fix(jaild): fail closed when the read-only remount fails (AB-003) - #304
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WHAT: In run_jailed's async-signal-safe pre_exec closure, the RO remount (MS_BIND|MS_REMOUNT|MS_RDONLY) discarded its return value — unlike every other syscall in the closure, which fails closed via the errno pipe. If the remount failed (EACCES/EPERM/EINVAL), the path was left READ-WRITE and the closure proceeded to pivot_root and exec: the jail's effective authority exceeded the caveat. Capture the return code and `return Err` on failure. WHY: #266 / AB-003 — a fail-open confinement breach (a would-be read-only path stays writable). Regression: a test-only fault seam (FAULT_RO_REMOUNT) forces the remount to fail; the privileged `ro_remount_failure_fails_closed` test (linux tier, like the other jaild E2E proofs) asserts run_jailed then fails closed rather than exec'ing. Reverting the `return Err` re-fails the test. Co-Authored-By: Claude Opus 4.8 <[email protected]>
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Summary
Fixes #266 / AB-003. In
run_jailed's async-signal-safepre_execclosure, the read-only remount (MS_BIND|MS_REMOUNT|MS_RDONLY) discarded its return value — unlike every other syscall in the closure, which fails closed via the errno pipe. If that remount failed (EACCES/EPERM/EINVAL), the path was left read-write and the closure proceeded topivot_rootandexec: the jail's effective authority exceeded the caveat. Now the return code is captured and the closurereturn Errs on failure.Test plan
ro_remount_failure_fails_closed(linux tier,#[ignore]d like the other jaild E2E proofs): a test-only fault seam (FAULT_RO_REMOUNT) forces the RO remount to fail, and assertsrun_jailedthen fails closed rather than exec'ing (baseline with no fault still runs). Reverting thereturn Errre-fails it.clippy -D warningsclean,cargo fmtclean, the 14 non-ignored jaild tests pass; full pre-push gate[pre-push] OK.Scope
Contained to
linux.rs. Minimal fail-closed change matching the closure's established errno-pipe pattern. (The related "failed initial bind is skipped" nuance from the issue is benign for RO mounts — a bind that fails leaves the path absent, which is fail-safe; the writable-path breach is the remount, which this closes.)Fixes #266
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