High [CVE-2024-21626] runc container breakout through process.cwd trickery and leaked fds

runc is a CLI tool for spawning and running containers on Linux according to the OCI specification.

In runc 1.1.11 and earlier, due to an internal file descriptor leak, an attacker could cause a newly-spawned container process (from runc exec) to have a working directory in the host filesystem namespace, allowing for a container escape by giving access to the host filesystem ("attack 2").

The same attack could be used by a malicious image to allow a container process to gain access to the host filesystem through runc run ("attack 1"). Variants of attacks 1 and 2 could be also be used to overwrite semi-arbitrary host binaries, allowing for complete container escapes ("attack 3a" and "attack 3b"). runc 1.1.12 includes patches for this issue.

While a user performs `O_CLOEXEC` all file descriptors before executing the container code, the file descriptor is open when performing `setcwd(2)`, which means that the reference can be kept alive in the container by configuring the working directory to be a path resolved through the file descriptor.

The non-dumpable bit is unset after `execve`, meaning there are multiple ways to attack this other than bad configurations. The only way to defend against it entirely is to close all unneeded file descriptors.