Is there a way to cancel unshare(2)?

Question

If I called unshare ./fooprogram, is there a way to cancel that unshare while fooprogram is running?

Answer 1

If there would be a way, that would be a kernel vulnerability.

At the time of writing of this comment, the latest kernel (5.7.2), has documentation on that matter:

unshare() system call adds a primitive to the Linux thread model that allows threads to selectively 'unshare' any resources that were being shared at the time of their creation.

[…]

unshare() augments the usefulness of Linux threads for applications that would like to control shared resources without creating a new process. unshare() is a natural addition to the set of available primitives on Linux that implement the concept of process/thread as a virtual machine.

And later continues:

unshare() can be used to implement polyinstantiated directories using the kernel's per-process namespace mechanism. Polyinstantiated directories, such as per-user and/or per-security context instance of /tmp, /var/tmp or per-security context instance of a user's home directory, isolate user processes when working with these directories. Using unshare(), a PAM module can easily setup a private namespace for a user at login.

The documentation does not explicitely says "unshare() can not be reverted", but it does mentions that one legitimate use of unshare() can be used for security purposes.

One could assume that being able to reverse it would defeat that purpose.