Re: [syzbot] [netfs?] divide error in netfs_submit_writethrough

[syzbot]

syzbot has found a reproducer for the following issue on:

HEAD commit:    a2c63a3f3d68 Merge tag 'bcachefs-2024-04-22' of https://ev..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11623dfd180000
kernel config:  https://syzkaller.appspot.com/x/.config?x=545d4b3e07d6ccbc
dashboard link: https://syzkaller.appspot.com/bug?extid=f3a09670f3d2a55b89b2
compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15ff809b180000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15aaab4f180000

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/7bc7510fe41f/non_bootable_disk-a2c63a3f.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/b64cb6a17a78/vmlinux-a2c63a3f.xz
kernel image: https://storage.googleapis.com/syzbot-assets/8984e0f657fd/bzImage-a2c63a3f.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+f3a09670f3d2a55b89b2@xxxxxxxxxxxxxxxxxxxxxxxxx

divide error: 0000 [#1] PREEMPT SMP KASAN NOPTI
CPU: 3 PID: 5183 Comm: syz-executor293 Not tainted 6.9.0-rc5-syzkaller-00025-ga2c63a3f3d68 #0
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
RIP: 0010:netfs_submit_writethrough+0x201/0x280 fs/netfs/output.c:427
Code: fc ff df 48 89 fa 48 c1 ea 03 0f b6 14 02 48 89 f8 83 e0 07 83 c0 03 38 d0 7c 04 84 d2 75 1a 8b 8b 0c 01 00 00 48 89 e8 31 d2 <48> f7 f1 48 89 c5 48 0f af e9 e9 1d ff ff ff e8 9b a3 b8 ff eb df
RSP: 0018:ffffc90003477760 EFLAGS: 00010246
RAX: 0000000000001000 RBX: ffff88802d072c00 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff82304719 RDI: ffff88802d072d0c
RBP: 0000000000001000 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000003 R12: 0000000000000000
R13: ffff8880324e05e0 R14: ffff88802d072d20 R15: 0000000000000000
FS:  0000555585aef480(0000) GS:ffff88806b500000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ffc516dc998 CR3: 0000000029bf4000 CR4: 0000000000350ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 netfs_advance_writethrough+0x13f/0x170 fs/netfs/output.c:449
 netfs_perform_write+0x1b9f/0x26b0 fs/netfs/buffered_write.c:385
 netfs_buffered_write_iter_locked+0x213/0x2c0 fs/netfs/buffered_write.c:454
 netfs_file_write_iter+0x1e0/0x470 fs/netfs/buffered_write.c:493
 v9fs_file_write_iter+0xa1/0x100 fs/9p/vfs_file.c:407
 call_write_iter include/linux/fs.h:2110 [inline]
 do_iter_readv_writev+0x504/0x780 fs/read_write.c:741
 vfs_writev+0x36f/0xdb0 fs/read_write.c:971
 do_pwritev+0x1b2/0x260 fs/read_write.c:1072
 __do_sys_pwritev2 fs/read_write.c:1131 [inline]
 __se_sys_pwritev2 fs/read_write.c:1122 [inline]
 __x64_sys_pwritev2+0xef/0x160 fs/read_write.c:1122
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xcf/0x260 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fecc4c95b59
Code: 48 83 c4 28 c3 e8 67 17 00 00 0f 1f 80 00 00 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 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fff380e4b18 EFLAGS: 00000216 ORIG_RAX: 0000000000000148
RAX: ffffffffffffffda RBX: 00007fff380e4b30 RCX: 00007fecc4c95b59
RDX: 0000000000000001 RSI: 0000000020000240 RDI: 0000000000000003
RBP: 00007fff380e4b38 R08: 0000000000000000 R09: 0000000000000015
R10: 0000000000000000 R11: 0000000000000216 R12: 0000000000000000
R13: 00007fff380e4d98 R14: 0000000000000001 R15: 0000000000000001
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:netfs_submit_writethrough+0x201/0x280 fs/netfs/output.c:427
Code: fc ff df 48 89 fa 48 c1 ea 03 0f b6 14 02 48 89 f8 83 e0 07 83 c0 03 38 d0 7c 04 84 d2 75 1a 8b 8b 0c 01 00 00 48 89 e8 31 d2 <48> f7 f1 48 89 c5 48 0f af e9 e9 1d ff ff ff e8 9b a3 b8 ff eb df
RSP: 0018:ffffc90003477760 EFLAGS: 00010246
RAX: 0000000000001000 RBX: ffff88802d072c00 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff82304719 RDI: ffff88802d072d0c
RBP: 0000000000001000 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000003 R12: 0000000000000000
R13: ffff8880324e05e0 R14: ffff88802d072d20 R15: 0000000000000000
FS:  0000555585aef480(0000) GS:ffff88806b500000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ffc516dc998 CR3: 0000000029bf4000 CR4: 0000000000350ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
----------------
Code disassembly (best guess), 2 bytes skipped:
   0:	df 48 89             	fisttps -0x77(%rax)
   3:	fa                   	cli
   4:	48 c1 ea 03          	shr    $0x3,%rdx
   8:	0f b6 14 02          	movzbl (%rdx,%rax,1),%edx
   c:	48 89 f8             	mov    %rdi,%rax
   f:	83 e0 07             	and    $0x7,%eax
  12:	83 c0 03             	add    $0x3,%eax
  15:	38 d0                	cmp    %dl,%al
  17:	7c 04                	jl     0x1d
  19:	84 d2                	test   %dl,%dl
  1b:	75 1a                	jne    0x37
  1d:	8b 8b 0c 01 00 00    	mov    0x10c(%rbx),%ecx
  23:	48 89 e8             	mov    %rbp,%rax
  26:	31 d2                	xor    %edx,%edx
* 28:	48 f7 f1             	div    %rcx <-- trapping instruction
  2b:	48 89 c5             	mov    %rax,%rbp
  2e:	48 0f af e9          	imul   %rcx,%rbp
  32:	e9 1d ff ff ff       	jmp    0xffffff54
  37:	e8 9b a3 b8 ff       	call   0xffb8a3d7
  3c:	eb df                	jmp    0x1d


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