[SRU][B][PATCH 0/1] Ubuntu 18.04 kernel 4.15.0-194 crashes on IPL (LP: 1994601)

[frank.heimes at canonical.com]

BugLink: https://bugs.launchpad.net/bugs/1994601

SRU Justification:

[ Impact ] 

 * Ubuntu 18.04 / bionic installations with latest kernel 4.15.0-194
   are no longer able to IPL (boot) on IBM z14 or newer hardware.

 * This issue got introduced by upstream commit e4f74400308c
   "s390/archrandom: simplify back to earlier design and initialize earlier"
   that was SRUed to 18.04/bionic based on LP#1989625,
   which made changes in the s390s IPL/boot area of kernel/arch/random.

 * The reason seems to be that the bad patch moves the decision about
   if arch randomness is available to setup.c function setup_randomness().
   This code uses a static_key s390_arch_random_available.
   But in the Canonical kernel the initialization function
   for the jump labels (where the static keys are based on)
   jump_label_init() is called in generic start_kernel()
   wheres in the upstream kernel the init function is
   called early in setup_arch().

 * Reverting this commit from bionic master-next makes bionic systems
   again bootable.
   (https://launchpad.net/~fheimes/+archive/ubuntu/test/)

[ Test Plan ]

 * An IBM z14 or LinuxONE II or newer system is needed.

 * Now install latest bionic on that system - doesn't if it's on LPAR, 
   z/VM or KVM.

 * After the installation (an the trigger of the post install reboot),
   the system will not come up.

 * To test a patched kernel with e4f74400308c can be tested in the
   following way:

 * Install 18.04 GA and prevent it from doing any kernel updates.

 * Means, install in 'island' mode
   or select in d-i 'Advanced Installation'
   and explicitly choose '4.15.0-50 generic' to install.

 * That allows the system to come up and to update the kernel to
   a modified one.

 * Then reboot and verify if the system comes up properly.

[ Where problems could occur ]

 * Problems could occur due to the fact that the commit
   was not cleanly reversible because of minor context changes.

 * Adjustments that were needed might break other things if not
   done carefully.

 * Further commits (applied after e4f74400308c) may still rely
   on the bad e4f74400308c commit - or even further patches
   (from upstream stable).

 * In worst case IPL / boot might get broken,
   even on hardware older than z14.

 * If the revert works fine can be easily tested and was tested based on
   https://launchpad.net/~fheimes/+archive/ubuntu/test/
   and the above test plan.

[ Other Info ]
 
 * Ubuntu 20.04 (focal, using legacy image with virt-install)
   was tested as well, but is not affected by this issue.

Frank Heimes (1):
  Revert "s390/archrandom: simplify back to earlier design and
    initialize earlier"

 arch/s390/crypto/arch_random.c     | 111 ++++++++++++++++++++++++++++-
 arch/s390/include/asm/archrandom.h |  27 +++----
 arch/s390/kernel/setup.c           |   5 --
 3 files changed, 121 insertions(+), 22 deletions(-)

-- 
2.25.1