Public bug reported: [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) CONFIG_SECURITY_DMESG_RESTRICT=y in the kernel. 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. 3) Add a commented out '# kernel.dmesg_restrict = 0' to /etc/sysctl.d/10-kernel-hardening.conf 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. $ 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 uncommenting the sysctl in /etc/sysctl.d/10-kernel-hardening.conf. ** Affects: linux (Ubuntu) Importance: Undecided Status: Fix Committed ** Affects: procps (Ubuntu) Importance: Undecided Assignee: Matthew Ruffell (mruffell) Status: In Progress ** Affects: util-linux (Ubuntu) Importance: Undecided Assignee: Matthew Ruffell (mruffell) Status: In Progress ** Affects: linux (Ubuntu Groovy) Importance: Undecided Status: Fix Committed ** Affects: procps (Ubuntu Groovy) Importance: Undecided Assignee: Matthew Ruffell (mruffell) Status: In Progress ** Affects: util-linux (Ubuntu Groovy) Importance: Undecided Assignee: Matthew Ruffell (mruffell) Status: In Progress ** Also affects: linux (Ubuntu Groovy) Importance: Undecided Status: New ** Changed in: linux (Ubuntu Groovy) Status: New => Fix Committed ** Also affects: procps (Ubuntu) Importance: Undecided Status: New ** Changed in: procps (Ubuntu Groovy) Status: New => In Progress ** Changed in: procps (Ubuntu Groovy) Assignee: (unassigned) => Matthew Ruffell (mruffell) ** Also affects: util-linux (Ubuntu) Importance: Undecided Status: New ** Changed in: util-linux (Ubuntu Groovy) Status: New => In Progress ** Changed in: util-linux (Ubuntu Groovy) Assignee: (unassigned) => Matthew Ruffell (mruffell) -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1886112 Title: Enabling DMESG_RESTRICT in Groovy Onward To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1886112/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
