Skip to content

RFE More detailed auditing of changes to system clock

Jump to bottom Edit New page
Ondrej Mosnáček edited this page Mar 4, 2019 · 1 revision

Feature Description

In order to ensure reliability of audit (and other) timestamps in the system, we need to provide a mechanism to log adjustments of system clock. This is also required by the Common Criteria security functional requirement (SFR) FPT_STM.1.

While it is possible to achieve this by simply logging all relevant syscalls, this approach has some disadvantages:

  • It is usually not possible to find out from the syscall records the amount by which the time was shifted (which appears to be required by the SFR as well, at least in some interpretations, see NIAP's Protection Profile for Certification Authorities, section 5.1, table 4).
  • Syscalls like adjtimex(2) or clock_adjtime(2) can be used also for read-only operations, which might flood the audit log with false positives.

Thus, a more focused solution is needed that would resolve at least the first issue. The ability to deal with the second issue will most likely require a separate RFE to allow more advanced audit message filtering.

Feature Design

The following syscalls can (at the time of writing) be used to modify the system time:

  • settimeofday(2)
  • stime(2)
  • adjtimex(2)
  • clock_settime(2)
  • clock_adjtime(2)

To cover the direct system clock adjustments done via these syscalls, we need to add audit hooks to two functions – do_adjtimex() and do_settimeofday64(). In the proposed solution, we log an AUDIT_TIME_INJOFFSET record with the offset (in the form of seconds + nanoseconds) by which the time was shifted. The old and new can be derived from the record timestamp and this offset.

To cover also changes that can indirectly affect the system clock, we need to log also some adjustments of internal NTP variables (done via adjtimex(2) or clock_adjtime(2)), which can be used for example to insert leap seconds. In the absence of logging this information, a rogue actor might abuse this interface to shift the clock without being detected.

For these changes we introduce another record type AUDIT_TIME_ADJNTPVAL, which logs the type of the variable that was modified and its old and new value.

In order to avoid flooding the logs with no-op records, modifications that do not change the values (offset is zero or new value equals the old value) should not generate an audit record. The records should also be generated only when the changes are triggered by a syscall and should be associated with the corresponding syscall record.

Record format summary:

AUDIT_TIME_INJOFFSET

  • sec – the 'seconds' part of the offset
  • nsec – the 'nanoseconds' part of the offset

AUDIT_TIME_ADJNTPVAL

  • op – which value was adjusted:
    • offset – corresponding to the time_offset NTP variable
    • freq – corresponding to the time_freq NTP variable
    • status – corresponding to the time_status NTP variable
    • adjust – corresponding to the time_adjust NTP variable
    • tick – corresponding to the tick_usec NTP variable
    • tai – corresponding to the timekeeping's TAI offset
  • old – the old value
  • new – the new value

Development Tasks

  • Write this page.
  • Implement the new record types, audit hooks that generate the new records, and insert calls to the right places in the kernel time management code.
  • Add an automated test to audit-testsuite that will test the new functionality.
  • Implement basic userspace support (to recognize the new record numbers).

Functional Testing and Verification

  • Calling the affected syscalls with parameters indicating a modifying operation must generate corresponding audit records.
  • Calling the affected syscalls with parameters indicating a read-only/no-change operation must not generate corresponding audit records.
  • The field values in the records must be correct.
  • Unsuccessfull syscalls (i.e. requested change was not acutally performed) must not generate the records.

Example Usage

See "Example Audit Records" below.

Example Audit Records

For example, running the following commands:

auditctl -D
auditctl -a exit,always -F arch=b64 -S adjtimex
chronyd -q

produces audit records like this:

type=TIME_ADJNTPVAL msg=audit(1530616044.507:5): op=adjust old=0 new=0
type=SYSCALL msg=audit(1530616044.507:5): arch=c000003e syscall=159 success=yes exit=5 a0=7fff57e78c00 a1=0 a2=4 a3=7f754ae28c0a items=0 ppid=626 pid=629 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=(none) ses=1 comm="chronyd" exe="/usr/sbin/chronyd" subj=system_u:system_r:kernel_t:s0 key=(null)
type=PROCTITLE msg=audit(1530616044.507:5): proctitle=6368726F6E7964002D71
type=SYSCALL msg=audit(1530616044.507:6): arch=c000003e syscall=159 success=yes exit=5 a0=7fff57e78c00 a1=1 a2=1 a3=7f754ae28c0a items=0 ppid=626 pid=629 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=(none) ses=1 comm="chronyd" exe="/usr/sbin/chronyd" subj=system_u:system_r:kernel_t:s0 key=(null)
type=PROCTITLE msg=audit(1530616044.507:6): proctitle=6368726F6E7964002D71
type=TIME_ADJNTPVAL msg=audit(1530616044.507:7): op=status old=64 new=8256
type=SYSCALL msg=audit(1530616044.507:7): arch=c000003e syscall=159 success=yes exit=5 a0=7fff57e78c00 a1=1 a2=1 a3=7f754ae28c0a items=0 ppid=626 pid=629 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=(none) ses=1 comm="chronyd" exe="/usr/sbin/chronyd" subj=system_u:system_r:kernel_t:s0 key=(null)
type=PROCTITLE msg=audit(1530616044.507:7): proctitle=6368726F6E7964002D71
type=TIME_ADJNTPVAL msg=audit(1530616044.507:8): op=status old=8256 new=8257
type=TIME_ADJNTPVAL msg=audit(1530616044.507:8): op=offset old=0 new=0
type=TIME_ADJNTPVAL msg=audit(1530616044.507:8): op=freq old=0 new=0
type=SYSCALL msg=audit(1530616044.507:8): arch=c000003e syscall=159 success=yes exit=5 a0=7fff57e78ab0 a1=0 a2=55e129c850c0 a3=7f754ae28c0a items=0 ppid=626 pid=629 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=(none) ses=1 comm="chronyd" exe="/usr/sbin/chronyd" subj=system_u:system_r:kernel_t:s0 key=(null)
type=PROCTITLE msg=audit(1530616044.507:8): proctitle=6368726F6E7964002D71
type=TIME_ADJNTPVAL msg=audit(1530616044.507:9): op=status old=8257 new=64
type=SYSCALL msg=audit(1530616044.507:9): arch=c000003e syscall=159 success=yes exit=5 a0=7fff57e78ab0 a1=0 a2=55e129c850c0 a3=7f754ae28c0a items=0 ppid=626 pid=629 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=(none) ses=1 comm="chronyd" exe="/usr/sbin/chronyd" subj=system_u:system_r:kernel_t:s0 key=(null)
type=PROCTITLE msg=audit(1530616044.507:9): proctitle=6368726F6E7964002D71
type=SYSCALL msg=audit(1530616044.507:10): arch=c000003e syscall=159 success=yes exit=5 a0=7fff57e78a70 a1=0 a2=55e129c850c0 a3=7f754ae28c0a items=0 ppid=626 pid=629 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=(none) ses=1 comm="chronyd" exe="/usr/sbin/chronyd" subj=system_u:system_r:kernel_t:s0 key=(null)
type=PROCTITLE msg=audit(1530616044.507:10): proctitle=6368726F6E7964002D71
type=TIME_ADJNTPVAL msg=audit(1530616044.511:11): op=freq old=0 new=49180377088000
type=TIME_ADJNTPVAL msg=audit(1530616044.511:11): op=tick old=10000 new=10000
type=SYSCALL msg=audit(1530616044.511:11): arch=c000003e syscall=159 success=yes exit=5 a0=7fff57e78ad0 a1=0 a2=2710 a3=f42f82a800000 items=0 ppid=626 pid=629 auid=0 uid=385 gid=382 euid=385 suid=385 fsuid=385 egid=382 sgid=382 fsgid=382 tty=(none) ses=1 comm="chronyd" exe="/usr/sbin/chronyd" subj=system_u:system_r:kernel_t:s0 key=(null)
type=PROCTITLE msg=audit(1530616044.511:11): proctitle=6368726F6E7964002D71
type=TIME_ADJNTPVAL msg=audit(1530616044.521:12): op=status old=64 new=64
type=SYSCALL msg=audit(1530616044.521:12): arch=c000003e syscall=159 success=yes exit=5 a0=7fff57e78b40 a1=1 a2=40 a3=f91f6ef84fbab items=0 ppid=626 pid=629 auid=0 uid=385 gid=382 euid=385 suid=385 fsuid=385 egid=382 sgid=382 fsgid=382 tty=(none) ses=1 comm="chronyd" exe="/usr/sbin/chronyd" subj=system_u:system_r:kernel_t:s0 key=(null)
type=PROCTITLE msg=audit(1530616044.521:12): proctitle=6368726F6E7964002D71
type=TIME_INJOFFSET msg=audit(1530616049.652:13): sec=-16 nsec=124887145
type=TIME_ADJNTPVAL msg=audit(1530616049.652:13): op=status old=64 new=8256
type=SYSCALL msg=audit(1530616049.652:13): arch=c000003e syscall=159 success=yes exit=5 a0=7fff57e78270 a1=1 a2=fffffffffffffff0 a3=137b828205ca12 items=0 ppid=626 pid=629 auid=0 uid=385 gid=382 euid=385 suid=385 fsuid=385 egid=382 sgid=382 fsgid=382 tty=(none) ses=1 comm="chronyd" exe="/usr/sbin/chronyd" subj=system_u:system_r:kernel_t:s0 key=(null)
type=PROCTITLE msg=audit(1530616049.652:13): proctitle=6368726F6E7964002D71
type=TIME_ADJNTPVAL msg=audit(1530616033.783:14): op=freq old=49180377088000 new=49180377088000
type=TIME_ADJNTPVAL msg=audit(1530616033.783:14): op=tick old=10000 new=10000
type=SYSCALL msg=audit(1530616033.783:14): arch=c000003e syscall=159 success=yes exit=5 a0=7fff57e78bc0 a1=0 a2=2710 a3=0 items=0 ppid=626 pid=629 auid=0 uid=385 gid=382 euid=385 suid=385 fsuid=385 egid=382 sgid=382 fsgid=382 tty=(none) ses=1 comm="chronyd" exe="/usr/sbin/chronyd" subj=system_u:system_r:kernel_t:s0 key=(null)
type=PROCTITLE msg=audit(1530616033.783:14): proctitle=6368726F6E7964002D71

The chronyd command that produced the above records executed the following adjtimex(2) syscalls (as per strace output):

adjtimex({modes=ADJ_OFFSET|0x8000, offset=0, freq=0, maxerror=16000000, esterror=16000000, status=STA_UNSYNC, constant=2, precision=1, tolerance=32768000, time={tv_sec=1530616044, tv_usec=507215}, tick=10000, ppsfreq=0, jitter=0, shift=0, stabil=0, jitcnt=0, calcnt=0, errcnt=0, stbcnt=0, tai=0}) = 5 (TIME_ERROR)
adjtimex({modes=ADJ_MAXERROR, offset=0, freq=0, maxerror=0, esterror=16000000, status=STA_UNSYNC, constant=2, precision=1, tolerance=32768000, time={tv_sec=1530616044, tv_usec=507438}, tick=10000, ppsfreq=0, jitter=0, shift=0, stabil=0, jitcnt=0, calcnt=0, errcnt=0, stbcnt=0, tai=0}) = 5 (TIME_ERROR)
adjtimex({modes=ADJ_SETOFFSET|ADJ_NANO, offset=0, freq=0, maxerror=16000000, esterror=16000000, status=STA_UNSYNC|STA_NANO, constant=2, precision=1, tolerance=32768000, time={tv_sec=1530616044, tv_usec=507604737}, tick=10000, ppsfreq=0, jitter=0, shift=0, stabil=0, jitcnt=0, calcnt=0, errcnt=0, stbcnt=0, tai=0}) = 5 (TIME_ERROR)
adjtimex({modes=ADJ_OFFSET|ADJ_STATUS, offset=0, freq=0, maxerror=16000000, esterror=16000000, status=STA_PLL|STA_UNSYNC|STA_NANO, constant=2, precision=1, tolerance=32768000, time={tv_sec=1530616044, tv_usec=507698330}, tick=10000, ppsfreq=0, jitter=0, shift=0, stabil=0, jitcnt=0, calcnt=0, errcnt=0, stbcnt=0, tai=0}) = 5 (TIME_ERROR)
adjtimex({modes=ADJ_STATUS, offset=0, freq=0, maxerror=16000000, esterror=16000000, status=STA_UNSYNC, constant=2, precision=1, tolerance=32768000, time={tv_sec=1530616044, tv_usec=507792}, tick=10000, ppsfreq=0, jitter=0, shift=0, stabil=0, jitcnt=0, calcnt=0, errcnt=0, stbcnt=0, tai=0}) = 5 (TIME_ERROR)
adjtimex({modes=0, offset=0, freq=0, maxerror=16000000, esterror=16000000, status=STA_UNSYNC, constant=2, precision=1, tolerance=32768000, time={tv_sec=1530616044, tv_usec=508000}, tick=10000, ppsfreq=0, jitter=0, shift=0, stabil=0, jitcnt=0, calcnt=0, errcnt=0, stbcnt=0, tai=0}) = 5 (TIME_ERROR)
adjtimex({modes=ADJ_FREQUENCY|ADJ_TICK, offset=0, freq=750433, maxerror=16000000, esterror=16000000, status=STA_UNSYNC, constant=2, precision=1, tolerance=32768000, time={tv_sec=1530616044, tv_usec=512146}, tick=10000, ppsfreq=0, jitter=0, shift=0, stabil=0, jitcnt=0, calcnt=0, errcnt=0, stbcnt=0, tai=0}) = 5 (TIME_ERROR)
adjtimex({modes=ADJ_MAXERROR|ADJ_ESTERROR|ADJ_STATUS, offset=0, freq=750433, maxerror=16000000, esterror=16000000, status=STA_UNSYNC, constant=2, precision=1, tolerance=32768000, time={tv_sec=1530616044, tv_usec=522506}, tick=10000, ppsfreq=0, jitter=0, shift=0, stabil=0, jitcnt=0, calcnt=0, errcnt=0, stbcnt=0, tai=0}) = 5 (TIME_ERROR)
adjtimex({modes=ADJ_SETOFFSET|ADJ_NANO, offset=0, freq=750433, maxerror=16000000, esterror=16000000, status=STA_UNSYNC|STA_NANO, constant=2, precision=1, tolerance=32768000, time={tv_sec=1530616033, tv_usec=778717675}, tick=10000, ppsfreq=0, jitter=0, shift=0, stabil=0, jitcnt=0, calcnt=0, errcnt=0, stbcnt=0, tai=0}) = 5 (TIME_ERROR)
adjtimex({modes=ADJ_FREQUENCY|ADJ_TICK, offset=0, freq=750433, maxerror=16000000, esterror=16000000, status=STA_UNSYNC|STA_NANO, constant=2, precision=1, tolerance=32768000, time={tv_sec=1530616033, tv_usec=784644657}, tick=10000, ppsfreq=0, jitter=0, shift=0, stabil=0, jitcnt=0, calcnt=0, errcnt=0, stbcnt=0, tai=0}) = 5 (TIME_ERROR)

(The struct timex fields above are from after the syscall was executed, so they contain the current (new) values as set from the kernel, except of the 'modes' field, which contains the original value sent by the caller.)

Bugzilla and Issue Trackers

Add a custom footer
Add a custom sidebar
Clone this wiki locally