Re: [PATCH v1 net] rtnetlink: Fix bad unlock balance in do_setlink().

[Stanislav Fomichev <stfomichev@gmail.com>]

On 04/07, Kuniyuki Iwashima wrote:
> When validate_linkmsg() fails in do_setlink(), we jump to the errout
> label and calls netdev_unlock_ops() even though we have not called
> netdev_lock_ops() as reported by syzbot.  [0]
> 
> Let's return an error directly in such a case.
> 
> [0]
> WARNING: bad unlock balance detected!
> 6.14.0-syzkaller-12504-g8bc251e5d874 #0 Not tainted
> 
> syz-executor814/5834 is trying to release lock (&dev_instance_lock_key) at:
> [<ffffffff89f41f56>] netdev_unlock include/linux/netdevice.h:2756 [inline]
> [<ffffffff89f41f56>] netdev_unlock_ops include/net/netdev_lock.h:48 [inline]
> [<ffffffff89f41f56>] do_setlink+0xc26/0x43a0 net/core/rtnetlink.c:3406
> but there are no more locks to release!
> 
> other info that might help us debug this:
> 1 lock held by syz-executor814/5834:
>  #0: ffffffff900fc408 (rtnl_mutex){+.+.}-{4:4}, at: rtnl_lock net/core/rtnetlink.c:80 [inline]
>  #0: ffffffff900fc408 (rtnl_mutex){+.+.}-{4:4}, at: rtnl_nets_lock net/core/rtnetlink.c:341 [inline]
>  #0: ffffffff900fc408 (rtnl_mutex){+.+.}-{4:4}, at: rtnl_newlink+0xd68/0x1fe0 net/core/rtnetlink.c:4064
> 
> stack backtrace:
> CPU: 0 UID: 0 PID: 5834 Comm: syz-executor814 Not tainted 6.14.0-syzkaller-12504-g8bc251e5d874 #0 PREEMPT(full)
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025
> Call Trace:
>  <TASK>
>  __dump_stack lib/dump_stack.c:94 [inline]
>  dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120
>  print_unlock_imbalance_bug+0x185/0x1a0 kernel/locking/lockdep.c:5296
>  __lock_release kernel/locking/lockdep.c:5535 [inline]
>  lock_release+0x1ed/0x3e0 kernel/locking/lockdep.c:5887
>  __mutex_unlock_slowpath+0xee/0x800 kernel/locking/mutex.c:907
>  netdev_unlock include/linux/netdevice.h:2756 [inline]
>  netdev_unlock_ops include/net/netdev_lock.h:48 [inline]
>  do_setlink+0xc26/0x43a0 net/core/rtnetlink.c:3406
>  rtnl_group_changelink net/core/rtnetlink.c:3783 [inline]
>  __rtnl_newlink net/core/rtnetlink.c:3937 [inline]
>  rtnl_newlink+0x1619/0x1fe0 net/core/rtnetlink.c:4065
>  rtnetlink_rcv_msg+0x80f/0xd70 net/core/rtnetlink.c:6955
>  netlink_rcv_skb+0x208/0x480 net/netlink/af_netlink.c:2534
>  netlink_unicast_kernel net/netlink/af_netlink.c:1313 [inline]
>  netlink_unicast+0x7f8/0x9a0 net/netlink/af_netlink.c:1339
>  netlink_sendmsg+0x8c3/0xcd0 net/netlink/af_netlink.c:1883
>  sock_sendmsg_nosec net/socket.c:712 [inline]
>  __sock_sendmsg+0x221/0x270 net/socket.c:727
>  ____sys_sendmsg+0x523/0x860 net/socket.c:2566
>  ___sys_sendmsg net/socket.c:2620 [inline]
>  __sys_sendmsg+0x271/0x360 net/socket.c:2652
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xf3/0x230 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f8427b614a9
> Code: 48 83 c4 28 c3 e8 37 17 00 00 0f 1f 80 00 00 00 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 b8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007fff9b59f3a8 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
> RAX: ffffffffffffffda RBX: 00007fff9b59f578 RCX: 00007f8427b614a9
> RDX: 0000000000000000 RSI: 0000200000000300 RDI: 0000000000000004
> RBP: 00007f8427bd4610 R08: 000000000000000c R09: 00007fff9b59f578
> R10: 000000000000001b R11: 0000000000000246 R12: 0000000000000001
> R13:
> 
> Fixes: 4c975fd70002 ("net: hold instance lock during NETDEV_REGISTER/UP")
> Reported-by: syzbot+45016fe295243a7882d3@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=45016fe295243a7882d3
> Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>

Acked-by: Stanislav Fomichev <sdf@fomichev.me>