Iwant to restrict both normal users and root user inside LXD container “hypervisor” from viewing and using dmesg command output. How do I prevent non-root (unprivileged) Linux users from viewing the Linux kernel’s log buffer?
One can use dmesg command see or control the kernel ring buffer. To display all messages from the kernel ring buffer just type the dmesg command: $ dmesg OR $ sudo dmesg Sample outputs:
Fig.01: dmesg command displaying the Linux kernel ring buffer
It is possible to see more human readable output with the following command: $ sudo dmesg -H --color
How do I restrict unprivileged access to kernel syslog?
Run the following sysctl command as root user: $ sudo sysctl -w kernel.dmesg_restrict=1
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kernel.dmesg_restrict=1
To view value, run: $ sysctl kernel.dmesg_restrict From the Linux kernel docs:
This toggle indicates whether unprivileged users are prevented from using dmesg(8) to view messages from the kernel’s log buffer. When dmesg_restrict is set to (0) there are no restrictions. When dmesg_restrict is set set to (1), users must have CAP_SYSLOG to use dmesg(8). The kernel config option CONFIG_SECURITY_DMESG_RESTRICT sets the default value of dmesg_restrict.
The kernel syslog contains debugging information that is often useful during exploitation of other vulnerabilities, such as kernel heap addresses. Rather than futilely attempt to sanitize hundreds (or thousands) of printk statements and simultaneously cripple useful debugging functionality, it is far simpler to create an option that prevents unprivileged users from reading the syslog.
This patch, loosely based on grsecurity’s GRKERNSEC_DMESG, creates the dmesg_restrict sysctl. When set to “0”, the default, no restrictions are enforced. When set to “1”, only users with CAP_SYS_ADMIN can read the kernel syslog via dmesg(8) or other mechanisms.
How do I clear the ring buffer?
Type the following command: $ sudo dmesg --clear You can also clear the ring buffer after first printing its contents on screen: $ sudo dmesg --read-clear
How can I disable the printing of messages to the console?
For security reason or to avoid too much message on the console, run: $ sudo dmesg --console-off To turn it on again pass the --console-on option: $ sudo dmesg --console-on
How do I show kernel messages only?
$ sudo dmesg --kernel To just display userspace messages, run: $ sudo dmesg --userspace I suggest you read dmesg man page or type the following command: $ dmesg -h
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Usage:<br/>dmesg<span style="box-sizing: inherit; color: #000066; font-weight: bold;"><span style="box-sizing: inherit;">[</span>options<span style="box-sizing: inherit;">]</span></span><br/> <br/>Display orcontrol the kernel ring buffer.<br/> <br/>Options:<br/>-C,--clear clear the kernel ring buffer<br/>-c,--read-clear read andclear all messages<br/>-D,--console-off disable printing messages toconsole<br/>-E,--console-on enable printing messages toconsole<br/>-F,--file<file>usethe file instead of the kernel log buffer<br/>-f,--facility<list>restrict output todefined facilities<br/>-H,--human human readable output<br/>-k,--kernel display kernel messages<br/>-L,--color<span style="box-sizing: inherit; color: #000066; font-weight: bold;"><span style="box-sizing: inherit;">[</span><span style="box-sizing: inherit;">=</span><when><span style="box-sizing: inherit;">]</span></span>colorize messages<span style="box-sizing: inherit;">(</span>auto,always ornever<span style="box-sizing: inherit;">)</span><br/>colors are enabled by default<br/>-l,--level<list>restrict output todefined levels<br/>-n,--console-level<level>set level of messages printed toconsole<br/>-P,--nopager donotpipe output intoapager<br/>-r,--raw print the raw message buffer<br/>-S,--syslog force tousesyslog<span style="box-sizing: inherit;">(</span><span style="box-sizing: inherit;">2</span><span style="box-sizing: inherit;">)</span>rather than/dev/kmsg<br/>-s,--buffer-size<size>buffer size toquery the kernel ring buffer<br/>-u,--userspace display userspace messages<br/>-w,--follow wait fornewmessages<br/>-x,--decode decode facility andlevel toreadable string<br/>-d,--show-delta show time delta between printed messages<br/>-e,--reltime show local time andtime delta inreadable format<br/>-T,--ctime show human readable timestamp<span style="box-sizing: inherit;">(</span>may be inaccurate!<span style="box-sizing: inherit;">)</span><br/>-t,--notime don'tprint messages timestamp<br/>--time-format<format>show time stamp using format:<br/><span style="box-sizing: inherit; color: #000066; font-weight: bold;"><span style="box-sizing: inherit;">[</span>delta|reltime|ctime|notime|iso<span style="box-sizing: inherit;">]</span></span><br/>Suspending/resume will make ctime andiso timestamps inaccurate.<br/> <br/>-h,--help display thishelp andexit<br/>-V,--version output version information andexit<br/> <br/>Supported log facilities:<br/>kern-kernel messages<br/>user-random user-level messages<br/>mail-mail system<br/>daemon-system daemons<br/>auth-security/authorization messages<br/>syslog-messages generated internally by syslogd<br/>lpr-line printer subsystem<br/>news-network news subsystem<br/> <br/>Supported log levels<span style="box-sizing: inherit;">(</span>priorities<span style="box-sizing: inherit;">)</span>:<br/>emerg-system isunusable<br/>alert-action must be taken immediately<br/>crit-critical conditions<br/>err-error conditions<br/>warn-warning conditions<br/>notice-normal but significant condition<br/>info-informational<br/>debug-debug-level messages<br/> <br/> <br/>Formore details see dmesg<span style="box-sizing: inherit;">(</span><span style="box-sizing: inherit;">1</span><span style="box-sizing: inherit;">)</span>.
