treewide: rename to hakurei

Fortify makes little sense for a container tool.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/hsu/main.go

@@ -0,0 +1,146 @@
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"log"
+	"os"
+	"path"
+	"slices"
+	"strconv"
+	"strings"
+	"syscall"
+)
+
+const (
+	hsuConfFile = "/etc/hsurc"
+	envShim     = "HAKUREI_SHIM"
+	envAID      = "HAKUREI_APP_ID"
+	envGroups   = "HAKUREI_GROUPS"
+
+	PR_SET_NO_NEW_PRIVS = 0x26
+)
+
+func main() {
+	log.SetFlags(0)
+	log.SetPrefix("hsu: ")
+	log.SetOutput(os.Stderr)
+
+	if os.Geteuid() != 0 {
+		log.Fatal("this program must be owned by uid 0 and have the setuid bit set")
+	}
+
+	puid := os.Getuid()
+	if puid == 0 {
+		log.Fatal("this program must not be started by root")
+	}
+
+	var toolPath string
+	pexe := path.Join("/proc", strconv.Itoa(os.Getppid()), "exe")
+	if p, err := os.Readlink(pexe); err != nil {
+		log.Fatalf("cannot read parent executable path: %v", err)
+	} else if strings.HasSuffix(p, " (deleted)") {
+		log.Fatal("hakurei executable has been deleted")
+	} else if p != mustCheckPath(hmain) && p != mustCheckPath(fpkg) {
+		log.Fatal("this program must be started by hakurei")
+	} else {
+		toolPath = p
+	}
+
+	// uid = 1000000 +
+	//   fid * 10000 +
+	//           aid
+	uid := 1000000
+
+	// refuse to run if hsurc is not protected correctly
+	if s, err := os.Stat(hsuConfFile); err != nil {
+		log.Fatal(err)
+	} else if s.Mode().Perm() != 0400 {
+		log.Fatal("bad hsurc perm")
+	} else if st := s.Sys().(*syscall.Stat_t); st.Uid != 0 || st.Gid != 0 {
+		log.Fatal("hsurc must be owned by uid 0")
+	}
+
+	// authenticate before accepting user input
+	if f, err := os.Open(hsuConfFile); err != nil {
+		log.Fatal(err)
+	} else if fid, ok := mustParseConfig(f, puid); !ok {
+		log.Fatalf("uid %d is not in the hsurc file", puid)
+	} else {
+		uid += fid * 10000
+	}
+
+	// allowed aid range 0 to 9999
+	if as, ok := os.LookupEnv(envAID); !ok {
+		log.Fatal("HAKUREI_APP_ID not set")
+	} else if aid, err := parseUint32Fast(as); err != nil || aid < 0 || aid > 9999 {
+		log.Fatal("invalid aid")
+	} else {
+		uid += aid
+	}
+
+	// pass through setup fd to shim
+	var shimSetupFd string
+	if s, ok := os.LookupEnv(envShim); !ok {
+		// hakurei requests target uid
+		// print resolved uid and exit
+		fmt.Print(uid)
+		os.Exit(0)
+	} else if len(s) != 1 || s[0] > '9' || s[0] < '3' {
+		log.Fatal("HAKUREI_SHIM holds an invalid value")
+	} else {
+		shimSetupFd = s
+	}
+
+	// supplementary groups
+	var suppGroups, suppCurrent []int
+
+	if gs, ok := os.LookupEnv(envGroups); ok {
+		if cur, err := os.Getgroups(); err != nil {
+			log.Fatalf("cannot get groups: %v", err)
+		} else {
+			suppCurrent = cur
+		}
+
+		// parse space-separated list of group ids
+		gss := bytes.Split([]byte(gs), []byte{' '})
+		suppGroups = make([]int, len(gss)+1)
+		for i, s := range gss {
+			if gid, err := strconv.Atoi(string(s)); err != nil {
+				log.Fatalf("cannot parse %q: %v", string(s), err)
+			} else if gid > 0 && gid != uid && gid != os.Getgid() && slices.Contains(suppCurrent, gid) {
+				suppGroups[i] = gid
+			} else {
+				log.Fatalf("invalid gid %d", gid)
+			}
+		}
+		suppGroups[len(suppGroups)-1] = uid
+	} else {
+		suppGroups = []int{uid}
+	}
+
+	// final bounds check to catch any bugs
+	if uid < 1000000 || uid >= 2000000 {
+		panic("uid out of bounds")
+	}
+
+	// careful! users in the allowlist is effectively allowed to drop groups via hsu
+
+	if err := syscall.Setresgid(uid, uid, uid); err != nil {
+		log.Fatalf("cannot set gid: %v", err)
+	}
+	if err := syscall.Setgroups(suppGroups); err != nil {
+		log.Fatalf("cannot set supplementary groups: %v", err)
+	}
+	if err := syscall.Setresuid(uid, uid, uid); err != nil {
+		log.Fatalf("cannot set uid: %v", err)
+	}
+	if _, _, errno := syscall.AllThreadsSyscall(syscall.SYS_PRCTL, PR_SET_NO_NEW_PRIVS, 1, 0); errno != 0 {
+		log.Fatalf("cannot set no_new_privs flag: %s", errno.Error())
+	}
+	if err := syscall.Exec(toolPath, []string{"hakurei", "shim"}, []string{envShim + "=" + shimSetupFd}); err != nil {
+		log.Fatalf("cannot start shim: %v", err)
+	}
+
+	panic("unreachable")
+}