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@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:
$ 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.
[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 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) /casper/ vmlinuz file=/cdrom/ preseed/ ubuntu. seed maybe-ubiquity quiet splash ---
$ 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@
[ 0.000000] kernel: Command line: BOOT_IMAGE=
If you install the package in the following ppa:
$ 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.