[Bug 1912122] Re: /var/log/dmesg is 0644, should be 0640 to match new DMESG_RESTRICT restrictions

[Launchpad Bug Tracker]

This bug was fixed in the package rsyslog - 8.2010.0-1ubuntu2

---------------
rsyslog (8.2010.0-1ubuntu2) hirsute; urgency=medium

  * debian/dmesg.service: Change /var/log/dmesg from 0644 to 0640
    to adhere to new DMESG_RESTRICT restrictions. (LP: #1912122)

 -- Matthew Ruffell <matthew.ruffell at canonical.com>  Mon, 18 Jan 2021
13:34:48 +1300

** Changed in: rsyslog (Ubuntu Hirsute)
       Status: In Progress => Fix Released

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Title:
  /var/log/dmesg is 0644, should be 0640 to match new DMESG_RESTRICT
  restrictions

Status in rsyslog package in Ubuntu:
  Fix Released
Status in rsyslog source package in Groovy:
  Won't Fix
Status in rsyslog source package in Hirsute:
  Fix Released

Bug description:
  [Impact]

  In bug 1886112, CONFIG_SECURITY_DMESG_RESTRICT was enabled on the
  Ubuntu kernel starting with Groovy and onward, in an effort to
  restrict access to the kernel log buffer from unprivileged users.

  It seems we have overlooked /var/log/dmesg, as it is still mode 0644,
  while /var/log/kern.log, /var/log/syslog are all 0640:

  $ ll /var/log
  -rw-r--r--   1 root              adm                 81768 Jan 18 09:09 dmesg
  -rw-r-----   1 syslog            adm                 24538 Jan 18 13:05 kern.log
  -rw-r-----   1 syslog            adm                213911 Jan 18 13:22 syslog

  Change /var/log/dmesg to 0640 to close the information leak.

  [Testcase]

  $ 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
  $ cat /var/log/dmesg
  [    0.000000] kernel: Linux version 5.8.0-36-generic (buildd at lgw01-amd64-011) (gcc (Ubuntu 10.2.1-2ubuntu3) 10.2.1 20201221, GNU ld (GNU Binutils for Ubuntu) 2.35.50.20210106) #40+21.04.1-Ubuntu SMP Thu Jan 7 11:35:09 UTC 2021 (Ubuntu 5.8.0-36.40+21.04.1-generic 5.8.18)
  [    0.000000] kernel: Command line: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed maybe-ubiquity quiet splash ---

  If you install the package in the following ppa:

  https://launchpad.net/~mruffell/+archive/ubuntu/lp1912122-test

  $ sudo systemctl daemon-reload
  $ sudo systemctl start dmesg.service

  $ 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
  $ cat /var/log/dmesg
  cat: /var/log/dmesg: Permission denied

  [Where problems could occur]

  Some users or log scraper programs might need to view the kernel log
  buffers, and in this case, their underlying service accounts should be
  added to the 'adm' group.

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