feat(sandbox): Landlock TCP port restriction for mandatory proxy#16
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When Platform mode is active, apply Landlock ABI v4 network rules to restrict TCP connect to only the proxy port (default 3128). This makes the loopback CONNECT proxy mandatory at the kernel level -- a process calling connect() to any other port gets EACCES before any packet is created. This closes the cooperative proxy gap: without this, processes ignoring HTTP_PROXY could bypass the proxy and connect directly. With this, enforcement is at the kernel LSM layer (Tier 1), same as Landlock filesystem restrictions. Graceful degradation: if the kernel does not support Landlock ABI v4 (RHEL 9.5 or earlier), the network rules are silently skipped and enforcement falls back to the cooperative proxy + NetworkPolicy. 1 file changed, +43/-3 lines. Compiles, tests pass, clippy clean. Ref: NVIDIA#899
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Add kernel-level network syscall interception using SECCOMP_RET_USER_NOTIF for Platform mode. Provides mandatory, syscall-level enforcement without any capabilities. DnsPinnedAllowlist: resolve domains to IPs at sandbox creation, freeze for session lifetime (DNS rebinding prevention). BPF filter intercepts: connect, sendto, sendmsg, recvfrom, recvmsg, bind. Validates AUDIT_ARCH to prevent x32/compat ABI bypass. Linux syscall wrappers: notification fd ioctls, pidfd_open/pidfd_getfd for on-behalf-of operations (TOCTOU-safe), read_process_memory with read_exact (no short reads), sockaddr parser (correct endianness for sa_family, port, flowinfo), verify_socket_fd (mitigates fd-swap race), deny/allow_connect response helpers. Code review fixes applied across all PRs: - PR #15: gateway propagates network_enforcement to DriverSandboxSpec - PR #15: driver uses typed enum comparison (not magic integer) - PR #16: saturating_sub prevents underflow in Landlock skipped count - PR #16: warn!() on TCP port restriction failure (was debug) - PR #17: BPF arch check, recvfrom/recvmsg/bind interception, verify_socket_fd, read_exact, allow_connect rename, flowinfo endianness, safety comments on all unsafe blocks 8 tests. Compiles, 949 tests pass, clippy clean. Ref: NVIDIA#899
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Summary
Add Landlock ABI v4 TCP port restriction to Platform mode. Restricts
connect()to proxy port only (3128). All other ports return EACCESat the kernel level. Makes the CONNECT proxy mandatory, not cooperative.
Depends on: #15 (Platform mode base)
1 file, +43/-3 lines. Compiles, tests pass, clippy clean.
Ref: NVIDIA#899
Assisted-By: Claude Code