nono-py has proxy-only network fallback bypass on older Linux kernels

Summary

On Linux kernels that do not support Landlock network rules, nono_py.sandboxed_exec() could run CapabilitySet.proxy_only(proxy) without supervising the seccomp-notify proxy-only fallback returned by the Rust core.

In that configuration, a sandboxed child process could remove HTTP_PROXY / HTTPS_PROXY environment variables or use raw sockets and then open direct TCP connections that should have been denied by proxy-only policy.

The issue affects proxy-only enforcement. It does not mean that all nono-py network blocking is ineffective. ECS validation showed caps.block_network() denied regular TCP and ECS metadata TCP on the tested Linux 6.1 host.

Impact

The intended proxy_only() security property is:

• child processes may connect only to the local nono proxy port
• the proxy enforces host allowlists and metadata/link-local denial
• direct TCP to any other target is denied

Before the fix, on kernels without Landlock AccessNet, the Python binding applied the sandbox and then executed the child, but did not install and supervise the proxy-only seccomp-notify fallback. A child could therefore bypass the proxy layer in that old-kernel path.

The highest-impact scenario is a sandboxed workload with access to cloud metadata discovery inputs, where direct TCP to a metadata endpoint could retrieve task or instance credentials after proxy environment variables are removed.

Affected Conditions

The issue requires all of the following:

• Linux runtime.
• Kernel without Landlock network support, such as Linux 6.1. Landlock network rules require Landlock ABI v4 / Linux 6.7 or newer.
• nono_py.sandboxed_exec() is used.
• The capability set uses caps.proxy_only(proxy).
• The child process removes or ignores proxy environment variables, or uses raw sockets.

macOS Seatbelt proxy-only enforcement is not affected by this Linux seccomp-notify fallback issue.

Affected Versions

Known affected builds include nono-py versions that expose and use through before the supervised fallback fix in this working tree.