[Bug 1886112] Re: Enabling DMESG_RESTRICT in Groovy Onward

[Matthew Ruffell]

I emailed Seth Forshee asking about what happens when Groovy's kernel
becomes Focal's HWE kernel, and he mentioned that the kernel team has
processes in place to handle config changes, and that it isn't a
problem.

So we will go with the more secure by default way, and enable
CONFIG_SECURITY_DMESG_RESTRICT in the kernel config.

I rebased the patches for procps and util-linux to the latest versions
in groovy and their debdiffs are attached below. Since things are quiet
on this bug I will write to ubuntu-devel shortly.

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https://bugs.launchpad.net/bugs/1886112

Title:
  Enabling DMESG_RESTRICT in Groovy Onward

Status in linux package in Ubuntu:
  Fix Committed
Status in procps package in Ubuntu:
  In Progress
Status in util-linux package in Ubuntu:
  In Progress
Status in linux source package in Groovy:
  Fix Committed
Status in procps source package in Groovy:
  In Progress
Status in util-linux source package in Groovy:
  In Progress

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.

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