fix(encode): create temp files in a private directory - #294
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The path is fully derived from PID plus a counter starting at 0, in a shared, world-writable directory. `create_dir_all` succeeds on an already-existing directory (or a symlink to one) and `fs::write` follows symlinks, so a local attacker who pre-creates `/tmp/rustmotion_audio_<pid>_0/audio.raw` as a symlink gets an arbitrary file truncated and overwritten with PCM bytes under the rendering user's identity; PID space is small enough to pre-seed exhaustively. The same class exists in `video_audio.rs:258`, `std::env::temp_dir().join(format!("rustmotion_vidaud_{:016x}.wav", hash))`, where `hash` comes from `DefaultHasher::new()` (SipHash seeded with fixed keys 0,0 — deterministic across processes and machines), so the path is exactly predictable. There, `if wav_path.exists() { return Some(wav_path) }` (line 298) means a pre-planted file is used verbatim as the render's audio, and the `.partial.wav` sibling is handed to `ffmpeg -y` (line 339-340), which follows a symlink at that path.
Refs #220
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The path is fully derived from PID plus a counter starting at 0, in a shared, world-writable directory. `create_dir_all` succeeds on an already-existing directory (or a symlink to one) and `fs::write` follows symlinks, so a local attacker who pre-creates `/tmp/rustmotion_audio_<pid>_0/audio.raw` as a symlink gets an arbitrary file truncated and overwritten with PCM bytes under the rendering user's identity; PID space is small enough to pre-seed exhaustively. The same class exists in `video_audio.rs:258`, `std::env::temp_dir().join(format!("rustmotion_vidaud_{:016x}.wav", hash))`, where `hash` comes from `DefaultHasher::new()` (SipHash seeded with fixed keys 0,0 — deterministic across processes and machines), so the path is exactly predictable. There, `if wav_path.exists() { return Some(wav_path) }` (line 298) means a pre-planted file is used verbatim as the render's audio, and the `.partial.wav` sibling is handed to `ffmpeg -y` (line 339-340), which follows a symlink at that path.
Refs #220
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Audit finding carried by this chantier. Refs #220 (RM-44).