Re: [syzbot] [driver-core?] [fs?] kernel BUG in sysfs_break_active_protection

Re: [syzbot] [driver-core?] [fs?] kernel BUG in sysfs_break_active_protection

[syzbot]

Hello,

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

Reported-by: syzbot+dcad997ed2b5bd95cc5e@syzkaller.appspotmail.com
Tested-by: syzbot+dcad997ed2b5bd95cc5e@syzkaller.appspotmail.com

Tested on:

commit:         7649ba73 Add linux-next specific files for 20261030
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
console output: https://syzkaller.appspot.com/x/log.txt?x=118aa28d580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=82467d52d846ff27
dashboard link: https://syzkaller.appspot.com/bug?extid=dcad997ed2b5bd95cc5e
compiler:       Debian clang version 22.1.8 (1~deb13u4), Debian LLD 22.1.8
patch:          https://syzkaller.appspot.com/x/patch.diff?x=13dccbe0580000

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

[syzbot] [driver-core?] [fs?] kernel BUG in sysfs_break_active_protection

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    7649ba73f9e7 Add linux-next specific files for 20261030
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
console output: https://syzkaller.appspot.com/x/log.txt?x=139805f1580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=82467d52d846ff27
dashboard link: https://syzkaller.appspot.com/bug?extid=dcad997ed2b5bd95cc5e
compiler:       Debian clang version 22.1.8 (1~deb13u4), Debian LLD 22.1.8
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=10a82cb1580000

AI-generated root cause summary:
In md_attr_store(), writing "clear" to array_state invokes
sysfs_break_active_protection() prior to acquiring all_mddevs_lock and
validating the device via mddev_get(). 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:
 <TASK>
 __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 <c3> 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
 </TASK>
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


---
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