What does enabling kernel.unprivileged_userns_clone do?

Enabling unprivileged user namespaces can make severe vulnerabilities in the Linux kernel much more easily exploitable. If you did not intend to enable it, you should ensure it is disabled. Numerous vulnerabilities that are found regularly are often only exploitable by unprivileged users if unprivileged user namespaces are supported and enabled by the kernel. Unless you truly need it, just disable it.

The reason for this is that much of the kernel that is only intended to be reachable by UID 0 is not audited particularly well, given that the code is typically considered to be trusted. That is, a bug that requires a UID of 0 is rarely considered a serious bug. Unfortunately, unprivileged user namespaces make it possible for unprivileged users to access this very same code and exploit security bugs.

A few examples of vulnerabilities only exploitable on systems with unprivileged user namespaces:

• https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-16120

• https://brauner.github.io/2019/02/12/privileged-containers.html

• https://www.halfdog.net/Security/2016/UserNamespaceOverlayfsXattrSetgidPrivilegeEscalation/

• https://www.halfdog.net/Security/2015/UserNamespaceOverlayfsSetuidWriteExec/

• https://googleprojectzero.blogspot.com/2017/05/exploiting-linux-kernel-via-packet.html

• https://seclists.org/fulldisclosure/2016/Feb/123

• https://seclists.org/oss-sec/2016/q4/607

• https://www.openwall.com/lists/oss-security/2015/12/31/5

• https://www.rapid7.com/db/modules/exploit/linux/local/nested_namespace_idmap_limit_priv_esc/

• https://lwn.net/Articles/543539/

• https://lwn.net/Articles/543442/

It disables a bit of "hardening" that Debian patches into their distribution kernel. If you're not running such a kernel, it will fail and not do anything, as such a setting doesn't even exist in the mainline Linux kernel. If you were running such a patched kernel, all it would do is disable the functionality of that patch, and let your kernel work like every other kernel, allowing unprivileged users to use unshare -U. Contrary to forest's answer, I don't believe this to be dangerous. In particular, if a user can sudo to root (as would be required to turn this off), they can already do everything that this would let them do.