CRIU: malicious process can hijack checkpoint injection and escalate privileges

CRIU is a checkpoint and restore tool for Linux. Prior to 4.2.1, CRIU injects a syscall;int3 sequence at the start of the first executable virtual memory area and resumes the target process to execute parasite code. Malicious code in the target process can keep an rseq critical section armed over that injection point so the kernel's restartable-sequence abort redirects execution to an attacker-controlled callback when CRIU resumes the process. In container snapshot environments, the hijacked parasite runs with seccomp disabled and a relaxed AppArmor profile and supplies trusted data to the privileged CRIU process, which can enable higher-privilege code execution. This issue is fixed in version 4.2.1.