Re: [syzbot] [net?] possible deadlock in __netdev_update_features

[Stanislav Fomichev <stfomichev@gmail.com>]

On 06/02, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    7d4e49a77d99 Merge tag 'mm-nonmm-stable-2025-05-31-15-28' ..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1298600c580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=2ea0d63949bc4278
> dashboard link: https://syzkaller.appspot.com/bug?extid=7e0f89fb6cae5d002de0
> compiler:       Debian clang version 20.1.6 (++20250514063057+1e4d39e07757-1~exp1~20250514183223.118), Debian LLD 20.1.6
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/eb4b617767b5/disk-7d4e49a7.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/d0be53c5da74/vmlinux-7d4e49a7.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/9a5769a0ff61/bzImage-7d4e49a7.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+7e0f89fb6cae5d002de0@syzkaller.appspotmail.com
> 
> netdevsim netdevsim1 netdevsim0: unset [0, 0] type 1 family 0 port 8472 - 0
> netdevsim netdevsim1 netdevsim0: unset [1, 0] type 2 family 0 port 6081 - 0
> ============================================
> WARNING: possible recursive locking detected
> 6.15.0-syzkaller-10769-g7d4e49a77d99 #0 Not tainted
> --------------------------------------------
> syz.1.2750/15558 is trying to acquire lock:
> ffff88805a35cd30 (&dev_instance_lock_key#20){+.+.}-{4:4}, at: netdev_lock include/linux/netdevice.h:2756 [inline]
> ffff88805a35cd30 (&dev_instance_lock_key#20){+.+.}-{4:4}, at: netdev_lock_ops include/net/netdev_lock.h:42 [inline]
> ffff88805a35cd30 (&dev_instance_lock_key#20){+.+.}-{4:4}, at: netdev_sync_lower_features net/core/dev.c:10549 [inline]
> ffff88805a35cd30 (&dev_instance_lock_key#20){+.+.}-{4:4}, at: __netdev_update_features+0xcb1/0x1a20 net/core/dev.c:10719
> 
> but task is already holding lock:
> ffff88805a35cd30 (&dev_instance_lock_key#20){+.+.}-{4:4}, at: netdev_lock include/linux/netdevice.h:2756 [inline]
> ffff88805a35cd30 (&dev_instance_lock_key#20){+.+.}-{4:4}, at: netdev_lock_ops include/net/netdev_lock.h:42 [inline]
> ffff88805a35cd30 (&dev_instance_lock_key#20){+.+.}-{4:4}, at: __dev_ethtool net/ethtool/ioctl.c:3227 [inline]
> ffff88805a35cd30 (&dev_instance_lock_key#20){+.+.}-{4:4}, at: dev_ethtool+0x716/0x1990 net/ethtool/ioctl.c:3490
> and the lock comparison function returns 0:
> 
> other info that might help us debug this:
>  Possible unsafe locking scenario:
> 
>        CPU0
>        ----
>   lock(&dev_instance_lock_key#20);
>   lock(&dev_instance_lock_key#20);
> 
>  *** DEADLOCK ***
> 
>  May be due to missing lock nesting notation
> 
> 2 locks held by syz.1.2750/15558:
>  #0: ffffffff8f50b248 (rtnl_mutex){+.+.}-{4:4}, at: dev_ethtool+0x1d0/0x1990 net/ethtool/ioctl.c:3489
>  #1: ffff88805a35cd30 (&dev_instance_lock_key#20){+.+.}-{4:4}, at: netdev_lock include/linux/netdevice.h:2756 [inline]
>  #1: ffff88805a35cd30 (&dev_instance_lock_key#20){+.+.}-{4:4}, at: netdev_lock_ops include/net/netdev_lock.h:42 [inline]
>  #1: ffff88805a35cd30 (&dev_instance_lock_key#20){+.+.}-{4:4}, at: __dev_ethtool net/ethtool/ioctl.c:3227 [inline]
>  #1: ffff88805a35cd30 (&dev_instance_lock_key#20){+.+.}-{4:4}, at: dev_ethtool+0x716/0x1990 net/ethtool/ioctl.c:3490
> 
> stack backtrace:
> CPU: 0 UID: 0 PID: 15558 Comm: syz.1.2750 Not tainted 6.15.0-syzkaller-10769-g7d4e49a77d99 #0 PREEMPT(full) 
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
>  print_deadlock_bug+0x28b/0x2a0 kernel/locking/lockdep.c:3044
>  check_deadlock kernel/locking/lockdep.c:3096 [inline]
>  validate_chain+0x1a3f/0x2140 kernel/locking/lockdep.c:3898
>  __lock_acquire+0xab9/0xd20 kernel/locking/lockdep.c:5240
>  lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5871
>  __mutex_lock_common kernel/locking/mutex.c:602 [inline]
>  __mutex_lock+0x182/0xe80 kernel/locking/mutex.c:747
>  netdev_lock include/linux/netdevice.h:2756 [inline]
>  netdev_lock_ops include/net/netdev_lock.h:42 [inline]
>  netdev_sync_lower_features net/core/dev.c:10549 [inline]
>  __netdev_update_features+0xcb1/0x1a20 net/core/dev.c:10719
>  netdev_change_features+0x72/0xd0 net/core/dev.c:10791
>  bond_compute_features+0x615/0x680 drivers/net/bonding/bond_main.c:1614
>  bond_slave_netdev_event drivers/net/bonding/bond_main.c:4112 [inline]
>  bond_netdev_event+0x72e/0xe80 drivers/net/bonding/bond_main.c:4157
>  notifier_call_chain+0x1b6/0x3e0 kernel/notifier.c:85
>  call_netdevice_notifiers_extack net/core/dev.c:2268 [inline]
>  call_netdevice_notifiers net/core/dev.c:2282 [inline]
>  netdev_features_change+0x85/0xc0 net/core/dev.c:1571
>  __dev_ethtool net/ethtool/ioctl.c:3457 [inline]
>  dev_ethtool+0x1520/0x1990 net/ethtool/ioctl.c:3490
>  dev_ioctl+0x392/0x1150 net/core/dev_ioctl.c:758
>  sock_do_ioctl+0x22c/0x300 net/socket.c:1204
>  sock_ioctl+0x576/0x790 net/socket.c:1311
>  vfs_ioctl fs/ioctl.c:51 [inline]
>  __do_sys_ioctl fs/ioctl.c:907 [inline]
>  __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:893
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7fa86e38e969
> 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:00007fa86b98f038 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> RAX: ffffffffffffffda RBX: 00007fa86e5b6320 RCX: 00007fa86e38e969
> RDX: 0000200000000080 RSI: 0000000000008946 RDI: 0000000000000006
> RBP: 00007fa86e410ab1 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> R13: 0000000000000000 R14: 00007fa86e5b6320 R15: 00007fa86e6dfa28
>  </TASK>
> 
> 
> ---
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I'll keep poking this, but I hope to get a reproducer at some point.
The features are evidently changed on the slave device (since it's the
netdevsim who's lock is grabbed twice), but I can't understand which
ethtool call leads to it.