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

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:00007fbc0bc37038 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 00007fbc0afe5fa0 RCX: 00007fbc0ad8f749
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000003
RBP: 00007fbc0bc37090 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
R13: 00007fbc0afe6038 R14: 00007fbc0afe5fa0 R15: 00007ffe094ab938
 </TASK>
Oops: divide error: 0000 [#1] SMP KASAN PTI
CPU: 0 UID: 0 PID: 6503 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+0x7a2/0x1270 net/ipv4/tcp_output.c:3333
Code: ff ff ff 44 89 e1 d3 e0 89 c1 f7 d1 41 01 ce 41 21 c6 e9 c5 00 00 00 e8 8c c6 01 f8 e9 b8 00 00 00 e8 82 c6 01 f8 44 89 f0 99 <f7> 7c 24 2c 41 29 d6 48 bb 00 00 00 00 00 fc ff df e9 9c 00 00 00
RSP: 0018:ffffc90002e67640 EFLAGS: 00010293
RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff88802e693c80
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffc90002e67730 R08: ffff888027d78143 R09: 1ffff11004faf028
R10: dffffc0000000000 R11: ffffed1004faf029 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: ffff88802fdc24f0
FS:  00007fbc0bc376c0(0000) GS:ffff888126139000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00005587d939f950 CR3: 000000007c31a000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 tcp_select_window net/ipv4/tcp_output.c:280 [inline]
 __tcp_transmit_skb+0xbc7/0x3aa0 net/ipv4/tcp_output.c:1565
 tcp_transmit_skb net/ipv4/tcp_output.c:1646 [inline]
 tcp_send_active_reset+0x2d1/0x5b0 net/ipv4/tcp_output.c:3828
 mptcp_do_fastclose+0x27e/0x380 net/mptcp/protocol.c:2801
 mptcp_disconnect+0x238/0x710 net/mptcp/protocol.c:3261
 mptcp_sendmsg_fastopen+0x2f8/0x580 net/mptcp/protocol.c:1775
 mptcp_sendmsg+0x1774/0x1980 net/mptcp/protocol.c:1854
 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: 0033:0x7fbc0ad8f749
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:00007fbc0bc37038 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 00007fbc0afe5fa0 RCX: 00007fbc0ad8f749
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000003
RBP: 00007fbc0bc37090 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
R13: 00007fbc0afe6038 R14: 00007fbc0afe5fa0 R15: 00007ffe094ab938
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:__tcp_select_window+0x7a2/0x1270 net/ipv4/tcp_output.c:3333
Code: ff ff ff 44 89 e1 d3 e0 89 c1 f7 d1 41 01 ce 41 21 c6 e9 c5 00 00 00 e8 8c c6 01 f8 e9 b8 00 00 00 e8 82 c6 01 f8 44 89 f0 99 <f7> 7c 24 2c 41 29 d6 48 bb 00 00 00 00 00 fc ff df e9 9c 00 00 00
RSP: 0018:ffffc90002e67640 EFLAGS: 00010293
RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff88802e693c80
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffc90002e67730 R08: ffff888027d78143 R09: 1ffff11004faf028
R10: dffffc0000000000 R11: ffffed1004faf029 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: ffff88802fdc24f0
FS:  00007fbc0bc376c0(0000) GS:ffff888126139000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00005587d939f950 CR3: 000000007c31a000 CR4: 00000000003526f0
----------------
Code disassembly (best guess), 1 bytes skipped:
   0:	ff c3                	inc    %ebx
   2:	66 2e 0f 1f 84 00 00 	cs nopw 0x0(%rax,%rax,1)
   9:	00 00 00
   c:	0f 1f 40 00          	nopl   0x0(%rax)
  10:	48 89 f8             	mov    %rdi,%rax
  13:	48 89 f7             	mov    %rsi,%rdi
  16:	48 89 d6             	mov    %rdx,%rsi
  19:	48 89 ca             	mov    %rcx,%rdx
  1c:	4d 89 c2             	mov    %r8,%r10
  1f:	4d 89 c8             	mov    %r9,%r8
  22:	4c 8b 4c 24 08       	mov    0x8(%rsp),%r9
  27:	0f 05                	syscall
* 29:	48 3d 01 f0 ff ff    	cmp    $0xfffffffffffff001,%rax <-- trapping instruction
  2f:	73 01                	jae    0x32
  31:	c3                   	ret
  32:	48 c7 c1 a8 ff ff ff 	mov    $0xffffffffffffffa8,%rcx
  39:	f7 d8                	neg    %eax
  3b:	64 89 01             	mov    %eax,%fs:(%rcx)
  3e:	48                   	rex.W


Tested on:

commit:         b3717b4d DO-NOT-MERGE: mptcp: enabled by default
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/matttbe/net-next.git b4/net-mptcp-clear-sched-rtx
console output: https://syzkaller.appspot.com/x/log.txt?x=1037ee92580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=14b6a9313e132a6b
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

Note: no patches were applied.