Users and permissions: accounts, login, enforced file permissions, setuid, su, passwd, doas

The kernel now checks owner/group/other on open, create, unlink, rename, exec
and path search; honours set-user-id and set-group-id; restricts kill, mount,
reboot and mknod; keeps supplementary groups; and implements chown. devtmpfs
nodes carry owners and modes that chmod/chown keep. Terminals run login, the
image has root and schos, X starts as the ordinary user.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EVRhxi9zpdW2uDUN4FVcao

docs/architecture/README.md

@@ -94,6 +94,7 @@ Read in this order if you are new:
 | [fat.md](fat.md) | FAT12/16/32 with long names: USB sticks, SD cards, the ESP. |
 | [devices.md](devices.md) | Buses, classes, binding, and the vtables consumers talk to. |
 | [usb.md](usb.md) | xHCI, the USB core, and the three class drivers. |
+| [users.md](users.md) | Accounts, login, and the permissions the kernel enforces. |
 | [udev.md](udev.md) | eudev on SchOS: the device table, the `/sys` tree, uevents, datagram sockets. |
 | [graphics.md](graphics.md) | Framebuffer, `/dev/fb0`, text rendering, the console, PSOGM and GMMOS. |
 | [desktop.md](desktop.md) | Xorg on the framebuffer, Window Maker, and what they stand on. |

docs/architecture/desktop.md

@@ -75,4 +75,5 @@ through ACPI, and reach X as a mouse: pointer, click and tap.
 - **Touchpads are mice**: no two-finger scroll, no gestures, no palm rejection.
 - **No resolution change from inside X.** The mode is the console's; `schmode` changes the
   `vgpud` head only, and the loader's menu is where a real panel's mode is chosen.
-- **Not a security boundary.** X runs as root on a system that enforces no file permissions.
+- **Runs as an ordinary user.** `startx` as `schos` works by group permission alone
+  ([users.md](users.md)); X itself is still a single seat with no logind or seatd.

docs/architecture/kernel.md

@@ -435,9 +435,8 @@ allocation over 8 MB mapped straight into a neighbouring live mapping.
 - **No page cache.** `mmap` of a file *reads the range in* at map time, and
   writable `MAP_SHARED` of a file is refused outright rather than faked. Demand
   paging covers anonymous memory only.
-- **Permissions are tracked but not enforced.** uid/gid are maintained honestly
-  so `getuid()`/`setuid()` stop lying, and file access does not consult them;
-  `setpriority` is the one place the uids are checked.
+- **Root is `euid == 0` and nothing finer.** No capabilities; file access, `kill`, `exec`
+  and the privileged calls are enforced as [users.md](users.md) says.
 - **`mprotect` is accepted and not enforced** (`sys_mprotect` returns 0). RELRO is not read-only,
   a `PROT_NONE` guard does not fault, and nothing has `NX` (`EFER.NXE` is never set), so every
   mapping is executable.

docs/architecture/scheduler.md

@@ -102,9 +102,8 @@ tracked. Two rules, with the two errnos POSIX gives them:
   alone. Without it nice is not a courtesy but a way for any process to
   outrank the rest of the system by asking.
 
-Both are inert today, because everything runs as root. They are written now
-because the alternative is growing a priority knob and a permission model in
-separate commits, and the window between the two is exactly the bug.
+Both bite now that there are users ([users.md](users.md)); root is exempt from
+both.
 
 ## Deliberately not a red-black tree
 

docs/architecture/users.md

@@ -0,0 +1,79 @@
+# Users and permissions
+
+SchOS has accounts, a login prompt, and file permissions that the kernel
+enforces. Until now every process was root and `uid` was bookkeeping; this is
+the part that makes it a multi-user system.
+
+## What you meet
+
+- **`login` on tty1–tty5.** `etc/schos-getty` takes the terminal back from
+  whoever had it and runs `login`, which asks who is there and starts their
+  shell; when the shell exits `schinit` starts the prompt again.
+- **tty6 is a root shell with no login**, on purpose: the way in when login
+  itself is what is broken.
+- **Two accounts in the image.** `root`, and `schos` (uid 1000, home
+  `/home/schos`, in `wheel tty video input audio cdrom`). Both have empty
+  passwords, so `login` lets them in without asking — it is a live image.
+  `passwd` sets one.
+- **`su` and `passwd`** (`cmd/su`, `cmd/passwd`) are setuid root and small
+  programs of their own, not toybox applets: toybox is one binary under every
+  name, and a setuid copy of it can be asked for any applet by naming it, as
+  root. `su` to root takes membership in `wheel`, whatever the password.
+- **`doas`** (Alpine's, setuid) with `permit nopass :wheel` in `/etc/doas.conf`.
+
+## What the kernel enforces
+
+| | |
+|---|---|
+| open, create, unlink, rename, link | the usual owner/group/other bits, `w`+`x` on the directory, and the sticky bit (`/tmp`) |
+| path walk | `x` on every directory searched |
+| `exec` | `x` on the file; set-user-id and set-group-id make the new image run as its owner or group, and the saved ids follow |
+| `chmod`, `chown`, `utimensat` | owner or root; a non-root `chown` drops the set-id bits |
+| `access` / `faccessat` | by the *real* ids (`AT_EACCESS` for the effective ones) |
+| `kill`, `tkill`, `tgkill` | root, or a target whose real or saved uid is one of yours. The kernel's own signals (`SIGPIPE`, the terminal's) are nobody's to refuse |
+| `mount`, `umount`, `chroot`, `reboot`, `shutdown`, `setgroups`, `mknod` of a device | root |
+| `setuid` family | root may set anything; anyone else only to an id they already hold |
+
+Root reads and writes everything and executes a file only if some execute bit
+is set. A process's supplementary groups (`setgroups`) are consulted along with
+its effective group. Ownership is stored by SchFS (it always had the fields);
+FAT volumes belong to everyone, since they cannot say otherwise, and `/proc`
+and `/sys` are root's.
+
+`inode_permission()` in `kernel/fs/fs.c` is the one check; the rest of this is
+call sites.
+
+## Devices
+
+`/dev` is built by the kernel and each node has the owner, group and mode in the
+`devnodes[]` table of `kernel/fs/devfs.c`: `null` and `zero` are 0666,
+terminals `root:tty 0660`, `fb0` `root:video`, input `root:input`, raw disks
+`root:disk`, `pci`, `dbg` and `netdev` root's alone. `chmod` and `chown` on a
+node are kept (login claims its terminal this way, and udev sets modes) in an
+override table applied whenever the inode is loaded. A new pseudo-terminal's
+slave belongs to whoever opened `ptmx`.
+
+udev would set a node it has no rule for to 0600, which made `/dev/null`
+root-only, so `etc/udev-schos.rules` (installed as `90-schos.rules`) names the
+kernel's devices with the answers the kernel gave.
+
+## The image
+
+Everything in the image is owned by root: the build host's user means nothing
+inside it. `mkschfs --owners FILE` takes the exceptions, one `path uid gid
+[mode]` per line (`tools/posix/mkaroot.sh` writes it: `/home/schos 1000 1000
+0700`, and the modes of `/tmp`, `/var/tmp` and `/root`). The setuid bits are set
+on `su`, `passwd` and `doas` in the root tree directly, because `apk` run as an
+ordinary user may have dropped them.
+
+## Not done
+
+- **No PAM and no `/etc/securetty`.** `login` is busybox's and checks
+  `/etc/shadow` itself.
+- **No capabilities.** Root is `euid == 0`, all of it.
+- **No per-user `/run`, no logind.** X as a non-root user works (`startx` as `schos`, under
+  QEMU: Window Maker up) by group permission alone — video, input and tty — and
+  `/etc/X11/Xwrapper.config` lets anyone start it, because the wrapper's "console users" test
+  looks for a Linux VT on a standard descriptor that xinit's server child does not keep.
+- **`/proc` and `/sys` hide nothing from other users.**
+- **`mprotect` is still not enforced** ([kernel.md](kernel.md)).