IPC Namespace

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Basic Information

An IPC (Inter-Process Communication) namespace is a Linux kernel feature that provides isolation of System V IPC objects, such as message queues, shared memory segments, and semaphores. This isolation ensures that processes in different IPC namespaces cannot directly access or modify each other's IPC objects, providing an additional layer of security and privacy between process groups.

How it works:

When a new IPC namespace is created, it starts with a completely isolated set of System V IPC objects. This means that processes running in the new IPC namespace cannot access or interfere with the IPC objects in other namespaces or the host system by default.
IPC objects created within a namespace are visible and accessible only to processes within that namespace. Each IPC object is identified by a unique key within its namespace. Although the key may be identical in different namespaces, the objects themselves are isolated and cannot be accessed across namespaces.
Processes can move between namespaces using the setns() system call or create new namespaces using the unshare() or clone() system calls with the CLONE_NEWIPC flag. When a process moves to a new namespace or creates one, it will start using the IPC objects associated with that namespace.

Lab:

Create different Namespaces

CLI

sudo unshare -i [--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

If you run the previous line without -f you will get that error.
The error is caused by the PID 1 process exits in the new namespace.

After bash start to run, bash will fork several new sub-processes to do somethings. If you run unshare without -f, bash will have the same pid as the current "unshare" process. The current "unshare" process call the unshare systemcall, create a new pid namespace, but the current "unshare" process is not in the new pid namespace. It is the desired behavior of linux kernel: process A creates a new namespace, the process A itself won't be put into the new namespace, only the sub-processes of process A will be put into the new namespace. So when you run:

unshare -p /bin/bash

The unshare process will exec /bin/bash, and /bin/bash forks several sub-processes, the first sub-process of bash will become PID 1 of the new namespace, and the subprocess will exit after it completes its job. So the PID 1 of the new namespace exits.

The PID 1 process has a special function: it should become all the orphan processes' parent process. If PID 1 process in the root namespace exits, kernel will panic. If PID 1 process in a sub namespace exits, linux kernel will call the disable_pid_allocation function, which will clean the PIDNS_HASH_ADDING flag in that namespace. When linux kernel create a new process, kernel will call alloc_pid function to allocate a PID in a namespace, and if the PIDNS_HASH_ADDING flag is not set, alloc_pid function will return a -ENOMEM error. That's why you got the "Cannot allocate memory" error.

You can resolve this issue by use the '-f' option:

unshare -fp /bin/bash

If you run unshare with '-f' option, unshare will fork a new process after it create the new pid namespace. And run /bin/bash in the new process. The new process will be the pid 1 of the new pid namespace. Then bash will also fork several sub-processes to do some jobs. As bash itself is the pid 1 of the new pid namespace, its sub-processes can exit without any problem.

Copied from https://stackoverflow.com/questions/44666700/unshare-pid-bin-bash-fork-cannot-allocate-memory

Docker

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

Check which namespace is your process in

ls -l /proc/self/ns/ipc
lrwxrwxrwx 1 root root 0 Apr  4 20:37 /proc/self/ns/ipc -> 'ipc:[4026531839]'

Find all IPC namespaces

{% code overflow="wrap" %}

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

{% endcode %}

Enter inside an IPC namespace

nsenter -i TARGET_PID --pid /bin/bash

Also, you can only enter in another process namespace if you are root. And you cannot enter in other namespace without a descriptor pointing to it (like /proc/self/ns/net).

Create IPC object

# Container
sudo unshare -i /bin/bash
ipcmk -M 100
Shared memory id: 0
ipcs -m

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status      
0x2fba9021 0          root       644        100        0    

# From the host
ipcs -m # Nothing is seen