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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:

  1. 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.

  2. 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.

  3. 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

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

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

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>

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

References

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