Time Namespace

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Basic Information

The time namespace in Linux allows for per-namespace offsets to the system monotonic and boot-time clocks. It is commonly used in Linux containers to change the date/time within a container and adjust clocks after restoring from a checkpoint or snapshot.

Lab:

Create different Namespaces

CLI

bash
sudo unshare -T [--mount-proc] /bin/bash

By mounting a new instance of the /proc filesystem if you use the param --mount-proc, you ensure that the new mount namespace has an accurate and isolated view of the process information specific to that namespace.

Error: bash: fork: Cannot allocate memory

When unshare is executed without the -f option, an error is encountered due to the way Linux handles new PID (Process ID) namespaces. The key details and the solution are outlined below:

  1. Problem Explanation:

    • The Linux kernel allows a process to create new namespaces using the unshare system call. However, the process that initiates the creation of a new PID namespace (referred to as the "unshare" process) does not enter the new namespace; only its child processes do.
    • Running %unshare -p /bin/bash% starts /bin/bash in the same process as unshare. Consequently, /bin/bash and its child processes are in the original PID namespace.
    • The first child process of /bin/bash in the new namespace becomes PID 1. When this process exits, it triggers the cleanup of the namespace if there are no other processes, as PID 1 has the special role of adopting orphan processes. The Linux kernel will then disable PID allocation in that namespace.

  2. Consequence:

    • The exit of PID 1 in a new namespace leads to the cleaning of the PIDNS_HASH_ADDING flag. This results in the alloc_pid function failing to allocate a new PID when creating a new process, producing the "Cannot allocate memory" error.

  3. Solution:

    • The issue can be resolved by using the -f option with unshare. This option makes unshare fork a new process after creating the new PID namespace.
    • Executing %unshare -fp /bin/bash% ensures that the unshare command itself becomes PID 1 in the new namespace. /bin/bash and its child processes are then safely contained within this new namespace, preventing the premature exit of PID 1 and allowing normal PID allocation.

By ensuring that unshare runs with the -f flag, the new PID namespace is correctly maintained, allowing /bin/bash and its sub-processes to operate without encountering the memory allocation error.

Docker

bash
docker run -ti --name ubuntu1 -v /usr:/ubuntu1 ubuntu bash

Check which namespace is your process in

bash
ls -l /proc/self/ns/time
lrwxrwxrwx 1 root root 0 Apr  4 21:16 /proc/self/ns/time -> 'time:[4026531834]'

Find all Time namespaces

bash
sudo find /proc -maxdepth 3 -type l -name time -exec readlink {} \; 2>/dev/null | sort -u
# Find the processes with an specific namespace
sudo find /proc -maxdepth 3 -type l -name time -exec ls -l  {} \; 2>/dev/null | grep <ns-number>

Enter inside a Time namespace

bash
nsenter -T TARGET_PID --pid /bin/bash

Manipulating Time Offsets

Starting with Linux 5.6, two clocks can be virtualised per time namespace:

  • CLOCK_MONOTONIC
  • CLOCK_BOOTTIME

Their per-namespace deltas are exposed (and can be modified) through the file /proc/<PID>/timens_offsets:

$ sudo unshare -Tr --mount-proc bash   # -T creates a new timens, -r drops capabilities
$ cat /proc/$$/timens_offsets
monotonic 0
boottime  0

The file contains two lines – one per clock – with the offset in nanoseconds. Processes that hold CAP_SYS_TIME in the time namespace can change the value:

# advance CLOCK_MONOTONIC by two days (172 800 s)
echo "monotonic 172800000000000" > /proc/$$/timens_offsets
# verify
$ cat /proc/$$/uptime   # first column uses CLOCK_MONOTONIC
172801.37  13.57

If you need the wall clock (CLOCK_REALTIME) to change as well you still have to rely on classic mechanisms (date, hwclock, chronyd, …); it is not namespaced.

unshare(1) helper flags (util-linux ≥ 2.38)

sudo unshare -T \
            --monotonic="+24h"  \
            --boottime="+7d"    \
            --mount-proc         \
            bash

The long options automatically write the chosen deltas to timens_offsets right after the namespace is created, saving a manual echo.

OCI & Runtime support

  • The OCI Runtime Specification v1.1 (Nov 2023) added a dedicated time namespace type and the linux.timeOffsets field so that container engines can request time virtualisation in a portable way.
  • runc >= 1.2.0 implements that part of the spec. A minimal config.json fragment looks like: 
    {
  "linux": {
    "namespaces": [
      {"type": "time"}
    ],
    "timeOffsets": {
      "monotonic": 86400,
      "boottime": 600
    }
  }
}
    Then run the container with runc run <id>.

NOTE: runc 1.2.6 (Feb 2025) fixed an "exec into container with private timens" bug that could lead to a hang and potential DoS. Make sure you are on ≥ 1.2.6 in production.

Security considerations

  1. Required capability – A process needs CAP_SYS_TIME inside its user/time namespace to change the offsets. Dropping that capability in the container (default in Docker & Kubernetes) prevents tampering.
  2. No wall-clock changes – Because CLOCK_REALTIME is shared with the host, attackers cannot spoof certificate lifetimes, JWT expiry, etc. via timens alone.
  3. Log / detection evasion – Software that relies on CLOCK_MONOTONIC (e.g. rate-limiters based on uptime) can be confused if the namespace user adjusts the offset. Prefer CLOCK_REALTIME for security-relevant timestamps.
  4. Kernel attack surface – Even with CAP_SYS_TIME removed, the kernel code remains accessible; keep the host patched. Linux 5.6 → 5.12 received multiple timens bug-fixes (NULL-deref, signedness issues).

Hardening checklist

  • Drop CAP_SYS_TIME in your container runtime default profile.
  • Keep runtimes updated (runc ≥ 1.2.6, crun ≥ 1.12).
  • Pin util-linux ≥ 2.38 if you rely on the --monotonic/--boottime helpers.
  • Audit in-container software that reads uptime or CLOCK_MONOTONIC for security-critical logic.

References

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Learn & practice AWS Hacking:HackTricks Training AWS Red Team Expert (ARTE)
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Learn & practice Az Hacking: HackTricks Training Azure Red Team Expert (AzRTE)

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