** Information type changed from Public to Public Security

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https://bugs.launchpad.net/bugs/1853861

Title:
  [SRU] Unattended-upgrades silently does not apply updates when
  MinimalSteps is disabled and there are autoremovable kernels

Status in unattended-upgrades package in Ubuntu:
  Confirmed

Bug description:
  [Impact]

   * When autoremovable kernel packages are present on the system, there are 
updates to apply and Unattended-Upgrade::MinimalSteps is set to "false", the 
autoremovable kernel packages are not removed and the updates are not applied.
   * The root cause is u-u not cleaning the dirty cache between operations and 
also relying on having a cache with packages marked to be installed when 
applying updates in one shot.
   * The fix is clearing the cache between operations and marking packages 
before installing them in one shot.

  [Test Case]

   * Install kernel-related packages, mark them as automatically installed to 
make them auto-removable ones.
   * Downgrade a few packages to a version lower than what is present in the 
security pocket.
   * Set Unattended-Upgrade::MinimalSteps to "false":
     # echo 'Unattended-Upgrade::MinimalSteps "false";' > 
/etc/apt/apt.conf.d/51unattended-upgrades-oneshot

   * Run u-u:
     # unattended-upgrade --verbose --debug

   * Observe fixed versions removing the kernel packages properly and
  also upgrading packages.

  [Regression Potential]

   * The changes introduce marking packages to install/upgrade and clearing the 
cache more often. The added operations slow down u-u, but clearing the cache 
adds a few 100 milliseconds on typical hardware and marking upgradable packages 
is also in the same range.
   * Functional regressions are unlikely due to those changes since the fixes 
are present in 19.04 and later releases and the extensive autopkgtest also 
covers when upgrades are performed in minimal steps.

  [Other Info]

   * While this bug has a security impact by holding back installation of 
security updates I don't recommend releasing the fix via the security pocket 
because this bug occurs only when the local configuration file of u-u is 
changed and u-u does not hold back upgrades with UCF-managed config file 
conflicts.
    See: https://github.com/mvo5/unattended-upgrades/issues/168

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