This bug was fixed in the package procps - 2:3.3.16-5ubuntu2
---------------
procps (2:3.3.16-5ubuntu2) groovy; urgency=medium
* debian/sysctl.d/10-kernel-hardening.conf:
- Add documentation for DMESG_RESTRICT feature, and allow users to
disable by uncommenting kernel.dmesg_restrict=0. (LP: #1886112)
-- Matthew Ruffell <matthew.ruff...@canonical.com> Thu, 23 Jul 2020
16:59:38 +1200
** Changed in: procps (Ubuntu Groovy)
Status: In Progress => Fix Released
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https://bugs.launchpad.net/bugs/1886112
Title:
Enabling DMESG_RESTRICT in Groovy Onward
Status in linux package in Ubuntu:
Fix Released
Status in procps package in Ubuntu:
Fix Released
Status in util-linux package in Ubuntu:
In Progress
Status in linux source package in Groovy:
Fix Released
Status in procps source package in Groovy:
Fix Released
Status in util-linux source package in Groovy:
Won't Fix
Bug description:
[Impact]
This bug implements the enablement of CONFIG_SECURITY_DMESG_RESTRICT
feature by default for Groovy onward, proposed to ubuntu-devel:
https://lists.ubuntu.com/archives/ubuntu-devel/2020-June/041063.html
The kernel log buffer contains a wealth of sensitive information, such
as detailed call traces and kernel addresses found in register dumps
in kernel oops messages.
Exploit developers and attackers can leverage these information leaks
to get past KASLR, and they can use the kernel log buffer to get
instant feedback on their privilege escalation attacks, as failures
will be shown as further oops messages, which attackers can use to fix
and tune their programs until they work.
Currently, if I create a new, unprivileged user on a Focal system,
they cannot access /var/log/kern.log, /var/log/syslog or see system
events in journalctl. But yet, they are given free reign to the kernel
log buffer.
$ sudo adduser dave
$ su dave
$ groups
dave
$ cat /var/log/kern.log
cat: /var/log/kern.log: Permission denied
$ cat /var/log/syslog
cat: /var/log/syslog: Permission denied
$ journalctl
Hint: You are currently not seeing messages from other users and the system.
Users in groups 'adm', 'systemd-journal' can see all messages.
Pass -q to turn off this notice.
Jun 16 23:44:59 ubuntu systemd[2328]: Reached target Main User Target.
Jun 16 23:44:59 ubuntu systemd[2328]: Startup finished in 69ms.
$ dmesg
[ 0.000000] Linux version 5.4.0-34-generic (buildd at lcy01-amd64-014)
(gcc version 9.3.0 (Ubuntu 9.3.0-10ubuntu2)) #38-Ubuntu SMP Mon May 25
15:46:55
UTC 2020 (Ubuntu 5.4.0-34.38-generic 5.4.41)
[ 0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-5.4.0-34-generic
root=UUID=f9f909c3-782a-43c2-a59d-c789656b4188 ro
...
I propose that we restrict access to dmesg to users in group 'adm'
like so:
1) Add kernel.dmesg_restrict = 1 to
/etc/sysctl.d/10-kernel-hardening.conf
2) Following changes to /bin/dmesg permissions in package 'util-linux'
- Ownership changes to root:adm
- Permissions changed to 0750 (-rwxr-x---)
- Add cap_syslog capability to binary.
For most users, they will use the initial admin account, which is in
the 'adm' group already, and will see no impact to these changes. If a
log scraper type program needs access to dmesg, the user the daemon
runs as can simply be added to the 'adm' group.
[Testcase]
Currently, all users can run /usr/bin/dmesg to view the kernel log
buffer:
$ dmesg
[ 0.000000] Linux version 5.4.0-34-generic (buildd at lcy01-amd64-014)
(gcc version 9.3.0 (Ubuntu 9.3.0-10ubuntu2)) #38-Ubuntu SMP Mon May 25
15:46:55
UTC 2020 (Ubuntu 5.4.0-34.38-generic 5.4.41)
[ 0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-5.4.0-34-generic
root=UUID=f9f909c3-782a-43c2-a59d-c789656b4188 ro
...
When the changes are applied, the default admin user will be able to
view dmesg (since they are in group 'adm'), while new unprivileged
users will not.
Test packages are available in the following ppa:
https://launchpad.net/~mruffell/+archive/ubuntu/lp1886112-test
$ whoami
ubuntu
$ groups
ubuntu adm cdrom sudo dip plugdev
$ dmesg
[ 0.000000] Linux version 5.4.0-34-generic (buildd at lcy01-amd64-014)
(gcc version 9.3.0 (Ubuntu 9.3.0-10ubuntu2)) #38-Ubuntu SMP Mon May 25
15:46:55
UTC 2020 (Ubuntu 5.4.0-34.38-generic 5.4.41)
[ 0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-5.4.0-34-generic
root=UUID=f9f909c3-782a-43c2-a59d-c789656b4188 ro
...
$ sudo adduser dave
$ su dave
$ groups
dave
$ dmesg
-bash: /usr/bin/dmesg: Permission denied
[Regression Potential]
Some users or log scraper type programs may need to view the kernel
log buffer, or have access to dmesg. In this case, the underlying
service user would need to be added to the 'adm' group.
Users have the ability to disable DMESG_RESTRICT by changing
kernel.dmesg_restrict sysctl in /etc/sysctl.d/10-kernel-hardening.conf
from '1' to '0', followed by a reboot.
To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1886112/+subscriptions
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