[syzbot ci] Re: udf: Fix race between file type conversion and writeback

[syzbot ci]

syzbot ci has tested the following series

[v1] udf: Fix race between file type conversion and writeback
https://lore.kernel.org/all/2026032316261...@suse.cz
* [PATCH 1/2] writeback: Export folio_prepare_writeback()
* [PATCH 2/2] udf: Fix race between file type conversion and writeback

and found the following issue:
general protection fault in folio_prepare_writeback

Full report is available here:
https://ci.syzbot.org/series/03e405d8-f247-471a-8469-f544c8393300

***

general protection fault in folio_prepare_writeback

tree: mm-new
URL: https://kernel.googlesource.com/pub/scm/linux/kernel/git/akpm/mm.git
base: af5802cff33fe3c557dff87cd3897d14241a7c6d
arch: amd64
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
config: https://ci.syzbot.org/builds/d1944db2-4f63-4e26-b642-d71f55382c9d/config
C repro: https://ci.syzbot.org/findings/87b82667-f800-480e-b52a-38decce9e6c4/c_repro
syz repro: https://ci.syzbot.org/findings/87b82667-f800-480e-b52a-38decce9e6c4/syz_repro

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000002: 0000 [#1] SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
CPU: 0 UID: 0 PID: 1860 Comm: kworker/u9:3 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
Workqueue: writeback wb_workfn (flush-7:0)
RIP: 0010:folio_prepare_writeback+0x32/0x280 mm/page-writeback.c:2371
Code: 56 41 55 41 54 53 50 48 89 d3 48 89 f5 49 89 fe 49 bd 00 00 00 00 00 fc ff df e8 f9 22 c2 ff 4c 8d 63 18 4c 89 e0 48 c1 e8 03 <42> 80 3c 28 00 74 08 4c 89 e7 e8 ef 6a 2c 00 4d 39 34 24 0f 85 bf
RSP: 0018:ffffc9000901f1e8 EFLAGS: 00010203
RAX: 0000000000000002 RBX: fffffffffffffffe RCX: ffff88810981ba80
RDX: 0000000000000000 RSI: ffffc9000901f4e0 RDI: ffff8881a659bc48
RBP: ffffc9000901f4e0 R08: ffff88810981ba80 R09: 0000000000000003
R10: 0000000000000406 R11: 0000000000000000 R12: 0000000000000016
R13: dffffc0000000000 R14: ffff8881a659bc48 R15: ffffc9000901f4e0
FS: 0000000000000000(0000) GS:ffff88818de5e000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00001b4fda9cd4b8 CR3: 0000000110906000 CR4: 00000000000006f0
Call Trace:
<TASK>
udf_writepages+0xce/0x3b0 fs/udf/inode.c:205
do_writepages+0x32e/0x550 mm/page-writeback.c:2554
__writeback_single_inode+0x133/0x11a0 fs/fs-writeback.c:1750
writeback_sb_inodes+0x992/0x1a20 fs/fs-writeback.c:2042
wb_writeback+0x456/0xb70 fs/fs-writeback.c:2227
wb_do_writeback fs/fs-writeback.c:2374 [inline]
wb_workfn+0x414/0xf50 fs/fs-writeback.c:2414
process_one_work kernel/workqueue.c:3276 [inline]
process_scheduled_works+0xb6e/0x18c0 kernel/workqueue.c:3359
worker_thread+0xa53/0xfc0 kernel/workqueue.c:3440
kthread+0x388/0x470 kernel/kthread.c:436
ret_from_fork+0x51e/0xb90 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
</TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:folio_prepare_writeback+0x32/0x280 mm/page-writeback.c:2371
Code: 56 41 55 41 54 53 50 48 89 d3 48 89 f5 49 89 fe 49 bd 00 00 00 00 00 fc ff df e8 f9 22 c2 ff 4c 8d 63 18 4c 89 e0 48 c1 e8 03 <42> 80 3c 28 00 74 08 4c 89 e7 e8 ef 6a 2c 00 4d 39 34 24 0f 85 bf
RSP: 0018:ffffc9000901f1e8 EFLAGS: 00010203
RAX: 0000000000000002 RBX: fffffffffffffffe RCX: ffff88810981ba80
RDX: 0000000000000000 RSI: ffffc9000901f4e0 RDI: ffff8881a659bc48
RBP: ffffc9000901f4e0 R08: ffff88810981ba80 R09: 0000000000000003
R10: 0000000000000406 R11: 0000000000000000 R12: 0000000000000016
R13: dffffc0000000000 R14: ffff8881a659bc48 R15: ffffc9000901f4e0
FS: 0000000000000000(0000) GS:ffff8882a945e000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00001b4fda9c7570 CR3: 0000000173314000 CR4: 00000000000006f0
----------------
Code disassembly (best guess):
0: 56 push %rsi
1: 41 55 push %r13
3: 41 54 push %r12
5: 53 push %rbx
6: 50 push %rax
7: 48 89 d3 mov %rdx,%rbx
a: 48 89 f5 mov %rsi,%rbp
d: 49 89 fe mov %rdi,%r14
10: 49 bd 00 00 00 00 00 movabs $0xdffffc0000000000,%r13
17: fc ff df
1a: e8 f9 22 c2 ff call 0xffc22318
1f: 4c 8d 63 18 lea 0x18(%rbx),%r12
23: 4c 89 e0 mov %r12,%rax
26: 48 c1 e8 03 shr $0x3,%rax
* 2a: 42 80 3c 28 00 cmpb $0x0,(%rax,%r13,1) <-- trapping instruction
2f: 74 08 je 0x39
31: 4c 89 e7 mov %r12,%rdi
34: e8 ef 6a 2c 00 call 0x2c6b28
39: 4d 39 34 24 cmp %r14,(%r12)
3d: 0f .byte 0xf
3e: 85 .byte 0x85
3f: bf .byte 0xbf


***

If these findings have caused you to resend the series or submit a
separate fix, please add the following tag to your commit message:
Tested-by: syz...@syzkaller.appspotmail.com

---
This report is generated by a bot. It may contain errors.
syzbot ci engineers can be reached at syzk...@googlegroups.com.

[Jan Kara]

On Mon 23-03-26 15:37:15, syzbot ci wrote:
> syzbot ci has tested the following series
>
> [v1] udf: Fix race between file type conversion and writeback
> https://lore.kernel.org/all/2026032316261...@suse.cz
> * [PATCH 1/2] writeback: Export folio_prepare_writeback()
> * [PATCH 2/2] udf: Fix race between file type conversion and writeback
>
> and found the following issue:
> general protection fault in folio_prepare_writeback
>
> Full report is available here:
> https://ci.syzbot.org/series/03e405d8-f247-471a-8469-f544c8393300

Bah, stupid me. The result of filemap_lock_folio() must be checked with
IS_ERR(), not against NULL. Will send v2.

Honza

--
Jan Kara <ja...@suse.com>
SUSE Labs, CR