So far, we opened a file descriptor refering to proc on the host inside the host namespace and handed that fd to the attached process in attach_child_main(). This was done to ensure that LSM labels were correctly setup. However, by exploiting a potential kernel bug, ptrace could be used to prevent the file descriptor from being closed which in turn could be used by an unprivileged container to gain access to the host namespace. Aside from this needing an upstream kernel fix, we should make sure that we don't pass the fd for proc itself to the attached process. However, we cannot completely prevent this, as the attached process needs to be able to change its apparmor profile by writing to /proc/self/attr/exec or /proc/self/attr/current. To minimize the attack surface, we only send the fd for /proc/self/attr/exec or /proc/self/attr/current to the attached process. To do this we introduce a little more IPC between the child and parent: * IPC mechanism: (X is receiver) * initial process intermediate attached * X <--- send pid of * attached proc, * then exit * send 0 ------------------------------------> X * [do initialization] * X <------------------------------------ send 1 * [add to cgroup, ...] * send 2 ------------------------------------> X * [set LXC_ATTACH_NO_NEW_PRIVS] * X <------------------------------------ send 3 * [open LSM label fd] * send 4 ------------------------------------> X * [set LSM label] * close socket close socket * run program The attached child tells the parent when it is ready to have its LSM labels set up. The parent then opens an approriate fd for the child PID to /proc/<pid>/attr/exec or /proc/<pid>/attr/current and sends it via SCM_RIGHTS to the child. The child can then set its LSM laben. Both sides then close the socket fds and the child execs the requested process. Signed-off-by: Christian Brauner <christian.brauner@canonical.com>