mesh-host/internal/accounts/exec.go

package accounts

import (
	"context"
	"errors"
	"fmt"
	"io/fs"
	"os"
	"os/exec"
	"path/filepath"
	"slices"
	"strconv"
	"strings"
	"syscall"
)

// Runner runs a command and answers what it printed — the apply's own (apply.ExecRunner).
type Runner func(ctx context.Context, name string, args ...string) (string, error)

// Exec reads the machine through its command lines and the process table. **Reads only**: `id`, `getent`,
// the machine's own manager's `systemctl show`, `sudo -l -U` (which lists, as root, what sudo would let an
// account run, and runs nothing), a status file under /proc and a secret's owner and mode (a test holds
// both).
type Exec struct {
	Run Runner
	// Proc is where the process table is; empty is /proc. A test points it at a directory of its own.
	Proc string
	// Stat is a file's owner and mode; nil is the machine's own (os.Stat). A test gives files of its own.
	Stat func(path string) (FileMode, error)
}

// FileMode is what decides whether an account reads a file: its owner, its group and its permission bits.
type FileMode struct {
	UID, GID int
	Perm     fs.FileMode
}

// Escalation is every way account can become root without a person (novox/hq ADR 0266): its uid, a group
// of RootGroups the user database lists it in, any sudo rule naming it or a group of it, and any of secrets
// it can read by owner, group or other bits. A secret not there yet is skipped: there is nothing to read.
// The parent directories are not walked, so a file the bits allow and a directory hides is still said:
// the judge errs toward saying a way that is not, never toward missing one that is.
func (e Exec) Escalation(ctx context.Context, account string, secrets []string) ([]string, error) {
	var ways []string
	uidOut, err := e.Run(ctx, "id", "-u", account)
	if err != nil {
		return nil, fmt.Errorf("the user database did not answer about %q: %w", account, err)
	}
	uid, err := strconv.Atoi(strings.TrimSpace(uidOut))
	if err != nil {
		return nil, fmt.Errorf("the user database gave %q as %q's number", strings.TrimSpace(uidOut), account)
	}
	if uid == 0 {
		ways = append(ways, "its uid is 0")
	}
	names, err := e.InDatabase(ctx, account)
	if err != nil {
		return nil, err
	}
	for _, g := range names {
		if slices.Contains(RootGroups, g) {
			ways = append(ways, "in the group "+g+", which grants root")
		}
	}
	listed, err := e.Run(ctx, "sudo", "-l", "-U", account)
	rules, err := SudoRules(listed, err)
	if err != nil {
		return nil, err
	}
	if len(rules) > 0 {
		ways = append(ways, "sudo grants it: "+strings.Join(rules, ", "))
	}
	if len(secrets) > 0 {
		gidsOut, err := e.Run(ctx, "id", "-G", account)
		if err != nil {
			return nil, fmt.Errorf("the user database did not answer about %q's groups: %w", account, err)
		}
		gids := map[int]bool{}
		for _, f := range strings.Fields(gidsOut) {
			if n, err := strconv.Atoi(f); err == nil {
				gids[n] = true
			}
		}
		stat := e.Stat
		if stat == nil {
			stat = statOf
		}
		for _, path := range secrets {
			m, err := stat(path)
			if errors.Is(err, fs.ErrNotExist) {
				continue
			}
			if err != nil {
				return nil, fmt.Errorf("the secret %s could not be read for its owner and mode: %w", path, err)
			}
			if Readable(m, uid, gids) {
				ways = append(ways, "it can read the secret "+path)
			}
		}
	}
	return ways, nil
}

// Readable is whether an account of uid, in the groups gids, reads a file of m by its permission bits, as
// the kernel decides it: the owner's bits for the owner (root reads everything), the group's for a member,
// the others' for anybody else.
func Readable(m FileMode, uid int, gids map[int]bool) bool {
	switch {
	case uid == 0:
		return true
	case m.UID == uid:
		return m.Perm&0o400 != 0
	case gids[m.GID]:
		return m.Perm&0o040 != 0
	default:
		return m.Perm&0o004 != 0
	}
}

// SudoRules is the rules `sudo -l -U <account>` lists, from what it printed and how it ended: none when
// the account "is not allowed to run sudo" or sudo is not on the machine; every indented line after "may
// run the following commands" otherwise. Any rule counts — the decision is no sudo for the account at
// all, so a rule that asks for a password the account was never given is still a rule somebody can give
// it one for. Output that says neither is an error: unread is never none.
func SudoRules(out string, err error) ([]string, error) {
	if err != nil && (errors.Is(err, exec.ErrNotFound) || errors.Is(err, fs.ErrNotExist)) {
		return nil, nil
	}
	text := out
	if err != nil {
		text += "\n" + err.Error()
	}
	if strings.Contains(text, "is not allowed to run sudo") {
		return nil, nil
	}
	if err != nil {
		return nil, fmt.Errorf("sudo did not list the account's rules: %w", err)
	}
	var rules []string
	listing := false
	for _, line := range strings.Split(out, "\n") {
		if strings.Contains(line, "may run the following commands") {
			listing = true
			continue
		}
		if listing && (strings.HasPrefix(line, " ") || strings.HasPrefix(line, "\t")) && strings.TrimSpace(line) != "" {
			rules = append(rules, strings.TrimSpace(line))
		}
	}
	if !listing {
		return nil, fmt.Errorf("sudo listed neither rules nor a refusal: %q", firstLine(out))
	}
	return rules, nil
}

func statOf(path string) (FileMode, error) {
	info, err := os.Stat(path)
	if err != nil {
		return FileMode{}, err
	}
	st, ok := info.Sys().(*syscall.Stat_t)
	if !ok {
		return FileMode{}, fmt.Errorf("%s has no owner this platform reports", path)
	}
	return FileMode{UID: int(st.Uid), GID: int(st.Gid), Perm: info.Mode().Perm()}, nil
}

// InDatabase is `id -nG`: every group the user database lists the account in.
func (e Exec) InDatabase(ctx context.Context, account string) ([]string, error) {
	out, err := e.Run(ctx, "id", "-nG", account)
	if err != nil {
		return nil, err
	}
	return strings.Fields(out), nil
}

// Session reads the account's own manager, user@<uid>.service, from the machine's manager — never from
// the account's, which asking would start — and the groups its process holds, from its status file.
func (e Exec) Session(ctx context.Context, account string, groups []string) (Session, error) {
	passwd, err := e.Run(ctx, "getent", "passwd", account)
	if err != nil {
		return Session{}, fmt.Errorf("the user database did not answer about %q: %w", account, err)
	}
	fields := strings.Split(strings.TrimSpace(passwd), ":")
	if len(fields) < 7 || fields[2] == "" {
		return Session{}, fmt.Errorf("the user database gave no number for %q", account)
	}
	shown, err := e.Run(ctx, "systemctl", "show", "--property=MainPID", "--value", "user@"+fields[2]+".service")
	if err != nil {
		return Session{}, fmt.Errorf("the machine's service manager did not say whether %q's own runs: %w", account, err)
	}
	pid := strings.TrimSpace(shown)
	if pid == "" || pid == "0" {
		return Session{}, nil
	}
	held, err := e.heldBy(pid)
	if err != nil {
		return Session{}, err
	}
	s := Session{Running: true, Has: map[string]bool{}}
	for _, g := range groups {
		entry, err := e.Run(ctx, "getent", "group", g)
		if err != nil {
			return Session{}, fmt.Errorf("the group database did not answer about %q: %w", g, err)
		}
		parts := strings.Split(strings.TrimSpace(entry), ":")
		if len(parts) < 3 {
			return Session{}, fmt.Errorf("the group database gave %q for %q, which is not a group entry", entry, g)
		}
		s.Has[g] = held[parts[2]]
	}
	return s, nil
}

// heldBy is every group id a process holds, from the Groups line of its status file.
func (e Exec) heldBy(pid string) (map[string]bool, error) {
	proc := e.Proc
	if proc == "" {
		proc = "/proc"
	}
	raw, err := os.ReadFile(filepath.Join(proc, pid, "status"))
	if errors.Is(err, os.ErrNotExist) {
		return nil, fmt.Errorf("the account's manager, process %s, ended while it was read", pid)
	}
	if err != nil {
		return nil, err
	}
	// Its supplementary groups, and its own group, which the supplementary list need not repeat.
	held := map[string]bool{}
	named := false
	for _, line := range strings.Split(string(raw), "\n") {
		if rest, ok := strings.CutPrefix(line, "Groups:"); ok {
			named = true
			for _, gid := range strings.Fields(rest) {
				held[gid] = true
			}
		}
		if rest, ok := strings.CutPrefix(line, "Gid:"); ok {
			if f := strings.Fields(rest); len(f) > 0 {
				held[f[0]] = true
			}
		}
	}
	if !named {
		return nil, fmt.Errorf("process %s's status names no groups", pid)
	}
	return held, nil
}