Bind Smart Policy entries to process instances to prevent PID reuse

[coderabbitai]

Summary

Bind Smart Policy authorization to a process instance instead of only a PID.

Rationale

SmartPolicyInterface::request_process_access checks that /proc/<pid> exists and then inserts a PID-only entry into the eBPF policy maps. The process can exit between the existence check and the insertion. If the PID is later reused, a stale policy entry can affect the new process.

Affected areas

• crates/cardwire-daemon/src/interface/smart.rs
• Smart Policy eBPF maps and their kernel enforcement logic

Required changes

• Capture a kernel-verifiable process-instance identity when a Smart Policy entry is created. A process start-time identity is one possible mechanism.
• Store this identity with the Smart Policy map entry.
• Validate the stored identity against the current process identity during kernel enforcement.
• Ignore or remove policy entries that do not match the current process instance.
• Preserve the intended behavior for Allow_dGPU, Force_dGPU, and GPU-specific policies.

Acceptance criteria

• A policy created for a process does not apply to a different process that later reuses the same PID.
• Kernel enforcement verifies process-instance identity before it applies a Smart Policy entry.
• The implementation includes tests or another documented validation method for PID reuse or stale-entry handling.

Backlinks

• Deferred from PR feat(cardwired): add a SmartPolicy interface #155: feat(cardwired): add a SmartPolicy interface #155
• Review discussion: feat(cardwired): add a SmartPolicy interface #155 (comment)

Requested by: @luytan