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Since sysfs writes only hold kernfs active references rather than a kobject reference, a concurrent deletion or teardown of the mddev can drop the final reference on mddev->kobj, reducing kobj->kref to zero before kernfs removal drains pending active accesses. Calling sysfs_break_active_protection() then executes kobject_get() on the zero-refcount object, triggering a refcount use-after-free saturation/BUG and potentially corrupting memory or crashing the kernel. full analysis: https://syzkaller.appspot.com/x/ai_root_cause?x=dcad997ed2b5bd95cc5e IMPORTANT: if you fix the issue, please add the following tag to the commit: Reported-by: syzbot+dcad997ed2b5bd95cc5e@syzkaller.appspotmail.com refcount_t: addition on 0; use-after-free. ------------[ cut here ]------------ kernel BUG at lib/refcount.c:34! Oops: invalid opcode: 0000 [#1] SMP KASAN PTI CPU: 1 UID: 0 PID: 6056 Comm: syz-executor398 Not tainted syzkaller #0 PREEMPT(full) Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/15/2026 RIP: 0010:refcount_warn_saturate+0xf0/0x110 lib/refcount.c:34 Code: 80 c1 03 38 c1 0f 8c 6a ff ff ff 4c 89 f7 e8 e7 44 51 fd e9 5d ff ff ff e8 dd 23 da fc 48 c7 c7 e0 a3 8c 8c e8 a1 d1 38 fc 90 <0f> 0b e8 c9 23 da fc 48 c7 c7 60 a4 8c 8c e8 8d d1 38 fc 90 0f 0b RSP: 0018:ffffc90002e5fb78 EFLAGS: 00010246 RAX: 000000000000002a RBX: 0000000000000002 RCX: 4550039d880cf800 RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000000 RBP: 0000000000000000 R08: ffffffff9014c84b R09: 1ffffffff2029909 R10: dffffc0000000000 R11: fffffbfff202990a R12: dffffc0000000000 R13: 1ffffffff22654c6 R14: ffff88807c40e128 R15: dffffc0000000000 FS: 00007f63dcdeb6c0(0000) GS:ffff88812350b000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 000055b9ba686138 CR3: 0000000074ef4000 CR4: 00000000003526f0 Call Trace: __refcount_add include/linux/refcount.h:-1 [inline] __refcount_inc include/linux/refcount.h:366 [inline] refcount_inc include/linux/refcount.h:383 [inline] kref_get include/linux/kref.h:45 [inline] kobject_get+0xfa/0x120 lib/kobject.c:643 sysfs_break_active_protection+0x28/0xa0 fs/sysfs/file.c:471 md_attr_store+0x28f/0x640 drivers/md/md.c:6167 kernfs_fop_write_iter+0x3a4/0x540 fs/kernfs/file.c:345 new_sync_write fs/read_write.c:595 [inline] vfs_write+0x612/0xba0 fs/read_write.c:687 ksys_pwrite64 fs/read_write.c:1504 [inline] __do_sys_pwrite64 fs/read_write.c:1512 [inline] __se_sys_pwrite64 fs/read_write.c:1509 [inline] __x64_sys_pwrite64+0x210/0x2c0 fs/read_write.c:1509 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline] do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84 entry_SYSCALL_64_after_hwframe+0x77/0x7f RIP: 0033:0x7f63de64ba9e Code: 08 0f 85 f5 e2 ff ff 49 89 fb 48 89 f0 48 89 d7 48 89 ce 4c 89 c2 4d 89 ca 4c 8b 44 24 08 4c 8b 4c 24 10 4c 89 5c 24 08 0f 05 90 41 57 41 56 4d 89 c6 41 55 4d 89 cd 41 54 55 53 48 83 ec 08 RSP: 002b:00007f63dcdeb0f8 EFLAGS: 00000246 ORIG_RAX: 0000000000000012 RAX: ffffffffffffffda RBX: 00007f63dcdeb6c0 RCX: 00007f63de64ba9e RDX: 0000000000000005 RSI: 00007f63de69003a RDI: 0000000000000006 RBP: 0000000000000006 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000246 R12: 00007f63de69003a R13: 0000000000000003 R14: 00007f63de6c3100 R15: 00007f63de6c4b4c Modules linked in: ---[ end trace 0000000000000000 ]--- RIP: 0010:refcount_warn_saturate+0xf0/0x110 lib/refcount.c:34 Code: 80 c1 03 38 c1 0f 8c 6a ff ff ff 4c 89 f7 e8 e7 44 51 fd e9 5d ff ff ff e8 dd 23 da fc 48 c7 c7 e0 a3 8c 8c e8 a1 d1 38 fc 90 <0f> 0b e8 c9 23 da fc 48 c7 c7 60 a4 8c 8c e8 8d d1 38 fc 90 0f 0b RSP: 0018:ffffc90002e5fb78 EFLAGS: 00010246 RAX: 000000000000002a RBX: 0000000000000002 RCX: 4550039d880cf800 RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000000 RBP: 0000000000000000 R08: ffffffff9014c84b R09: 1ffffffff2029909 R10: dffffc0000000000 R11: fffffbfff202990a R12: dffffc0000000000 R13: 1ffffffff22654c6 R14: ffff88807c40e128 R15: dffffc0000000000 FS: 00007f63dcdeb6c0(0000) GS:ffff88812350b000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f94a870e7d0 CR3: 0000000074ef4000 CR4: 00000000003526f0 --- 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. 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