffffed100621d1b9 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
FS:  00007f86321dd6c0(0000) GS:ffff888126235000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000000180 CR3: 000000007e366000 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+0x286/0x390 net/mptcp/protocol.c:2896
 mptcp_disconnect+0x238/0x730 net/mptcp/protocol.c:3380
 mptcp_sendmsg_fastopen+0x2f8/0x580 net/mptcp/protocol.c:1801
 mptcp_sendmsg+0x1774/0x1980 net/mptcp/protocol.c:1880
 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: 0033:0x7f863138f749
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:00007f86321dd038 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 00007f86315e5fa0 RCX: 00007f863138f749
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000003
RBP: 00007f86321dd090 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
R13: 00007f86315e6038 R14: 00007f86315e5fa0 R15: 00007ffd93c62a18
 </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 da 10 01 f8 e9 9c 00 00 00 e8 d0 10 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:ffffc900030a7640 EFLAGS: 00010293
RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff8880258abc80
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffc900030a7730 R08: ffff8880310e8dc3 R09: 1ffff1100621d1b8
R10: dffffc0000000000 R11: ffffed100621d1b9 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
FS:  00007f86321dd6c0(0000) GS:ffff888126235000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f4311661286 CR3: 000000007e366000 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 da 10 01 f8       	call   0xf80110f4
  1a:	e9 9c 00 00 00       	jmp    0xbb
  1f:	e8 d0 10 01 f8       	call   0xf80110f4
  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


Tested on:

commit:         ab084f0b drivers: net: fbnic: Return the true error in..
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git main
console output: https://syzkaller.appspot.com/x/log.txt?x=1317ee92580000
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
patch:          https://syzkaller.appspot.com/x/patch.diff?x=16cb0e12580000