ProtectClock setting

The property ProtectClock is a systemd unit setting used for sandboxing. It is available since systemd 245.

Purpose: limit access to clock information

Why and when to use ProtectClock

The setting ProtectClock reduces access the clock information.

Configuration options of ProtectClock

When this setting is not configured or disabled (e.g. no) access to clock information is not restricted. When set (e.g. true), the service will no longer have access to clock information. A system function call like clock_adjtime(2) will fail.

Generic advice

For most services ProtectClock=yes can be used, unless it really depends on retrieving clock information.

Testing

To see if a program works with this property, consider using the systemd-run command.

systemd-run --pty --property=ProtectClock=yes /path/to/PROGRAM

Another option is to use strace and monitor for syscalls related to the clock.

#include <stdio.h>
#include <sys/timex.h>

int main(int argc, char *argv[]) {
    struct ntptimeval timestate = {0};
    // Try to get the time using the syscall clock_adjtime
    ntp_gettime(&timestate);
    // Returns Success if it worked
    perror("clock_adjtime");
}

Compile the program:

gcc protectclock.c

Then run it:

./a.out

To look at the syscalls being used:

strace ./a.out

Example when the clock is accessible:

clock_adjtime(CLOCK_REALTIME, {modes=0, offset=479672, freq=634399, maxerror=175500, esterror=0, status=STA_PLL|STA_NANO, constant=7, precision=1, tolerance=32768000, time={tv_sec=1731537097, tv_usec=153901179}, tick=10000, ppsfreq=0, jitter=0, shift=0, stabil=0, jitcnt=0, calcnt=0, errcnt=0, stbcnt=0, tai=0}) = 0 (TIME_OK)

Values

This setting expects a boolean (yes or no).

• no: normal access allowed to clock information - default
• yes: processes can't retrieve or adjust clock information

Example to show the current value of ProtectClock for the dmesg service:

Related hardening profiles

The systemd unit setting ProtectClock is used in the following hardening profiles.

• Nginx hardening profile