Possibly related: when the problem is triggered, I notice that my guest
instances start with no /etc/resolv.conf and no inet address.

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https://bugs.launchpad.net/bugs/1642767

Title:
  starting any container with umask 007 breaks lxc-stop and prevents
  host system shutdown

Status in lxc package in Ubuntu:
  New

Bug description:
  If I run lxc-start with umask 007 (or any other value that masks the
  world-execute bit), my host system enters a state with the following
  problems:

  * lxc-stop hangs forever instead of stopping any container, even one that 
wasn't started with umask 007.
  * lxc-stop --kill --nolock hangs in the same way.
  * Attempts to reboot or shut down my host system fail, requiring a hard reset 
to recover.

  When lxc-stop hangs, messages like these appear in syslog every couple
  of minutes:

  Nov 17 01:22:11 hostbox kernel: [ 3360.091624] INFO: task systemd:12179 
blocked for more than 120 seconds.
  Nov 17 01:22:11 hostbox kernel: [ 3360.091629]       Tainted: P           OE  
 4.4.0-47-generic #68-Ubuntu
  Nov 17 01:22:11 hostbox kernel: [ 3360.091631] "echo 0 > 
/proc/sys/kernel/hung_task_timeout_secs" disables this message.
  Nov 17 01:22:11 hostbox kernel: [ 3360.091633] systemd         D 
ffff8800c6febb58     0 12179  12168 0x00000104
  Nov 17 01:22:11 hostbox kernel: [ 3360.091638]  ffff8800c6febb58 
ffff8800d318d280 ffff88040c649b80 ffff8800d318d280
  Nov 17 01:22:11 hostbox kernel: [ 3360.091641]  ffff8800c6fec000 
ffff8800345bc088 ffff8800345bc070 ffffffff00000000
  Nov 17 01:22:11 hostbox kernel: [ 3360.091644]  fffffffe00000001 
ffff8800c6febb70 ffffffff81830f15 ffff8800d318d280
  Nov 17 01:22:11 hostbox kernel: [ 3360.091647] Call Trace:
  Nov 17 01:22:11 hostbox kernel: [ 3360.091653]  [<ffffffff81830f15>] 
schedule+0x35/0x80
  Nov 17 01:22:11 hostbox kernel: [ 3360.091657]  [<ffffffff81833b62>] 
rwsem_down_write_failed+0x202/0x350
  Nov 17 01:22:11 hostbox kernel: [ 3360.091662]  [<ffffffff812899a0>] ? 
kernfs_sop_show_options+0x40/0x40
  Nov 17 01:22:11 hostbox kernel: [ 3360.091666]  [<ffffffff81403fa3>] 
call_rwsem_down_write_failed+0x13/0x20
  Nov 17 01:22:11 hostbox kernel: [ 3360.091669]  [<ffffffff8183339d>] ? 
down_write+0x2d/0x40
  Nov 17 01:22:11 hostbox kernel: [ 3360.091672]  [<ffffffff812104a0>] 
grab_super+0x30/0xa0
  Nov 17 01:22:11 hostbox kernel: [ 3360.091674]  [<ffffffff81210a32>] 
sget_userns+0x152/0x450
  Nov 17 01:22:11 hostbox kernel: [ 3360.091677]  [<ffffffff81289a20>] ? 
kernfs_sop_show_path+0x50/0x50
  Nov 17 01:22:11 hostbox kernel: [ 3360.091680]  [<ffffffff81289c8e>] 
kernfs_mount_ns+0x7e/0x230
  Nov 17 01:22:11 hostbox kernel: [ 3360.091685]  [<ffffffff811187ab>] 
cgroup_mount+0x2eb/0x7f0
  Nov 17 01:22:11 hostbox kernel: [ 3360.091687]  [<ffffffff81211af8>] 
mount_fs+0x38/0x160
  Nov 17 01:22:11 hostbox kernel: [ 3360.091691]  [<ffffffff8122db57>] 
vfs_kern_mount+0x67/0x110
  Nov 17 01:22:11 hostbox kernel: [ 3360.091694]  [<ffffffff81230329>] 
do_mount+0x269/0xde0
  Nov 17 01:22:11 hostbox kernel: [ 3360.091698]  [<ffffffff812311cf>] 
SyS_mount+0x9f/0x100
  Nov 17 01:22:11 hostbox kernel: [ 3360.091701] [<ffffffff81834ff2>] 
entry_SYSCALL_64_fastpath+0x16/0x71

  When system shutdown hangs, similar messages appear on the console
  every couple of minutes.

  I'm running lxc 2.0.5-0ubuntu1~ubuntu16.04.2 on xubuntu 16.04.1 LTS
  amd64.

  My containers are all unprivileged.

  I can reproduce this at will with either an old-ish container or a
  fresh new one.

  My umask at container creation time does not seem to matter.  As far
  as I have seen, my umask only matters the first time I start a
  container in my login session.

  I can work around the bug by manually setting my umask to something
  more permissive before I start my first container of the day, and then
  setting it back again, but that's rather a hassle.  (Even worse, it's
  very easy to forget this workaround and be left with containers that
  can't be stopped and a host system that won't shut down cleanly.)

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