CVE-2021-33910: 
NixOS vulnerability analysis and mitigation

CVE-2021-33910 affects systemd prior to versions 246.15, 247.8, 248.5, and 249.1. The vulnerability involves a Memory Allocation with an Excessive Size Value in basic/unit-name.c, specifically in the unitnamepath_escape function that uses strdupa and alloca for pathname controlled by a local attacker. The issue was discovered by Qualys Research Labs and was introduced in systemd v220 (April 2015) (Qualys Advisory).

The vulnerability exists in systemd's handling of mountpoint paths. When systemd monitors and parses /proc/self/mountinfo, it passes each mountpoint path through mountsetupunit(), which ultimately calls unitnamepathescape(). This function uses strdupa() to allocate memory on the stack for the pathname. If the total path length exceeds 8MB (the default RLIMITSTACK), it causes a stack exhaustion leading to a crash (Qualys Advisory).

When successfully exploited, this vulnerability results in a crash of systemd (PID 1) which leads to a kernel panic and brings down the entire operating system. Since systemd runs as the 'global init' process with PID 1, its crash forces the kernel to panic (Qualys Advisory).

The vulnerability has been fixed in systemd versions 246.15, 247.8, 248.5, and 249.1. The fix involves replacing the use of strdupa() with strdup() to allocate memory on the heap instead of the stack. Users should upgrade to the fixed versions available through their distribution's package management system (Debian Advisory, Fedora Update).