initial commit

.gitignore

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+example/example

LICENSE

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+Copyright (C) 2016 Marius Schellenberger
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+    * Redistributions in binary form must reproduce the above copyright
+      notice, this list of conditions and the following disclaimer in the
+      documentation and/or other materials provided with the distribution.
+    * The names of the authors and/or contributors may not be used to
+      endorse or promote products derived from this software without
+      specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL ston1th BE LIABLE FOR ANY
+DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

README.md

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+# godrop - drop privileges
+
+Godrop is a simple library to drop privileges on linux maschines.

example/main.go

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+package main
+
+import (
+	"fmt"
+	"net"
+	"net/http"
+	"os"
+
+	"git.giftfish.de/ston1th/godrop"
+)
+
+func main() {
+	cfg := godrop.Config{
+		User:  "nobody",
+		Group: "nobody",
+	}
+	err := godrop.Drop(cfg, func() (net.Listener, error) { return net.Listen("tcp", ":80") })
+	if err != nil {
+		fmt.Println(err)
+	}
+
+	l, err := godrop.GetListener()
+	if err != nil {
+		fmt.Println("Failed to listen on FD 3:", err)
+		os.Exit(1)
+	}
+
+	http.Serve(l, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprintf(w, "pid %d\nuid %d\ngid %d", os.Getpid(), os.Getuid(), os.Getgid())
+	}))
+}

godrop.go

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+package godrop
+
+import (
+	"errors"
+	"io/ioutil"
+	"net"
+	"os"
+	"os/exec"
+	"regexp"
+	"strconv"
+	"syscall"
+)
+
+func readFile(name, file string) (id int, err error) {
+	id = -1
+	if name == "" {
+		return
+	}
+	f, err := os.Open(file)
+	if err != nil {
+		return
+	}
+	b, err := ioutil.ReadAll(f)
+	if err != nil {
+		return
+	}
+	r := regexp.MustCompile(name + ":.*:(\\d+)")
+	m := r.FindAllStringSubmatch(string(b), 1)
+	if len(m) == 1 {
+		if len(m[0]) == 2 {
+			return strconv.Atoi(m[0][1])
+		}
+	}
+	return
+}
+
+// UID of user
+func UID(user string) (int, error) {
+	return readFile(user, "/etc/passwd")
+}
+
+// GID of group
+func GID(group string) (int, error) {
+	return readFile(group, "/etc/group")
+}
+
+type Config struct {
+	User   string
+	Group  string
+	Chroot string
+}
+
+// Drop will spawn a new process, hand over the listening socket file descriptor and terminate itself after
+func Drop(c Config, f func() (net.Listener, error)) error {
+	uid, err := UID(c.User)
+	if err != nil {
+		return err
+	}
+	gid, err := GID(c.Group)
+	if err != nil {
+		return err
+	}
+	switch os.Getuid() {
+	case 0:
+		ln, err := f()
+		if err != nil {
+			return err
+		}
+
+		l, ok := ln.(*net.TCPListener)
+		if !ok {
+			return errors.New("godrop: interface conversion failed")
+		}
+		f, err := l.File()
+		if err != nil {
+			return err
+		}
+
+		cmd := exec.Command(os.Args[0], os.Args[1:]...)
+		cmd.ExtraFiles = []*os.File{f}
+		cmd.SysProcAttr = &syscall.SysProcAttr{
+			Chroot: c.Chroot,
+			Credential: &syscall.Credential{
+				Uid: uint32(uid),
+				Gid: uint32(gid),
+			},
+			Setsid: true,
+		}
+
+		if err := cmd.Start(); err != nil {
+			return err
+		}
+
+		cmd.Process.Release()
+		os.Exit(0)
+	case uid:
+		return nil
+	}
+	return errors.New("godrop: droping priviledges failed")
+}
+
+// MultiDrop will spawn a new process, hand over the all listening sockets and terminate itself after
+func MultiDrop(c Config, f func() ([]net.Listener, error)) error {
+	uid, err := UID(c.User)
+	if err != nil {
+		return err
+	}
+	gid, err := GID(c.Group)
+	if err != nil {
+		return err
+	}
+	switch os.Getuid() {
+	case 0:
+		cmd := exec.Command(os.Args[0], os.Args[1:]...)
+		ln, err := f()
+		if err != nil {
+			return err
+		}
+		for _, v := range ln {
+			l, ok := v.(*net.TCPListener)
+			if !ok {
+				return errors.New("godrop: interface conversion failed")
+			}
+			f, err := l.File()
+			if err != nil {
+				return err
+			}
+			cmd.ExtraFiles = append(cmd.ExtraFiles, f)
+		}
+
+		cmd.SysProcAttr = &syscall.SysProcAttr{
+			Chroot: c.Chroot,
+			Credential: &syscall.Credential{
+				Uid: uint32(uid),
+				Gid: uint32(gid),
+			},
+			Setsid: true,
+		}
+
+		if err := cmd.Start(); err != nil {
+			return err
+		}
+
+		cmd.Process.Release()
+		os.Exit(0)
+	case uid:
+		return nil
+	}
+	return errors.New("godrop: droping priviledges failed")
+}
+
+// GetListener returns the listener socket of file descriptor 3
+func GetListener() (net.Listener, error) {
+	return net.FileListener(os.NewFile(3, "[socket]"))
+}
+
+// GetListenerFd returns the listener socket of the given file descriptor
+func GetListenerFd(fd int) (net.Listener, error) {
+	return net.FileListener(os.NewFile(uintptr(fd), "[socket]"))
+}

godrop_test.go

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+package godrop
+
+import "testing"
+
+func TestUID(t *testing.T) {
+	uid, err := UID("root")
+	if err != nil {
+		t.Error(err)
+	}
+	if uid != 0 {
+		t.Error("expected uid '0' got", uid)
+	}
+}
+
+func TestGID(t *testing.T) {
+	gid, err := GID("root")
+	if err != nil {
+		t.Error(err)
+	}
+	if gid != 0 {
+		t.Error("expected gid '0' got", gid)
+	}
+}