** Changed in: linux (Ubuntu Bionic)
Status: Triaged => Fix Committed
** Changed in: linux (Ubuntu Disco)
Status: Triaged => Fix Committed
** Changed in: linux (Ubuntu Eoan)
Status: Triaged => Fix Committed
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https://bugs.launchpad.net/bugs/1851380
Title:
root can lift kernel lockdown
Status in linux package in Ubuntu:
In Progress
Status in linux source package in Xenial:
Invalid
Status in linux source package in Bionic:
Fix Committed
Status in linux source package in Disco:
Fix Committed
Status in linux source package in Eoan:
Fix Committed
Status in linux source package in Focal:
In Progress
Bug description:
SRU Justification
Impact: The kernel lockdown support adds a sysrq to allow a physically
present user to disable lockdown from the keyboard. A bug in the
implementation makes it possible to also lift lockdown by writing to
/proc/sysrq-trigger.
Fix: Correct the logic to disallow disabling lockdown via /proc/sysrq-
trigger.
Test Case: Write "x" to /proc/sysrq-trigger. When working properly
there should be no messages in dmesg about lifting lockdown, and
lockdown restrictions (e.g. loading unsigned modules) should remain in
effect.
Regression Potential: Anyone using /proc/sysrq-trigger to disable
lockdown will no longer be able to do so. Implementation bugs could
prevent use of the sysrq from the keyboard from disabling lockdown,
but this has been confrimed to still work with the fix in place.
---
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
userspace.
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
operation.
This should only be possible by physically pressing sysrq+x on an attached
keyboard.
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
(https://gehrcke.de/2019/09/running-an-ebpf-program-may-require-
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 ==
SYSRQ_FROM_SYNTHETIC) &&
556 │ op_p->enable_mask & SYSRQ_DISABLE_USERSPACE)
557 │ printk("This sysrq operation is disabled from
userspace.\n");
558 │ /*
559 │ * Should we check for enabled operations
(/proc/sysrq-trigger
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
AudioDevicesInUse:
USER PID ACCESS COMMAND
/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
RelatedPackageVersions:
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.modalias:
dmi:bvnLENOVO:bvrJHET69WW(1.69):bd09/26/2018:svnLENOVO:pn20E8S00600:pvrThinkPad11e:rvnLENOVO:rnIntelpoweredclassmatePC:rvrSDK0E50510WIN:cvnLENOVO:ct10:cvrNone:
dmi.product.family: ThinkPad 11e
dmi.product.name: 20E8S00600
dmi.product.version: ThinkPad 11e
dmi.sys.vendor: LENOVO
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