[Bug 2017401] Re: Unexpected / unwanted unattended-upgrades behaviour after kernel upgrade when Livepatch enabled

[mev]

Hi Robie -

I think that's a separate issue!

As far as I know, there's no way of measuring the sufficiency of
Livepatch. A new kernel will usually have fixes for other bugs and less-
impactful vulnerabilities on top of what Livepatch is taking care of,
and it's impossible to know whether those fixes will be of significance
to the user.

If a user wants to rely on Livepatch entirely (or 'until further
notice'), unattended-upgrades can be set to blacklist automatic kernel
upgrades.

I certainly don't see any value, in any circumstance, of installing a
new kernel automatically but not alerting the user that it's not
running, which is one of the things this bug is about, as well as
failing to reboot automatically when unattended-upgrades has been set to
do so.

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https://bugs.launchpad.net/bugs/2017401

Title:
  Unexpected / unwanted unattended-upgrades behaviour after kernel
  upgrade when Livepatch enabled

Status in unattended-upgrades package in Ubuntu:
  Confirmed

Bug description:
  Following the resolution for bug #1747499, after a kernel upgrade when
  Livepatch is enabled, the current behaviour in unattended-upgrades
  (2.3ubuntu0.2 and later) is not to touch /var/run/reboot-required so
  as not to confuse users with two separate messages calling for a
  restart in motd. This functionality is implemented in the script at
  /etc/kernel/postinst.d/unattended-upgrades.

  While this works as intended in terms of suppressing an extra message
  in motd, it defeats the ability of unattended-upgrades to restart
  automatically with the new kernel, which is reliant on
  /var/run/reboot-required being present.

  This is unexpected / unwanted behaviour in scenarios where a)
  Livepatch is being used to provide fast-response kernel patching; and
  b) Unattended-Upgrade::Automatic-Reboot is set to true, to enable
  automatic reboots during a regular maintenance window. In this case,
  without administrative intervention, the system could never boot into
  the new kernel even though it would be expected to, leaving Livepatch
  to do all the heavy lifting indefinitely, and unnecessarily.

  I believe this counts as a regression caused by the resolution to bug
  #1747499. It also has the potential to be a security threat if
  Livepatch doesn't work comprehensively for a particular kernel flaw,
  and an administrator is reliant on expected behaviour according to
  unattended-upgrades settings.

  Potential options for a fix that come to mind:
  1. Revert to original behaviour in /etc/kernel/postinst.d/unattended-upgrades, and change the ***System restart required*** message to be less alarming or confusing when the cause is a kernel upgrade that's being patched by Livepatch.
  2. Add an extra configuration setting (eg Unattended-Upgrade::Automatic-Reboot-After-Livepatch) that triggers a reboot when it's 'recommended' by Livepatch, not reliant on the presence of /var/run/reboot-required.
  3. Add support in /etc/kernel/postinst.d/unattended-upgrades for an extra file somewhere. When present, /var/run/reboot-required is always touched, even if Livepatch is enabled.

  (This is my first time reporting a bug in this system, and I apologise
  if I haven't followed the usual descriptive format.)

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