if-up might fail when triggered before temporary directory available

It seems that if-up.d is activated before any temporary directory is available. On affected setup that caused a bash here-tag to fail, thus security settings were not applied to the interface leaving it open to attacks.

Documentation seems to be unclear, if any script should expect, that temporary directory would work. At least here it was quite unexpected that data processing in scripts without explicit request for disk operation failed due to that.

In my opinion, a CAVEAT in the interfaces (5) documentation might be sufficient in most cases.

A workaround for the affected setup was to create a separate temporary directory on /var/run (which is a tmpfs instance) and export it using TEMPDIR.

To reproduce:

cat <<OUTEREOF > /etc/network/if-up.d/aaa-early
#!/bin/bash

echo "mounts"
cat /proc/mounts
echo "touch"
touch /tmp/xxx
cat <<EOF
Here document test
EOF
echo "mounts again"
cat /proc/mounts
OUTEREOF
chmod 0755 /etc/network/if-up.d/aaa-early

touch /forcefsck
reboot

# cat /var/log/upstart/network-interface-eth0.log

mounts
...
tmpfs /run tmpfs rw,nosuid,noexec,relatime,size=24784k,mode=755 0 0
/dev/disk/by-uuid/19c0ddc6-c305-4a8a-86f2-fb470ae5b39c / ext4 ro,relatime,data=ordered 0 0
...
touch
touch: cannot touch '/tmp/xxx': Read-only file system
/etc/network/if-up.d/aaa-early: line 7: cannot create temp file for here-document: Read-only file system
mounts again
...
/dev/disk/by-uuid/19c0ddc6-c305-4a8a-86f2-fb470ae5b39c / ext4 ro,relatime,data=ordered 0 0
...

# lsb_release -rd
Description: Ubuntu 14.04.2 LTS
Release: 14.04

# apt-cache policy ifupdown
ifupdown:
  Installed: 0.7.47.2ubuntu4.1
  Candidate: 0.7.47.2ubuntu4.1
  Version table:
 *** 0.7.47.2ubuntu4.1 0
        500 http://ubuntu-proxy-ehealth.d03.arc.local/ubuntu/ trusty-updates/main i386 Packages
        100 /var/lib/dpkg/status
     0.7.47.2ubuntu4 0
        500 http://ubuntu-proxy-ehealth.d03.arc.local/ubuntu/ trusty/main i386 Packages