** Information type changed from Private Security to Public Security

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  root can lift kernel lockdown

Status in linux package in Ubuntu:
  In Progress
Status in linux source package in Xenial:
Status in linux source package in Bionic:
Status in linux source package in Disco:
Status in linux source package in Eoan:
Status in linux source package in Focal:
  In Progress

Bug description:
  Echoing "x" into /proc/sysrq-trigger disables kernel lockdown, even
  though it shouldn't.

  If I'm not mistaken, kernel lockdown is meant to create a barrier
  between root and the kernel that can only be broken with physical
  access to the system. It is automatically enabled when the system is
  booted with UEFI Secure Boot, which is the case for me.

  This should show the bug:

  # echo "x" > /proc/sysrq-trigger 
  Nov 05 14:58:15 panzersperre kernel: sysrq: SysRq : 
  Nov 05 14:58:15 panzersperre kernel: This sysrq operation is disabled from 
  Nov 05 14:58:15 panzersperre kernel: Disabling Secure Boot restrictions
  Nov 05 14:58:15 panzersperre kernel: Lifting lockdown

  Note that it first says that the operation is disabled and then performs this 
  This should only be possible by physically pressing sysrq+x on an attached 

  I'm doing this on 4.15.0-68-generic on Ubuntu 18.04.3 LTS.
  I have kernel.sysrq set to 1 - this is important to be able to trigger this 
bug. (But I don't think it disqualifies this issue as non-security relevant 
because root can trivially execute `sysctl kernel.sysrq=1`.)

  I first learned about this by reading a blog post
  lifting-the-kernel-lockdown/), so I'm not the first to notice this
  behavior (even though this post doesn't say it's a bug).

  Looking through drivers/tty/sysrq.c, I guess the problem is caused by
  this if condition in __handle_sysrq:

   554   │         /* Ban synthetic events from some sysrq functionality */
   555   │         if ((from == SYSRQ_FROM_PROC || from == 
   556   │             op_p->enable_mask & SYSRQ_DISABLE_USERSPACE)
   557   │             printk("This sysrq operation is disabled from 
   558   │         /*
   559   │          * Should we check for enabled operations 
   560   │          * should not) and is the invoked operation enabled?
   561   │          */
   562   │         if (from == SYSRQ_FROM_KERNEL || 
sysrq_on_mask(op_p->enable_mask)) {
   563   │             pr_cont("%s\n", op_p->action_msg);
   564   │             console_loglevel = orig_log_level;
   565   │             op_p->handler(key);
   566   │         } else {
   567   │             pr_cont("This sysrq operation is disabled.\n");
   568   │         }

  Note that `op_p->enable_mask & SYSRQ_DISABLE_USERSPACE` just causes a
  printk and no change of behavior.

  ProblemType: Bug
  DistroRelease: Ubuntu 18.04
  Package: linux-image-4.15.0-68-generic 4.15.0-68.77
  ProcVersionSignature: Ubuntu 4.15.0-68.77-generic 4.15.18
  Uname: Linux 4.15.0-68-generic x86_64
  ApportVersion: 2.20.9-0ubuntu7.8
  Architecture: amd64
   /dev/snd/controlC1:  niklas     2442 F.... pulseaudio
   /dev/snd/controlC0:  niklas     2442 F.... pulseaudio
  CurrentDesktop: KDE
  Date: Tue Nov  5 14:58:33 2019
  InstallationDate: Installed on 2015-12-11 (1424 days ago)
  InstallationMedia: Kubuntu 14.04.3 LTS "Trusty Tahr" - Beta amd64 (20150805)
  MachineType: LENOVO 20E8S00600
  ProcFB: 0 inteldrmfb
  ProcKernelCmdLine: BOOT_IMAGE=/@/boot/vmlinuz-4.15.0-68-generic 
root=UUID=67485aa6-c665-4c53-bf41-328307d0cbf0 ro rootflags=subvol=@ quiet 
splash kaslr i915.alpha_support=1 vt.handoff=1
   linux-restricted-modules-4.15.0-68-generic N/A
   linux-backports-modules-4.15.0-68-generic  N/A
   linux-firmware                             1.173.11
  SourcePackage: linux
  UpgradeStatus: Upgraded to bionic on 2018-07-05 (487 days ago)
  dmi.bios.date: 09/26/2018
  dmi.bios.vendor: LENOVO
  dmi.bios.version: JHET69WW (1.69 )
  dmi.board.asset.tag: Not Available
  dmi.board.name: Intel powered classmate PC
  dmi.board.vendor: LENOVO
  dmi.board.version: SDK0E50510 WIN
  dmi.chassis.asset.tag: No Asset Information
  dmi.chassis.type: 10
  dmi.chassis.vendor: LENOVO
  dmi.chassis.version: None
  dmi.product.family: ThinkPad 11e
  dmi.product.name: 20E8S00600
  dmi.product.version: ThinkPad 11e
  dmi.sys.vendor: LENOVO

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