[syzbot] [fuse?] WARNING: refcount bug in process_scheduled_works

[syzbot] [fuse?] WARNING: refcount bug in process_scheduled_works

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    ced1b9e0392d Merge tag 'ata-6.17-rc1' of git://git.kernel...
git tree:       upstream
console+strace: https://syzkaller.appspot.com/x/log.txt?x=15c89782580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=d51af648924b64c9
dashboard link: https://syzkaller.appspot.com/bug?extid=9921e319bd6168140b40
compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=177f7034580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=165e44a2580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/f1495c1de592/disk-ced1b9e0.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/982f755305ce/vmlinux-ced1b9e0.xz
kernel image: https://storage.googleapis.com/syzbot-assets/b17cd424f373/bzImage-ced1b9e0.xz

The issue was bisected to:

commit 9e59d609763f70a992a8f3808dabcce60f14eb5c
Author: Xiao Ni <xni@redhat.com>
Date:   Wed Jun 11 07:31:06 2025 +0000

    md: call del_gendisk in control path

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=14ab54a2580000
final oops:     https://syzkaller.appspot.com/x/report.txt?x=16ab54a2580000
console output: https://syzkaller.appspot.com/x/log.txt?x=12ab54a2580000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+9921e319bd6168140b40@syzkaller.appspotmail.com
Fixes: 9e59d609763f ("md: call del_gendisk in control path")

------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 1 PID: 24 at lib/refcount.c:28 refcount_warn_saturate+0x11a/0x1d0 lib/refcount.c:28
Modules linked in:
CPU: 1 UID: 0 PID: 24 Comm: kworker/1:0 Not tainted 6.16.0-syzkaller-00857-gced1b9e0392d #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
Workqueue: md_misc mddev_delayed_delete
RIP: 0010:refcount_warn_saturate+0x11a/0x1d0 lib/refcount.c:28
Code: 00 83 e2 8b e8 b7 5b bf fc 90 0f 0b 90 90 eb d7 e8 bb 6d fb fc c6 05 09 32 c7 0a 01 90 48 c7 c7 60 83 e2 8b e8 97 5b bf fc 90 <0f> 0b 90 90 eb b7 e8 9b 6d fb fc c6 05 e6 31 c7 0a 01 90 48 c7 c7
RSP: 0018:ffffc900001e7a68 EFLAGS: 00010246
RAX: 3c6a9b38a742a100 RBX: 0000000000000003 RCX: ffff88801d298000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000002
RBP: ffffc900001e7c70 R08: ffff8880b8724293 R09: 1ffff110170e4852
R10: dffffc0000000000 R11: ffffed10170e4853 R12: ffff8880b8739700
R13: ffff88801d2ab218 R14: ffff8880331dc130 R15: 0000000000000001
FS:  0000000000000000(0000) GS:ffff888125d07000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000056071c71c048 CR3: 000000000df38000 CR4: 0000000000350ef0
Call Trace:
 <TASK>
 process_one_work kernel/workqueue.c:3238 [inline]
 process_scheduled_works+0xae1/0x17b0 kernel/workqueue.c:3321
 worker_thread+0x8a0/0xda0 kernel/workqueue.c:3402
 kthread+0x711/0x8a0 kernel/kthread.c:464
 ret_from_fork+0x3fc/0x770 arch/x86/kernel/process.c:148
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
For information about bisection process see: https://goo.gl/tpsmEJ#bisection

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want syzbot to run the reproducer, reply with:
#syz test: git://repo/address.git branch-or-commit-hash
If you attach or paste a git patch, syzbot will apply it before testing.

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup

Re: [syzbot] [fuse?] WARNING: refcount bug in process_scheduled_works

[Hillf Danton]

> Date: Tue, 29 Jul 2025 14:58:32 -0700	[thread overview]
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    ced1b9e0392d Merge tag 'ata-6.17-rc1' of git://git.kernel...
> git tree:       upstream
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=15c89782580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=d51af648924b64c9
> dashboard link: https://syzkaller.appspot.com/bug?extid=9921e319bd6168140b40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=177f7034580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=165e44a2580000

#syz test

--- x/drivers/md/md.c
+++ y/drivers/md/md.c
@@ -632,10 +632,13 @@ static void mddev_delayed_delete(struct
 
 static void __mddev_put(struct mddev *mddev)
 {
+	struct kobject *kobj = &mddev->kobj;
+
 	if (mddev->raid_disks || !list_empty(&mddev->disks) ||
 	    mddev->ctime || mddev->hold_active)
 		return;
 
+	BUG_ON(kref_read(&kobj->kref) < 1);
 	/*
 	 * Call queue_work inside the spinlock so that flush_workqueue() after
 	 * mddev_find will succeed in waiting for the work to be done.
@@ -710,6 +713,7 @@ int mddev_init(struct mddev *mddev)
 
 	INIT_WORK(&mddev->sync_work, md_start_sync);
 	INIT_WORK(&mddev->del_work, mddev_delayed_delete);
+	kobject_init(&mddev->kobj, &md_ktype);
 
 	return 0;
 }
@@ -5982,7 +5986,6 @@ struct mddev *md_alloc(dev_t dev, char *
 	if (error)
 		goto out_put_disk;
 
-	kobject_init(&mddev->kobj, &md_ktype);
 	error = kobject_add(&mddev->kobj, &disk_to_dev(disk)->kobj, "%s", "md");
 	if (error) {
 		/*
--

Re: [syzbot] [fuse?] WARNING: refcount bug in process_scheduled_works

[syzbot]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
kernel BUG in mddev_put

------------[ cut here ]------------
kernel BUG at drivers/md/md.c:641!
Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
CPU: 1 UID: 0 PID: 6615 Comm: syz.0.17 Not tainted 6.16.0-syzkaller-gbeb6c8326eb4-dirty #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
RIP: 0010:__mddev_put drivers/md/md.c:641 [inline]
RIP: 0010:mddev_put+0x1e9/0x1f0 drivers/md/md.c:660
Code: f9 e9 3f ff ff ff 44 89 f1 80 e1 07 80 c1 03 38 c1 0f 8c 6d ff ff ff 4c 89 f7 e8 12 f3 e0 f9 e9 60 ff ff ff e8 48 e0 7e f9 90 <0f> 0b 0f 1f 44 00 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90
RSP: 0018:ffffc90002ee7cb0 EFLAGS: 00010293
RAX: ffffffff8840dae8 RBX: ffff888033654000 RCX: ffff88802f4bda00
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: ffff888033654133 R09: 1ffff110066ca826
R10: dffffc0000000000 R11: ffffed10066ca827 R12: ffff888027267000
R13: ffff888148c55090 R14: ffff888033654130 R15: dffffc0000000000
FS:  0000555567c20500(0000) GS:ffff888125d86000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000007 CR3: 0000000075a4e000 CR4: 0000000000350ef0
Call Trace:
 <TASK>
 bdev_release+0x536/0x650 block/bdev.c:-1
 blkdev_release+0x15/0x20 block/fops.c:699
 __fput+0x44c/0xa70 fs/file_table.c:468
 task_work_run+0x1d4/0x260 kernel/task_work.c:227
 resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
 exit_to_user_mode_loop+0xec/0x110 kernel/entry/common.c:43
 exit_to_user_mode_prepare include/linux/irq-entry-common.h:208 [inline]
 syscall_exit_to_user_mode_work include/linux/entry-common.h:175 [inline]
 syscall_exit_to_user_mode include/linux/entry-common.h:210 [inline]
 do_syscall_64+0x2bd/0x3b0 arch/x86/entry/syscall_64.c:100
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fe9c578e9a9
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:00007ffe99d6fa48 EFLAGS: 00000246 ORIG_RAX: 00000000000001b4
RAX: 0000000000000000 RBX: 000000000002511a RCX: 00007fe9c578e9a9
RDX: 0000000000000000 RSI: 000000000000001e RDI: 0000000000000003
RBP: 00007fe9c59b7ba0 R08: 0000000000000001 R09: 0000000899d6fd3f
R10: 00007fe9c5600000 R11: 0000000000000246 R12: 00007fe9c59b5fac
R13: 00007fe9c59b5fa0 R14: ffffffffffffffff R15: 00007ffe99d6fb60
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:__mddev_put drivers/md/md.c:641 [inline]
RIP: 0010:mddev_put+0x1e9/0x1f0 drivers/md/md.c:660
Code: f9 e9 3f ff ff ff 44 89 f1 80 e1 07 80 c1 03 38 c1 0f 8c 6d ff ff ff 4c 89 f7 e8 12 f3 e0 f9 e9 60 ff ff ff e8 48 e0 7e f9 90 <0f> 0b 0f 1f 44 00 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90
RSP: 0018:ffffc90002ee7cb0 EFLAGS: 00010293
RAX: ffffffff8840dae8 RBX: ffff888033654000 RCX: ffff88802f4bda00
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: ffff888033654133 R09: 1ffff110066ca826
R10: dffffc0000000000 R11: ffffed10066ca827 R12: ffff888027267000
R13: ffff888148c55090 R14: ffff888033654130 R15: dffffc0000000000
FS:  0000555567c20500(0000) GS:ffff888125d86000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000007 CR3: 0000000075a4e000 CR4: 0000000000350ef0


Tested on:

commit:         beb6c832 Merge tag 'uml-for-linux-6.17-rc1' of git://g..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=152d0834580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=dea300d7050f7861
dashboard link: https://syzkaller.appspot.com/bug?extid=9921e319bd6168140b40
compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
patch:          https://syzkaller.appspot.com/x/patch.diff?x=13a3b1bc580000

Re: [syzbot] [fuse?] WARNING: refcount bug in process_scheduled_works

[Hillf Danton]

> Date: Tue, 29 Jul 2025 14:58:32 -0700	[thread overview]
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    ced1b9e0392d Merge tag 'ata-6.17-rc1' of git://git.kernel...
> git tree:       upstream
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=15c89782580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=d51af648924b64c9
> dashboard link: https://syzkaller.appspot.com/bug?extid=9921e319bd6168140b40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=177f7034580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=165e44a2580000

#syz test

--- x/drivers/md/md.c
+++ y/drivers/md/md.c
@@ -632,10 +632,13 @@ static void mddev_delayed_delete(struct
 
 static void __mddev_put(struct mddev *mddev)
 {
+	struct kobject *kobj = &mddev->kobj;
+
 	if (mddev->raid_disks || !list_empty(&mddev->disks) ||
 	    mddev->ctime || mddev->hold_active)
 		return;
 
+	BUG_ON(kref_read(&kobj->kref) < 1);
 	/*
 	 * Call queue_work inside the spinlock so that flush_workqueue() after
 	 * mddev_find will succeed in waiting for the work to be done.
@@ -5983,6 +5986,7 @@ struct mddev *md_alloc(dev_t dev, char *
 		goto out_put_disk;
 
 	kobject_init(&mddev->kobj, &md_ktype);
+	kobject_get(&mddev->kobj);
 	error = kobject_add(&mddev->kobj, &disk_to_dev(disk)->kobj, "%s", "md");
 	if (error) {
 		/*
--

Re: [syzbot] [fuse?] WARNING: refcount bug in process_scheduled_works

[syzbot]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
kernel BUG in mddev_put

------------[ cut here ]------------
kernel BUG at drivers/md/md.c:641!
Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
CPU: 0 UID: 0 PID: 6690 Comm: syz.0.18 Not tainted 6.16.0-syzkaller-04405-g4b290aae788e-dirty #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
RIP: 0010:__mddev_put drivers/md/md.c:641 [inline]
RIP: 0010:mddev_put+0x1e9/0x1f0 drivers/md/md.c:660
Code: f9 e9 3f ff ff ff 44 89 f1 80 e1 07 80 c1 03 38 c1 0f 8c 6d ff ff ff 4c 89 f7 e8 c2 ed e0 f9 e9 60 ff ff ff e8 f8 da 7e f9 90 <0f> 0b 0f 1f 44 00 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90
RSP: 0018:ffffc90002f27cb0 EFLAGS: 00010293
RAX: ffffffff8840e0e8 RBX: ffff888032332000 RCX: ffff88807b13da00
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: ffff888032332133 R09: 1ffff11006466426
R10: dffffc0000000000 R11: ffffed1006466427 R12: ffff88806364e000
R13: ffff888148c47310 R14: ffff888032332130 R15: dffffc0000000000
FS:  000055558e36e500(0000) GS:ffff888125c86000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000007 CR3: 000000005c370000 CR4: 0000000000350ef0
Call Trace:
 <TASK>
 bdev_release+0x536/0x650 block/bdev.c:-1
 blkdev_release+0x15/0x20 block/fops.c:699
 __fput+0x44c/0xa70 fs/file_table.c:468
 task_work_run+0x1d4/0x260 kernel/task_work.c:227
 resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
 exit_to_user_mode_loop+0xec/0x110 kernel/entry/common.c:43
 exit_to_user_mode_prepare include/linux/irq-entry-common.h:208 [inline]
 syscall_exit_to_user_mode_work include/linux/entry-common.h:175 [inline]
 syscall_exit_to_user_mode include/linux/entry-common.h:210 [inline]
 do_syscall_64+0x2bd/0x3b0 arch/x86/entry/syscall_64.c:100
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f9d1d58e9a9
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:00007ffee3402d38 EFLAGS: 00000246 ORIG_RAX: 00000000000001b4
RAX: 0000000000000000 RBX: 0000000000025060 RCX: 00007f9d1d58e9a9
RDX: 0000000000000000 RSI: 000000000000001e RDI: 0000000000000003
RBP: 00007f9d1d7b7ba0 R08: 0000000000000001 R09: 00000008e340302f
R10: 00007f9d1d400000 R11: 0000000000000246 R12: 00007f9d1d7b5fac
R13: 00007f9d1d7b5fa0 R14: ffffffffffffffff R15: 00007ffee3402e50
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:__mddev_put drivers/md/md.c:641 [inline]
RIP: 0010:mddev_put+0x1e9/0x1f0 drivers/md/md.c:660
Code: f9 e9 3f ff ff ff 44 89 f1 80 e1 07 80 c1 03 38 c1 0f 8c 6d ff ff ff 4c 89 f7 e8 c2 ed e0 f9 e9 60 ff ff ff e8 f8 da 7e f9 90 <0f> 0b 0f 1f 44 00 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90
RSP: 0018:ffffc90002f27cb0 EFLAGS: 00010293
RAX: ffffffff8840e0e8 RBX: ffff888032332000 RCX: ffff88807b13da00
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: ffff888032332133 R09: 1ffff11006466426
R10: dffffc0000000000 R11: ffffed1006466427 R12: ffff88806364e000
R13: ffff888148c47310 R14: ffff888032332130 R15: dffffc0000000000
FS:  000055558e36e500(0000) GS:ffff888125c86000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000007 CR3: 000000005c370000 CR4: 0000000000350ef0


Tested on:

commit:         4b290aae Merge tag 'sysctl-6.17-rc1' of git://git.kern..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11948834580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=dea300d7050f7861
dashboard link: https://syzkaller.appspot.com/bug?extid=9921e319bd6168140b40
compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
patch:          https://syzkaller.appspot.com/x/patch.diff?x=1394ccf0580000

Re: [syzbot] [fuse?] WARNING: refcount bug in process_scheduled_works

[Hillf Danton]

> Date: Tue, 29 Jul 2025 14:58:32 -0700	[thread overview]
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    ced1b9e0392d Merge tag 'ata-6.17-rc1' of git://git.kernel...
> git tree:       upstream
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=15c89782580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=d51af648924b64c9
> dashboard link: https://syzkaller.appspot.com/bug?extid=9921e319bd6168140b40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=177f7034580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=165e44a2580000

Test Yu Kuai's fix (https://lore.kernel.org/lkml/20250730073321.2583158-1-yukuai1@huaweicloud.com/)

#syz test

--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -636,6 +636,12 @@ static void __mddev_put(struct mddev *mddev)
 	    mddev->ctime || mddev->hold_active)
 		return;
 
+	/*
+	 * If array is freed by stopping array, MD_DELETED is set by
+	 * do_md_stop(), MD_DELETED is still set here in cause mddev is freed
+	 * directly by closing a mddev that is created by create_on_open.
+	 */
+	set_bit(MD_DELETED, &mddev->flags);
 	/*
 	 * Call queue_work inside the spinlock so that flush_workqueue() after
 	 * mddev_find will succeed in waiting for the work to be done.
-- 

Re: [syzbot] [fuse?] WARNING: refcount bug in process_scheduled_works

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+9921e319bd6168140b40@syzkaller.appspotmail.com
Tested-by: syzbot+9921e319bd6168140b40@syzkaller.appspotmail.com

Tested on:

commit:         4b290aae Merge tag 'sysctl-6.17-rc1' of git://git.kern..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=13420ca2580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=dea300d7050f7861
dashboard link: https://syzkaller.appspot.com/bug?extid=9921e319bd6168140b40
compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
patch:          https://syzkaller.appspot.com/x/patch.diff?x=12908834580000

Note: testing is done by a robot and is best-effort only.