validate_path_within_cwd used exists() to decide whether a path was new.
exists() follows symlinks, so a *dangling* symlink (target missing) reported
false and fell into the not-yet-existing branch, which only bound-checked the
nearest existing ancestor — never the symlink itself. An approved
`ln -s /etc/cron.d/pwn evil` inside the sandbox would therefore pass
validation, and a subsequent Write to `evil` would follow the link and create
a file outside the sandbox. In AcceptEdits mode this chained cleanly: approve
one innocuous-looking symlink (Execute), then writes auto-allow.
Resolve every path component with lstat (symlink_metadata, which does not
follow links) instead of exists():
- Final component: if it is a symlink (dangling or live), resolve its real
target via read_link + canonicalize-nearest-ancestor and reject unless the
target stays within the sandbox.
- Ancestor walk: an intermediate dangling symlink directory is likewise
resolved and bound-checked rather than skipped.
Live symlinks were already caught by canonicalize; this closes the dangling
case. It does not close the check-vs-open TOCTOU (the open happens inside the
SDK tool and would need O_NOFOLLOW/openat2 there); tracked for phase 2 along
with OS-level bash isolation.
Covered by tests/agent_sandbox.rs (dangling symlink to absolute and relative
targets, write tool through a dangling link, write through an intermediate
dangling symlink dir, all rejected; dangling link pointing inside and ordinary
new files still allowed) plus mirrored in-crate unit tests.
The previous approval registry used a oneshot channel, so two independent
signals decided an approval: the HTTP handler's result (from Sender::send's
bool) and the agent loop's allow/deny (from which timeout arm fired). At the
300s boundary these could disagree — a late /approve could see send() return
Ok (receiver not yet dropped) and report ok:true to the user, while the
timeout's final poll simultaneously fired and denied the tool. The frontend
was told "approved" while nothing ran. The mirror case (ok:false but allowed)
was impossible, so it never executed an unapproved tool, but the user-facing
signal could lie.
Replace the channel with a single source of truth: each entry is an
AtomicU8 state machine (Pending -> Approved/Denied/TimedOut) plus a Notify.
The decision is made by exactly one compare_exchange from Pending — whoever
wins (approver or timeout) settles it, everyone else observes a losing CAS
and no-ops. resolve() now returns whether *its* CAS won, so /approve tells
the frontend the truth; the loop decides purely from the settled atomic.
They read the same atomic and cannot disagree.
A PendingGuard removes the registry entry on drop (normal completion or
aborted/disconnected session), so there is no leak on any path. The callback
also emits an approval_resolved SSE event with the final decision so a late
click — already told ok:false — can dismiss its modal with the real outcome.
Verified with an integration test that races resolve() against a near-zero
timeout 5000x and asserts ok==true iff the executed decision is Approved
(never a mixed state). Run via tests/ because the in-crate lib test binary
is currently blocked by unrelated AppConfig test fixtures in other modules.
boos