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package main
import (
"bytes"
"fmt"
"log"
"os"
"path"
"slices"
"strconv"
"strings"
"syscall"
)
const (
compPoison = "INVALIDINVALIDINVALIDINVALIDINVALID"
fsuConfFile = "/etc/fsurc"
envShim = "FORTIFY_SHIM"
envAID = "FORTIFY_APP_ID"
envGroups = "FORTIFY_GROUPS"
PR_SET_NO_NEW_PRIVS = 0x26
)
var (
Fmain = compPoison
)
func main() {
log.SetFlags(0)
log.SetPrefix("fsu: ")
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 fmain string
if p, ok := checkPath(Fmain); !ok {
log.Fatal("invalid fortify path, this copy of fsu is not compiled correctly")
} else {
fmain = p
}
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("fortify executable has been deleted")
} else if p != fmain {
log.Fatal("this program must be started by fortify")
}
// uid = 1000000 +
// fid * 10000 +
// aid
uid := 1000000
// authenticate before accepting user input
if fid, ok := parseConfig(fsuConfFile, puid); !ok {
log.Fatalf("uid %d is not in the fsurc file", puid)
} else {
uid += fid * 10000
}
// allowed aid range 0 to 9999
if as, ok := os.LookupEnv(envAID); !ok {
log.Fatal("FORTIFY_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 {
// fortify 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("FORTIFY_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 fsu
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(fmain, []string{"fortify", "shim"}, []string{envShim + "=" + shimSetupFd}); err != nil {
log.Fatalf("cannot start shim: %v", err)
}
panic("unreachable")
}
func checkPath(p string) (string, bool) {
return p, p != compPoison && p != "" && path.IsAbs(p)
}
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