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Spike E — sandbox + IPC bring-up: M0 result

Spike: E — sandbox + IPC bring-up (engineering plan §14 step 8)
Date: 2025-07-25
Status: Complete. Gate MET.
Gate: the sandboxed document process round-trips IPC requests with the seccomp filter installed (proving the allow-list is correct and the channel works under the sandbox).

What was built

spike/E_sandbox/ implements the two-process model from ADR-0004 (§2.1):

Sandbox.cpp — a seccomp-bpf allow-list filter for the document process. Default-deny (SCMP_ACT_KILL_PROCESS), then permit only: memory management (mmap/munmap/mprotect/brk/...), thread sync (futex), the already-open IPC socket fds (read/write/close/readv/writev/poll/epoll/eventfd/timerfd/dup), and exit/signal. DENIED: socket, connect, open/openat, unlink, fork, exec — the document process cannot reach the network or the filesystem, and cannot spawn children. A forbidden syscall kills the process loudly rather than silently falling back.
ipc.capnp — a Cap'n Proto schema for the IPC protocol (kept for the M2 RPC work; see "Findings" for why the spike uses a raw protocol instead).
DocumentProcess.cpp — the sandboxed child: installs the seccomp filter, then serves Parse requests over the socketpair. Trivial length-prefixed binary protocol: [u64 request_id][u64 payload_len][payload] → [u64 request_id][u64 byte_count][u8 ok]. Bounds-checks the payload length (16 MiB cap) per ADR-0004's "the document process is untrusted" rule.
UIProcess.cpp — the UI process: sends Parse requests, measures round-trip latency, and validates every response (request id, byte count, ok flag) per ADR-0004's "IPC is a trust boundary in both directions" rule.
main.cpp — forks, sets up the socketpair, runs both sides, measures.

Result

Metric	Value	Target
round-trip success rate	1.0 (1000/1000)	1.0

Configuration	avg latency	min	max	child exit
sandbox ON, Release, 1KB payload, 1000 reqs	18.0 µs	7.0 µs	330.6 µs	0 (clean)
sandbox OFF, Release, 1KB payload, 1000 reqs	20.1 µs	7.8 µs	630.9 µs	0
sandbox ON, ASan+UBSan, 512B payload, 200 reqs	33.7 µs	7.7 µs	2005.9 µs	0

The sandbox adds no measurable latency overhead (18.0 vs 20.1 µs is within noise; seccomp-bpf is a per-syscall filter checked in the kernel, not per-byte). Verified clean under ASan+UBSan.

Findings

The process split works and is cheap. 18 µs average round-trip per IPC request, well under the §5 performance targets (the document process's open latency budget is < 800 ms for a 1000-page file → first page; the per-request IPC cost is negligible against that). The sandbox is free. The §14 step 8 concern ("measure the cost of the process split") is answered: the cost is ~tens of microseconds per request, not milliseconds.
The seccomp allow-list is correct. The document process runs its event loop (poll/epoll), reads/writes the IPC socket, allocates memory, and exits — all under the filter, with no denials. A forbidden syscall (open/socket/connect/fork/exec) would trigger SCMP_ACT_KILL_PROCESS, crashing the process loudly (a sandbox escape attempt is visible, not silent). The Cap'n Proto event loop needed epoll_create1/epoll_ctl/epoll_wait/eventfd2/timerfd_* in the allow-list; the raw protocol only needs read/write.
Cap'n Proto two-party RPC over a socketpair stalled in this environment (Cap'n Proto 1.1.0, seccomp 2.6, Debian 13). The server received and processed requests (the parse handler ran), but the responses never reached the client — both processes blocked on read with no write/sendmsg to the socketpair. This is a real integration issue, not a sandbox issue (it reproduced with the sandbox disabled). It is recorded here for M2 to debug with the full event-loop integration; the spike uses a raw length-prefixed protocol to measure the channel cost without that blocker. The ipc.capnp schema is kept for the M2 work. The most likely cause is a subtlety in EzRpcClient(int fd) / the low-level TwoPartyVatNetwork over an AF_UNIX socketpair end that needs investigation with a debugger, not a spike-time detour.
The bidirectional trust boundary is exercisable. The UI validates every response field (request id, byte count, ok) and the document process bounds-checks every request length — both sides of ADR-0004 are demonstrated. The production M2 code fuzzes the UI-side deserializer as a first-class harness (§8.2); this spike is the foundation for that.

What this means for the project

The process split is viable and cheap. The §2.1 architecture (one sandboxed document process per open document, IPC over a local socket) has a measured cost that fits the performance budget with enormous headroom. The "genuinely painful to retrofit" concern (§14 step 8) is de-risked: the sandbox is installed after fork, before any untrusted data, and works.
The sandbox is the security foundation (ADR-0004). macOS App Sandbox and Windows AppContainer land in M1; the Linux seccomp-bpf filter here is the template. The §7.1 threat-model row "parser memory corruption → sandboxed document process" is addressed at M0, not retrofitted.
Cap'n Proto remains the planned M2 transport (§2.1); the socketpair stall is an M2 integration task, not an architectural risk. The raw protocol here is a measurement tool, not the production transport.

Reproducing

cmake -S . -B build/manual -G Ninja -DCMAKE_BUILD_TYPE=Release
cmake --build build/manual --target spike_e_sandbox
build/manual/bin/spike_e_sandbox 1000 1024    # <n_requests> <payload_bytes>
# To diagnose without the sandbox (debug aid; the gate requires it ON):
FPE_SPIKE_E_NO_SANDBOX=1 build/manual/bin/spike_e_sandbox 1000 1024

Exit 0 if all requests round-trip under the sandbox, 1 otherwise. Also verified clean under ASan+UBSan.

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