PlayOS Security Model

Cross-references: architecture.md §13–15, runtime-ipc.md §3, sprints/Sprint-12.md

Table of Contents

1. Trust Zones
2. Component Privilege Levels
3. Game Restrictions
4. IPC Access Control
5. Filesystem Access Control
6. seccomp Filter Policy
7. Landlock Filesystem Restrictions
8. Input Security
9. System Image Integrity
10. Secure Boot Chain
11. Development vs Production
12. Post-MVP Hardening Roadmap

1. Trust Zones

┌───────────────────────────────────────────────────────────────┐
│  Zone 1: Kernel                                               │
│  Linux kernel + drivers                                       │
│  Full hardware access                                         │
└───────────────────┬───────────────────────────────────────────┘
                    │
┌───────────────────▼───────────────────────────────────────────┐
│  Zone 2: Trusted System Components                            │
│  playos-init (root)                                           │
│  playos-compositor (display + input caps)                     │
│  playos-shell (playos-trusted group)                          │
│  playos-overlay (playos-trusted group)                        │
│                                                               │
│  ← communicate via /run/playos/ UNIX sockets →               │
└───────────────────┬───────────────────────────────────────────┘
                    │ public libplayos C ABI only
┌───────────────────▼───────────────────────────────────────────┐
│  Zone 3: Untrusted Game Process                               │
│  User: playos-game (unprivileged)                             │
│  No access to Zone 2 sockets                                  │
│  No DRM primary nodes                                         │
│  No raw input devices                                         │
│  Restricted to own save/cache directories                     │
│  seccomp + Landlock enforced                                  │
└───────────────────────────────────────────────────────────────┘

2. Component Privilege Levels

3. Game Restrictions

A normal game must not be able to:

4. IPC Access Control

Control socket (/run/playos/control.sock)

• Owner: root:playos-trusted, mode 0660
• Who can connect: playos-shell, playos-overlay (both in playos-trusted)
• Who cannot: playos-game — enforced by UNIX group check at connect(2) time

Compositor socket (/run/playos/compositor.sock)

• Owner: root:playos-trusted, mode 0660
• Who can connect: playos-runtime internal client only
• Who cannot: playos-game

Lifecycle fd (PLAYOS_LIFECYCLE_FD)

• Direction: Write end held by playos-init; read end passed to game
• Game can: Read lifecycle events (single-byte values)
• Game cannot: Write to the fd; the write end is close()d in the game process before exec
• Not a socket: Cannot be used to connect to any IPC endpoint

5. Filesystem Access Control

System partition (/)

• Mounted read-only via MS_RDONLY (Sprint 11)
• dm-verity hash tree appended to system image (Sprint 12+ — planned, not yet implemented)
• Any write attempt returns EROFS

Data partition (/data)

• Mounted read-write, owned by root
• Per-game directories: chown playos-game:playos-game /data/saves/<id> and cache/<id>
• Other directories (config/, games/, logs/) owned by playos-system, not writable by games

Device nodes

Note: Games access GPU rendering through the Wayland EGL surface, not directly through render nodes.

6. seccomp Filter Policy

Applied to game processes via libseccomp before execve(). Default action: SECCOMP_RET_ERRNO(EPERM).

Allowed syscalls (core game set)

# Memory management
mmap, munmap, mprotect, mremap, madvise, brk

# File I/O
read, readv, write, writev, open, openat, close, stat, fstat, lstat,
newfstatat, statx, lseek, dup, dup2, ioctl (restricted — see below),
fcntl, access, faccessat, getdents64, getcwd

# Network (AF_UNIX only for Wayland socket)
socket (AF_UNIX only — enforced by arg filter), connect, bind,
accept, sendmsg, recvmsg, sendto, recvfrom, shutdown, getsockname, getpeername

# Processes and threads
exit, exit_group, clone, clone3, fork, execve (restricted — no setuid),
wait4, waitid, getpid, getppid, gettid, set_tid_address, prctl (restricted)

# Synchronization
futex, futex_waitv, nanosleep, clock_nanosleep

# Signals
rt_sigaction, rt_sigprocmask, rt_sigreturn, sigaltstack, kill (self only)

# Time
clock_gettime, clock_getres, gettimeofday, time

# Misc
getrandom, getuid, getgid, geteuid, getegid, uname, sysinfo,
pread64, pwrite64, eventfd2, epoll_create1, epoll_ctl, epoll_wait,
pipe2, timerfd_create, timerfd_settime, timerfd_gettime,
inotify_init1, inotify_add_watch, inotify_rm_watch,
mlock, munlock, memfd_create

Blocked syscalls (fatal SIGSYS or EPERM)

mount, umount2, umount           # no filesystem mounting
init_module, finit_module, delete_module  # no kernel modules
reboot                           # no direct reboot
ptrace                           # no process tracing
setuid, setgid, setresuid, setresgid, setfsuid, setfsgid  # no privilege escalation
capset, prctl(PR_SET_SECCOMP)    # no capability changes
sysctl, nfsservctl               # no kernel parameter changes
kexec_load, kexec_file_load      # no kernel replacement
iopl, ioperm                     # no direct I/O port access
perf_event_open                  # no performance counters (in retail builds)

ioctl restrictions

ioctl is allowed only with these device categories:

• Wayland socket (AF_UNIX)
• /dev/dri/renderD* (DRI render node — needed for EGL)
• /dev/dri/card* — blocked (prevents DRM master)

7. Landlock Filesystem Restrictions

Requires Linux ≥ 5.13 (ROG Ally ships with kernels that support this). Falls back to logging-only if unavailable.

Allowed paths for game processes

Denied (implicitly — not in allowed set)

• /data/games/<other-game-id>/ — other games
• /data/saves/<other-game-id>/ — other games' saves
• /data/config/ — system configuration
• /data/log/ — system logs (games write via playos_log(), not direct fs access)
• /run/playos/control.sock — control IPC
• /run/playos/compositor.sock — compositor control
• /sys/, /proc/<other-pid>/ — system and process snooping
• /dev/dri/card* — DRM primary nodes

8. Input Security

Current gap (pre-Sprint 12): The target model below is not yet implemented. Today reserved buttons are stripped only by a software bitmask in libplayos (playos_input.c), and games are spawned via a plain fork()+exec() from PID 1 (root) with no credential drop, so a game can open /dev/input/event* directly and read the reserved buttons — bypassing the mask. Sprint 12 closes this gap (see Sprint-12.md §Input Device Isolation).

Reserved system actions (PLAYOS_BUTTON_SYSTEM, PLAYOS_BUTTON_QUICK_MENU) are intercepted at the Wayland compositor's libinput layer before any event reaches a client. They are never present in the game's input stream.

Input routing hierarchy:

libinput event
    │
    ▼ playos-compositor intercepts
    ├── reserved action → PlayOS only (never to client)
    ├── overlay visible → overlay Wayland client
    ├── game foreground → game Wayland client (filtered: no reserved keys)
    └── otherwise      → shell Wayland client

Games receive input exclusively through the Wayland seat (not raw evdev). They cannot open /dev/input/event* (not in the input group; Landlock also blocks the path).

9. System Image Integrity

Sprint 11 (initial): Read-only mount

mount(device, "/", "ext4", MS_RDONLY, NULL);

Any write to the system partition returns EROFS.

Post-Sprint 12 (production): dm-verity

# At build time (in playos-refdistro release pipeline):
veritysetup format system.img system.img.verity > system.verity.superblock

At boot, playos-init:

1. Sets up a dm-verity device over the system partition
2. Mounts the dm-verity device read-only
3. If hash verification fails for any block, the kernel returns I/O errors (enforced by dm-verity)
4. playos-init monitors for dm-verity errors; repeated errors trigger A/B rollback

10. Secure Boot Chain

Target signing chain (post-MVP, Sprint 12 foundations)

UEFI Secure Boot (platform key)
    └── signs BOOTX64.EFI

BOOTX64.EFI (Linux EFI stub)
    └── kernel + embedded initramfs (verified by EFI stub signature)

Kernel (IMA or dm-verity)
    └── system partition hash tree (dm-verity root hash embedded in initramfs)

A/B update bundles
    └── signed with PlayOS update key (RAUC bundle signature)

Development key setup (Sprint 12)

• Self-signed certificate used for development and CI builds
• Production: HSM-backed key, never leaves the signing server
• sbsign used in the release pipeline

Recovery

If Secure Boot verification fails:

1. UEFI firmware refuses to boot the artifact
2. User must boot into UEFI Secure Boot key management to enroll the PlayOS development key (dev builds)
3. Production: chain-of-trust failure surfaces as boot failure → A/B rollback → recovery mode

11. Development vs Production

The post-build production lint CI step asserts:

• No /bin/sh or /bin/busybox in the image
• No open listening TCP sockets
• No gdbserver, strace, or debug tools
• System partition is read-only
• All EFI artifacts are signed

12. Post-MVP Hardening Roadmap

In priority order after v0.1.0:

1. dm-verity for system partition integrity (Sprint 12 gap)
2. Signed game manifests (Ed25519 — warn-only in Sprint 12, enforced post-MVP)
3. User namespaces for additional game isolation if needed
4. Hardware-backed keys for update signing
5. IMA/EVM for individual file integrity in the initramfs
6. Audit logging for privileged IPC commands
7. Network namespace for games (when networking is introduced)
8. Mandatory access control (SELinux or AppArmor) if seccomp + Landlock proves insufficient