132 lines
3.0 KiB
Go
132 lines
3.0 KiB
Go
package main
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import (
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"bufio"
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"log"
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"os"
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"path"
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"strconv"
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"strings"
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"syscall"
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)
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const (
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fsuConfFile = "/etc/fsurc"
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envShim = "FORTIFY_SHIM"
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envAID = "FORTIFY_APP_ID"
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fpPoison = "INVALIDINVALIDINVALIDINVALIDINVALID"
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)
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// FortifyPath is the path to fortify, set at compile time.
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var FortifyPath = fpPoison
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func main() {
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log.SetFlags(0)
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log.SetPrefix("fsu: ")
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log.SetOutput(os.Stderr)
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if os.Geteuid() != 0 {
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log.Fatal("this program must be owned by uid 0 and have the setuid bit set")
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}
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puid := os.Getuid()
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if puid == 0 {
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log.Fatal("this program must not be started by root")
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}
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// check compiled in fortify path
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if FortifyPath == fpPoison || !path.IsAbs(FortifyPath) {
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log.Fatal("invalid fortify path, this copy of fsu is not compiled correctly")
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}
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// uid = 1000000 +
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// fid * 10000 +
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// aid
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uid := 1000000
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// authenticate before accepting user input
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if fid, ok := parseConfig(fsuConfFile, puid); !ok {
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log.Fatalf("uid %d is not in the fsurc file", puid)
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} else {
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uid += fid * 10000
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}
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// pass through setup path to shim
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var shimSetupPath string
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if s, ok := os.LookupEnv(envShim); !ok {
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log.Fatal("FORTIFY_SHIM not set")
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} else if !path.IsAbs(s) {
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log.Fatal("FORTIFY_SHIM is not absolute")
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} else {
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shimSetupPath = s
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}
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// allowed aid range 0 to 9999
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if as, ok := os.LookupEnv(envAID); !ok {
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log.Fatal("FORTIFY_APP_ID not set")
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} else if aid, err := strconv.Atoi(as); err != nil || aid < 0 || aid > 9999 {
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log.Fatal("invalid aid")
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} else {
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uid += aid
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}
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if err := syscall.Setresgid(uid, uid, uid); err != nil {
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log.Fatalf("cannot set gid: %v", err)
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}
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if err := syscall.Setresuid(uid, uid, uid); err != nil {
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log.Fatalf("cannot set uid: %v", err)
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}
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if err := syscall.Exec(FortifyPath, []string{"fortify", "shim"}, []string{envShim + "=" + shimSetupPath}); err != nil {
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log.Fatalf("cannot start shim: %v", err)
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}
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panic("unreachable")
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}
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func parseConfig(p string, puid int) (fid int, ok bool) {
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// refuse to run if fsurc is not protected correctly
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if s, err := os.Stat(p); err != nil {
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log.Fatal(err)
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} else if s.Mode().Perm() != 0400 {
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log.Fatal("bad fsurc perm")
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} else if st := s.Sys().(*syscall.Stat_t); st.Uid != 0 || st.Gid != 0 {
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log.Fatal("fsurc must be owned by uid 0")
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}
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if r, err := os.Open(p); err != nil {
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log.Fatal(err)
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return -1, false
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} else {
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s := bufio.NewScanner(r)
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var line int
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for s.Scan() {
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line++
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// <puid> <fid>
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lf := strings.SplitN(s.Text(), " ", 2)
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if len(lf) != 2 {
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log.Fatalf("invalid entry on line %d", line)
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}
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var puid0 int
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if puid0, err = strconv.Atoi(lf[0]); err != nil || puid0 < 1 {
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log.Fatalf("invalid parent uid on line %d", line)
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}
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ok = puid0 == puid
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if ok {
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// allowed fid range 0 to 99
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if fid, err = strconv.Atoi(lf[1]); err != nil || fid < 0 || fid > 99 {
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log.Fatalf("invalid fortify uid on line %d", line)
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}
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return
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}
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}
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if err = s.Err(); err != nil {
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log.Fatalf("cannot read fsurc: %v", err)
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}
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return -1, false
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}
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}
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