Replace the shell-string launcher in buildProcessRunner with a direct
exec.Command. The previous sh -c wrapper relied on shellQuote escaping
for every MachineConfig field that flowed into the launch script; any
future field that ever carried an attacker-controlled value would have
become RCE-as-root. The new path passes binary path and flags as
separate argv entries, so there is no shell to interpret anything.
The wrapper also did two things the shell can no longer do for us:
1. umask 077 — moved to syscall.Umask in cmd/bangerd/main.go so every
firecracker child (and any other file the daemon creates) inherits
0600 by default. Single-user dev sandbox state should be private.
2. chown_watcher — the SDK's HTTP probe inside Machine.Start connects
to the API socket the moment it appears. Under sudo the socket is
created root-owned and the daemon's connect(2) gets EACCES, so the
post-Start EnsureSocketAccess never runs. The shell papered over
this with a backgrounded chown loop. Replaced by
fcproc.EnsureSocketAccessForAsync: same race-window guarantee, in
pure Go, kicked off in LaunchFirecracker right before Start and
awaited right after.
Tests updated: shell-substring assertions replaced with cmd-arg
assertions, plus a new fcproc test pinning the async chown sequence.
Smoke (full systemd two-service install + KVM scenarios) passes.
Thales Maciel
d73efe6fbc