fsu: implement simple setuid user switcher

Contains path to fortify set at compile time, authenticates based on a simple uid range assignment file which also acts as the allow list.

Signed-off-by: Ophestra Umiker <cat@ophivana.moe>

cmd/fsu/main.go

@@ -0,0 +1,131 @@
+package main
+
+import (
+	"bufio"
+	"log"
+	"os"
+	"path"
+	"strconv"
+	"strings"
+	"syscall"
+)
+
+const (
+	fsuConfFile = "/etc/fsurc"
+	envShim     = "FORTIFY_SHIM"
+	envAID      = "FORTIFY_APP_ID"
+
+	fpPoison = "INVALIDINVALIDINVALIDINVALIDINVALID"
+)
+
+// FortifyPath is the path to fortify, set at compile time.
+var FortifyPath = fpPoison
+
+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")
+	}
+
+	// check compiled in fortify path
+	if FortifyPath == fpPoison || !path.IsAbs(FortifyPath) {
+		log.Fatal("invalid fortify path, this copy of fsu is not compiled correctly")
+	}
+
+	// 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
+	}
+
+	// pass through setup path to shim
+	var shimSetupPath string
+	if s, ok := os.LookupEnv(envShim); !ok {
+		log.Fatal("FORTIFY_SHIM not set")
+	} else if !path.IsAbs(s) {
+		log.Fatal("FORTIFY_SHIM is not absolute")
+	} else {
+		shimSetupPath = s
+	}
+
+	// allowed aid range 0 to 9999
+	if as, ok := os.LookupEnv(envAID); !ok {
+		log.Fatal("FORTIFY_APP_ID not set")
+	} else if aid, err := strconv.Atoi(as); err != nil || aid < 0 || aid > 9999 {
+		log.Fatal("invalid aid")
+	} else {
+		uid += aid
+	}
+
+	if err := syscall.Setresgid(uid, uid, uid); err != nil {
+		log.Fatalf("cannot set gid: %v", err)
+	}
+	if err := syscall.Setresuid(uid, uid, uid); err != nil {
+		log.Fatalf("cannot set uid: %v", err)
+	}
+	if err := syscall.Exec(FortifyPath, []string{"fortify", "shim"}, []string{envShim + "=" + shimSetupPath}); err != nil {
+		log.Fatalf("cannot start shim: %v", err)
+	}
+
+	panic("unreachable")
+}
+
+func parseConfig(p string, puid int) (fid int, ok bool) {
+	// refuse to run if fsurc is not protected correctly
+	if s, err := os.Stat(p); err != nil {
+		log.Fatal(err)
+	} else if s.Mode().Perm() != 0400 {
+		log.Fatal("bad fsurc perm")
+	} else if st := s.Sys().(*syscall.Stat_t); st.Uid != 0 || st.Gid != 0 {
+		log.Fatal("fsurc must be owner by uid 0")
+	}
+
+	if r, err := os.Open(p); err != nil {
+		log.Fatal(err)
+		return -1, false
+	} else {
+		s := bufio.NewScanner(r)
+		var line int
+		for s.Scan() {
+			line++
+
+			// <puid> <fid>
+			lf := strings.SplitN(s.Text(), " ", 2)
+			if len(lf) != 2 {
+				log.Fatalf("invalid entry on line %d", line)
+			}
+
+			var puid0 int
+			if puid0, err = strconv.Atoi(lf[0]); err != nil || puid0 < 1 {
+				log.Fatalf("invalid parent uid on line %d", line)
+			}
+
+			ok = puid0 == puid
+			if ok {
+				// allowed fid range 0 to 99
+				if fid, err = strconv.Atoi(lf[1]); err != nil || fid < 0 || fid > 99 {
+					log.Fatalf("invalid fortify uid on line %d", line)
+				}
+				return
+			}
+		}
+		if err = s.Err(); err != nil {
+			log.Fatalf("cannot read fsurc: %v", err)
+		}
+		return -1, false
+	}
+}

flake.nix

@@ -34,9 +34,7 @@
 
       devShells = forAllSystems (system: {
         default = nixpkgsFor.${system}.mkShell {
-          buildInputs =
+          buildInputs = with nixpkgsFor.${system}; self.packages.${system}.fortify.buildInputs;
-            with nixpkgsFor.${system};
-            self.packages.${system}.fortify.buildInputs;
         };
 
         withPackage = nixpkgsFor.${system}.mkShell {

package.nix

@@ -20,6 +20,8 @@ buildGoModule rec {
     "-w"
     "-X"
     "main.Version=v${version}"
+    "-X"
+    "main.FortifyPath=${placeholder "out"}/bin/fortify"
   ];
 
   buildInputs = [
@@ -36,5 +38,7 @@ buildGoModule rec {
         xdg-dbus-proxy
       ]
     }
+
+    mv $out/bin/fsu $out/bin/.fsu
   '';
 }