Re: [syzbot] [nfs?] INFO: task hung in nfsd_nl_listener_get_doit

[Jeff Layton <jlayton@kernel.org>]

On Sat, 2024-06-15 at 03:39 -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    cea2a26553ac mailmap: Add my outdated addresses to the map..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=169fd8ee980000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=fa0ce06dcc735711
> dashboard link: https://syzkaller.appspot.com/bug?extid=4207adf14e7c0981d28d
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/1f7ce933512f/disk-cea2a265.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/0ce3b9940616/vmlinux-cea2a265.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/19e24094ea37/bzImage-cea2a265.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+4207adf14e7c0981d28d@syzkaller.appspotmail.com
> 
> INFO: task syz-executor.1:17770 blocked for more than 143 seconds.
>       Not tainted 6.10.0-rc3-syzkaller-00022-gcea2a26553ac #0
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> task:syz-executor.1  state:D stack:23800 pid:17770 tgid:17767 ppid:11381  flags:0x00000006
> Call Trace:
>  <TASK>
>  context_switch kernel/sched/core.c:5408 [inline]
>  __schedule+0x17e8/0x4a20 kernel/sched/core.c:6745
>  __schedule_loop kernel/sched/core.c:6822 [inline]
>  schedule+0x14b/0x320 kernel/sched/core.c:6837
>  schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:6894
>  __mutex_lock_common kernel/locking/mutex.c:684 [inline]
>  __mutex_lock+0x6a4/0xd70 kernel/locking/mutex.c:752
>  nfsd_nl_listener_get_doit+0x115/0x5d0 fs/nfsd/nfsctl.c:2124
>  genl_family_rcv_msg_doit net/netlink/genetlink.c:1115 [inline]
>  genl_family_rcv_msg net/netlink/genetlink.c:1195 [inline]
>  genl_rcv_msg+0xb16/0xec0 net/netlink/genetlink.c:1210
>  netlink_rcv_skb+0x1e5/0x430 net/netlink/af_netlink.c:2564
>  genl_rcv+0x28/0x40 net/netlink/genetlink.c:1219
>  netlink_unicast_kernel net/netlink/af_netlink.c:1335 [inline]
>  netlink_unicast+0x7ec/0x980 net/netlink/af_netlink.c:1361
>  netlink_sendmsg+0x8db/0xcb0 net/netlink/af_netlink.c:1905
>  sock_sendmsg_nosec net/socket.c:730 [inline]
>  __sock_sendmsg+0x223/0x270 net/socket.c:745
>  ____sys_sendmsg+0x525/0x7d0 net/socket.c:2585
>  ___sys_sendmsg net/socket.c:2639 [inline]
>  __sys_sendmsg+0x2b0/0x3a0 net/socket.c:2668
>  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>  do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f24ed27cea9
> RSP: 002b:00007f24ee0080c8 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
> RAX: ffffffffffffffda RBX: 00007f24ed3b3f80 RCX: 00007f24ed27cea9
> RDX: 0000000000000000 RSI: 0000000020000100 RDI: 0000000000000005
> RBP: 00007f24ed2ebff4 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> 
> 
> ---
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We've had number of these reports recently. I think I understand what's
happening but I'm not sure how to fix it. The problem manifests as a
stuck nfsd_mutex:

nfsd_nl_rpc_status_get_start takes the nfsd_mutex, and it's released in
nfsd_nl_rpc_status_get_done. These are the ->start and ->done
operations for the rpc_status_get dumpit routine.

I think syzbot is triggering one of the two "goto errout_skb"
conditions in netlink_dump (not sure which). In those cases we end up
returning from that function without calling ->done, which would lead
to the hung mutex like we see here.

Is this a bug in the netlink code, or is the rpc_status_get dumpit
routine not using ->start and ->done correctly?

Thanks,
-- 
Jeff Layton <jlayton@kernel.org>