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tailscale/tailfs/tailfsimpl/remote_impl.go

413 lines
11 KiB
Go

// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
package tailfsimpl
import (
"bufio"
"context"
"encoding/hex"
"fmt"
"log"
"math"
"net"
"net/http"
"net/netip"
"net/url"
"os"
"os/exec"
"os/user"
"slices"
"strings"
"sync"
"time"
"github.com/tailscale/xnet/webdav"
"tailscale.com/safesocket"
"tailscale.com/tailfs"
"tailscale.com/tailfs/tailfsimpl/compositedav"
"tailscale.com/tailfs/tailfsimpl/dirfs"
"tailscale.com/tailfs/tailfsimpl/shared"
"tailscale.com/types/logger"
)
func NewFileSystemForRemote(logf logger.Logf) *FileSystemForRemote {
if logf == nil {
logf = log.Printf
}
fs := &FileSystemForRemote{
logf: logf,
lockSystem: webdav.NewMemLS(),
children: make(map[string]*compositedav.Child),
userServers: make(map[string]*userServer),
}
return fs
}
// FileSystemForRemote implements tailfs.FileSystemForRemote.
type FileSystemForRemote struct {
logf logger.Logf
lockSystem webdav.LockSystem
// mu guards the below values. Acquire a write lock before updating any of
// them, acquire a read lock before reading any of them.
mu sync.RWMutex
fileServerAddr string
shares []*tailfs.Share
children map[string]*compositedav.Child
userServers map[string]*userServer
}
// SetFileServerAddr implements tailfs.FileSystemForRemote.
func (s *FileSystemForRemote) SetFileServerAddr(addr string) {
s.mu.Lock()
s.fileServerAddr = addr
s.mu.Unlock()
}
// SetShares implements tailfs.FileSystemForRemote. Shares must be sorted
// according to tailfs.CompareShares.
func (s *FileSystemForRemote) SetShares(shares []*tailfs.Share) {
userServers := make(map[string]*userServer)
if tailfs.AllowShareAs() {
// Set up per-user server by running the current executable as an
// unprivileged user in order to avoid privilege escalation.
executable, err := os.Executable()
if err != nil {
s.logf("can't find executable: %v", err)
return
}
for _, share := range shares {
p, found := userServers[share.As]
if !found {
p = &userServer{
logf: s.logf,
username: share.As,
executable: executable,
}
userServers[share.As] = p
}
p.shares = append(p.shares, share)
}
for _, p := range userServers {
go p.runLoop()
}
}
children := make(map[string]*compositedav.Child, len(shares))
for _, share := range shares {
children[share.Name] = s.buildChild(share)
}
s.mu.Lock()
s.shares = shares
oldUserServers := s.userServers
oldChildren := s.children
s.children = children
s.userServers = userServers
s.mu.Unlock()
s.stopUserServers(oldUserServers)
s.closeChildren(oldChildren)
}
func (s *FileSystemForRemote) buildChild(share *tailfs.Share) *compositedav.Child {
return &compositedav.Child{
Child: &dirfs.Child{
Name: share.Name,
},
BaseURL: fmt.Sprintf("http://%v/%v", hex.EncodeToString([]byte(share.Name)), url.PathEscape(share.Name)),
Transport: &http.Transport{
Dial: func(_, shareAddr string) (net.Conn, error) {
shareNameHex, _, err := net.SplitHostPort(shareAddr)
if err != nil {
return nil, fmt.Errorf("unable to parse share address %v: %w", shareAddr, err)
}
// We had to encode the share name in hex to make sure it's a valid hostname
shareNameBytes, err := hex.DecodeString(shareNameHex)
if err != nil {
return nil, fmt.Errorf("unable to decode share name from host %v: %v", shareNameHex, err)
}
shareName := string(shareNameBytes)
s.mu.RLock()
var share *tailfs.Share
i, shareFound := slices.BinarySearchFunc(s.shares, shareName, func(s *tailfs.Share, name string) int {
return strings.Compare(s.Name, name)
})
if shareFound {
share = s.shares[i]
}
userServers := s.userServers
fileServerAddr := s.fileServerAddr
s.mu.RUnlock()
if !shareFound {
return nil, fmt.Errorf("unknown share %v", shareName)
}
var addr string
if !tailfs.AllowShareAs() {
addr = fileServerAddr
} else {
userServer, found := userServers[share.As]
if found {
userServer.mu.RLock()
addr = userServer.addr
userServer.mu.RUnlock()
}
}
if addr == "" {
return nil, fmt.Errorf("unable to determine address for share %v", shareName)
}
_, err = netip.ParseAddrPort(addr)
if err == nil {
// this is a regular network address, dial normally
return net.Dial("tcp", addr)
}
// assume this is a safesocket address
return safesocket.Connect(addr)
},
},
}
}
// ServeHTTPWithPerms implements tailfs.FileSystemForRemote.
func (s *FileSystemForRemote) ServeHTTPWithPerms(permissions tailfs.Permissions, w http.ResponseWriter, r *http.Request) {
isWrite := writeMethods[r.Method]
if isWrite {
share := shared.CleanAndSplit(r.URL.Path)[0]
switch permissions.For(share) {
case tailfs.PermissionNone:
// If we have no permissions to this share, treat it as not found
// to avoid leaking any information about the share's existence.
http.Error(w, "not found", http.StatusNotFound)
return
case tailfs.PermissionReadOnly:
http.Error(w, "permission denied", http.StatusForbidden)
return
}
}
s.mu.RLock()
childrenMap := s.children
s.mu.RUnlock()
children := make([]*compositedav.Child, 0, len(childrenMap))
// filter out shares to which the connecting principal has no access
for name, child := range childrenMap {
if permissions.For(name) == tailfs.PermissionNone {
continue
}
children = append(children, child)
}
h := compositedav.Handler{
Logf: s.logf,
}
h.SetChildren("", children...)
h.ServeHTTP(w, r)
}
func (s *FileSystemForRemote) stopUserServers(userServers map[string]*userServer) {
for _, server := range userServers {
if err := server.Close(); err != nil {
s.logf("error closing tailfs user server: %v", err)
}
}
}
func (s *FileSystemForRemote) closeChildren(children map[string]*compositedav.Child) {
for _, child := range children {
child.CloseIdleConnections()
}
}
// Close() implements tailfs.FileSystemForRemote.
func (s *FileSystemForRemote) Close() error {
s.mu.Lock()
userServers := s.userServers
children := s.children
s.userServers = make(map[string]*userServer)
s.children = make(map[string]*compositedav.Child)
s.mu.Unlock()
s.stopUserServers(userServers)
s.closeChildren(children)
return nil
}
// userServer runs tailscaled serve-tailfs to serve webdav content for the
// given Shares. All Shares are assumed to have the same Share.As, and the
// content is served as that Share.As user.
type userServer struct {
logf logger.Logf
shares []*tailfs.Share
username string
executable string
// mu guards the below values. Acquire a write lock before updating any of
// them, acquire a read lock before reading any of them.
mu sync.RWMutex
cmd *exec.Cmd
addr string
closed bool
}
func (s *userServer) Close() error {
s.mu.Lock()
cmd := s.cmd
s.closed = true
s.mu.Unlock()
if cmd != nil && cmd.Process != nil {
return cmd.Process.Kill()
}
// not running, that's okay
return nil
}
func (s *userServer) runLoop() {
maxSleepTime := 30 * time.Second
consecutiveFailures := float64(0)
var timeOfLastFailure time.Time
for {
s.mu.RLock()
closed := s.closed
s.mu.RUnlock()
if closed {
return
}
err := s.run()
now := time.Now()
timeSinceLastFailure := now.Sub(timeOfLastFailure)
timeOfLastFailure = now
if timeSinceLastFailure < maxSleepTime {
consecutiveFailures++
} else {
consecutiveFailures = 1
}
sleepTime := time.Duration(math.Pow(2, consecutiveFailures)) * time.Millisecond
if sleepTime > maxSleepTime {
sleepTime = maxSleepTime
}
s.logf("user server % v stopped with error %v, will try again in %v", s.executable, err, sleepTime)
time.Sleep(sleepTime)
}
}
// Run runs the user server using the configured executable. This function only
// works on UNIX systems, but those are the only ones on which we use
// userServers anyway.
func (s *userServer) run() error {
// set up the command
args := []string{"serve-tailfs"}
for _, s := range s.shares {
args = append(args, s.Name, s.Path)
}
var cmd *exec.Cmd
if s.canSudo() {
s.logf("starting TailFS file server as user %q", s.username)
allArgs := []string{"-n", "-u", s.username, s.executable}
allArgs = append(allArgs, args...)
cmd = exec.Command("sudo", allArgs...)
} else {
// If we were root, we should have been able to sudo as a specific
// user, but let's check just to make sure, since we never want to
// access shared folders as root.
err := s.assertNotRoot()
if err != nil {
return err
}
s.logf("starting TailFS file server as ourselves")
cmd = exec.Command(s.executable, args...)
}
stdout, err := cmd.StdoutPipe()
if err != nil {
return fmt.Errorf("stdout pipe: %w", err)
}
defer stdout.Close()
stderr, err := cmd.StderrPipe()
if err != nil {
return fmt.Errorf("stderr pipe: %w", err)
}
defer stderr.Close()
err = cmd.Start()
if err != nil {
return fmt.Errorf("start: %w", err)
}
s.mu.Lock()
s.cmd = cmd
s.mu.Unlock()
// read address
stdoutScanner := bufio.NewScanner(stdout)
stdoutScanner.Scan()
if stdoutScanner.Err() != nil {
return fmt.Errorf("read addr: %w", stdoutScanner.Err())
}
addr := stdoutScanner.Text()
// send the rest of stdout and stderr to logger to avoid blocking
go func() {
for stdoutScanner.Scan() {
s.logf("tailscaled serve-tailfs stdout: %v", stdoutScanner.Text())
}
}()
stderrScanner := bufio.NewScanner(stderr)
go func() {
for stderrScanner.Scan() {
s.logf("tailscaled serve-tailfs stderr: %v", stderrScanner.Text())
}
}()
s.mu.Lock()
s.addr = strings.TrimSpace(addr)
s.mu.Unlock()
return cmd.Wait()
}
var writeMethods = map[string]bool{
"PUT": true,
"POST": true,
"COPY": true,
"LOCK": true,
"UNLOCK": true,
"MKCOL": true,
"MOVE": true,
"PROPPATCH": true,
}
// canSudo checks wether we can sudo -u the configured executable as the
// configured user by attempting to call the executable with the '-h' flag to
// print help.
func (s *userServer) canSudo() bool {
ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
defer cancel()
if err := exec.CommandContext(ctx, "sudo", "-n", "-u", s.username, s.executable, "-h").Run(); err != nil {
return false
}
return true
}
// assertNotRoot returns an error if the current user has UID 0 or if we cannot
// determine the current user.
//
// On Linux, root users will always have UID 0.
//
// On BSD, root users should always have UID 0.
func (s *userServer) assertNotRoot() error {
u, err := user.Current()
if err != nil {
return fmt.Errorf("assertNotRoot failed to find current user: %s", err)
}
if u.Uid == "0" {
return fmt.Errorf("%q is root", u.Name)
}
return nil
}