[Bug 271474] Possible to "lose" a tap(4) interface in a jail

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=271474

Stephen Fox <stephen.j.fox.jr@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |stephen.j.fox.jr@gmail.com

--- Comment #5 from Stephen Fox <stephen.j.fox.jr@gmail.com> ---
I ran into this issue as well under in a similar scenario (trying to run
a bhyve VM from a jail). While trying to understand this issue, I have been
doing a lot of "ls /dev" and "ls /dev/tapN"... and I realized that
"ls -l /dev/tapN" creates an entry in "/dev"...

```
root@x:/etc/jail.conf.d # ifconfig tap4141
ifconfig: interface tap4141 does not exist
root@x:/etc/jail.conf.d # ls -l /dev | grep tap4141
root@x:/etc/jail.conf.d # ls -l /dev/tap4141
crw-------  1 uucp dialer 0x70 Apr 20 12:53 /dev/tap4141
root@x:/etc/jail.conf.d # ifconfig tap4141
tap4141: flags=8802<BROADCAST,SIMPLEX,MULTICAST> metric 0 mtu 1500
        options=80000<LINKSTATE>
        ether 58:9c:fc:10:97:4a
        groups: tap
        media: Ethernet 1000baseT <full-duplex>
        status: no carrier
        nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
```

Needless to say - this is not the behavior I expected.

The steps to reproduce the issue described by Joshua appear to be:

```
service jail start lose-tap-example
jexec lose-tap-example ls -l /dev/tap41
service jail stop lose-tap-example
```

Here is the jail configuration file ("/etc/jail.conf.d/lose-tap-example.conf"):

```
lose-tap-example {
  path = "/zroot/jails/${name}";
  mount.devfs;
  vnet;

  exec.start += "/bin/sh /etc/rc";
  exec.stop += "/bin/sh /etc/rc.shutdown";
}
```

The host system and jail versions:

```
# freebsd-version -uk
14.0-RELEASE-p5
14.0-RELEASE-p5
# jexec lose-tap-example freebsd-version -u
14.0-RELEASE-p6
```

And here is what it looks like from a shell for more context:

```
root@x:~ # ls -l /dev | grep tap
root@x:~ # service jail start lose-tap-example
Starting jails: lose-tap-example.
root@x:~ # jexec lose-tap-example ifconfig
lo0: flags=1008049<UP,LOOPBACK,RUNNING,MULTICAST,LOWER_UP> metric 0 mtu 16384
        options=680003<RXCSUM,TXCSUM,LINKSTATE,RXCSUM_IPV6,TXCSUM_IPV6>
        inet 127.0.0.1 netmask 0xff000000
        inet6 ::1 prefixlen 128
        inet6 fe80::1%lo0 prefixlen 64 scopeid 0x3
        groups: lo
        nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>
root@x:~ # jexec lose-tap-example ls -l /dev/tap41
ls: /dev/tap41: No such file or directory
root@x:~ # jexec lose-tap-example ifconfig
lo0: flags=1008049<UP,LOOPBACK,RUNNING,MULTICAST,LOWER_UP> metric 0 mtu 16384
        options=680003<RXCSUM,TXCSUM,LINKSTATE,RXCSUM_IPV6,TXCSUM_IPV6>
        inet 127.0.0.1 netmask 0xff000000
        inet6 ::1 prefixlen 128
        inet6 fe80::1%lo0 prefixlen 64 scopeid 0x3
        groups: lo
        nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>
tap41: flags=8802<BROADCAST,SIMPLEX,MULTICAST> metric 0 mtu 1500
        options=80000<LINKSTATE>
        ether 52:72:e6:7e:7c:ab
        groups: tap
        media: Ethernet 1000baseT <full-duplex>
        status: no carrier
        nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
root@x:~ # service jail stop lose-tap-example
Stopping jails: lose-tap-example.
root@x:~ # ifconfig
vtnet0: flags=1008843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST,LOWER_UP> metric 0
mtu 1500
        options=80028<VLAN_MTU,JUMBO_MTU,LINKSTATE>
        ether (...)
        inet (...)  netmask 0xffffff00 broadcast (...)
        media: Ethernet autoselect (10Gbase-T <full-duplex>)
        status: active
        nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
lo0: flags=1008049<UP,LOOPBACK,RUNNING,MULTICAST,LOWER_UP> metric 0 mtu 16384
        options=680003<RXCSUM,TXCSUM,LINKSTATE,RXCSUM_IPV6,TXCSUM_IPV6>
        inet 127.0.0.1 netmask 0xff000000
        inet6 ::1 prefixlen 128
        inet6 fe80::1%lo0 prefixlen 64 scopeid 0x2
        groups: lo
        nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>
root@x:~ # ifconfig tap41 create
ifconfig: interface tap41 already exists
root@x:~ # rm /dev/tap41
root@x:~ # ifconfig tap41 create
ifconfig: interface tap41 already exists
```

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