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Ineffective attempt making sftp work with pam

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Signed-off-by: Percy Wegmann <percy@tailscale.com>
This commit is contained in:
Percy Wegmann 2024-05-01 19:31:15 -05:00
parent 2cc47f441c
commit e5a7b55ea2
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GPG Key ID: 29D8CDEB4C13D48B
3 changed files with 135 additions and 68 deletions
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116
ssh/tailssh/incubator.go
View File

@ -104,6 +104,12 @@ func (ss *sshSession) newIncubatorCommand() (cmd *exec.Cmd) {
remoteUser = strings.Join(ci.node.Tags().AsSlice(), ",")
}
incubatorArgs := ss.buildIncubatorArgs(lu, gids, remoteUser, ci.src.Addr().String(), isSFTP, isShell, name, args)
log.Printf("ZZZZ incubator args are: %+s", incubatorArgs)
return exec.CommandContext(ss.ctx, ss.conn.srv.tailscaledPath, incubatorArgs...)
}
func (ss *sshSession) buildIncubatorArgs(lu *userMeta, gids, remoteUser, remoteIP string, isSFTP, isShell bool, cmdName string, cmdArgs []string) []string {
incubatorArgs := []string{
"be-child",
"ssh",
@ -112,7 +118,7 @@ func (ss *sshSession) newIncubatorCommand() (cmd *exec.Cmd) {
"--groups=" + gids,
"--local-user=" + lu.Username,
"--remote-user=" + remoteUser,
"--remote-ip=" + ci.src.Addr().String(),
"--remote-ip=" + remoteIP,
"--has-tty=false", // updated in-place by startWithPTY
"--tty-name=", // updated in-place by startWithPTY
}
@ -122,19 +128,34 @@ func (ss *sshSession) newIncubatorCommand() (cmd *exec.Cmd) {
}
if isSFTP {
// We have two ways of running an SFTP server.
// 1. Immediately launch incubator as SFTP server
// 2. Launch incubator to spawn itself as SFTP server inside of a login
// shell.
// The second method is preferred because it allows PAM authentication
// to run.
su, _ := findSUCommand(ss.logf)
if su == "" {
// We can't use su, just serve sftp directly
incubatorArgs = append(incubatorArgs, "--sftp")
} else {
return incubatorArgs
}
// We can use su, tell incubator to spawn itself
cmdName = ss.conn.srv.tailscaledPath
cmdArgs = ss.buildIncubatorArgs(lu, gids, remoteUser, remoteIP, false, false, "", nil)
}
if isShell {
incubatorArgs = append(incubatorArgs, "--shell")
}
incubatorArgs = append(incubatorArgs, "--cmd="+name)
if len(args) > 0 {
incubatorArgs = append(incubatorArgs, "--cmd="+cmdName)
if len(cmdArgs) > 0 {
incubatorArgs = append(incubatorArgs, "--")
incubatorArgs = append(incubatorArgs, args...)
incubatorArgs = append(incubatorArgs, cmdArgs...)
}
}
return exec.CommandContext(ss.ctx, ss.conn.srv.tailscaledPath, incubatorArgs...)
return incubatorArgs
}
var debugIncubator bool
@ -229,6 +250,7 @@ func beIncubator(args []string) error {
defer logFile.Close()
}
}
logf("ZZZZ Being incubator with args %+s", args)
if ia.isSFTP {
return dropPrivilegesAndHandleSFTP(logf, ia)
@ -364,43 +386,15 @@ func tryLoginCmd(logf logger.Logf, ia incubatorArgs) (bool, error) {
// an su command which accepts the right flags, we'll use su instead of login
// when no TTY is available.
func tryLoginWithSU(logf logger.Logf, ia incubatorArgs) (bool, error) {
// Currently, we only support falling back to su on Linux. This
// potentially could work on BSDs as well, but requires testing.
if runtime.GOOS != "linux" {
return false, nil
}
su, err := exec.LookPath("su")
if err != nil {
logf("can't find su command")
return false, nil
}
// Get help text to inspect supported flags.
out, err := exec.Command(su, "-h").CombinedOutput()
if err != nil {
logf("%s doesn't support -h, don't use", su)
return false, nil
}
supportsFlag := func(flag string) bool {
return bytes.Contains(out, []byte(flag))
}
// Make sure su supports the necessary flags.
if !supportsFlag("--login") {
logf("%s doesn't support --login, don't use", su)
return false, nil
}
if !supportsFlag("--command") {
logf("%s doesn't support --command, don't use", su)
su, supportsPTY := findSUCommand(logf)
if su == "" {
return false, nil
}
loginArgs := []string{
"--login",
}
if ia.hasTTY && supportsFlag("--pty") {
if ia.hasTTY && supportsPTY {
// Allocate a pseudo terminal for improved security. In particular,
// this can help avoid TIOCSTI ioctl terminal injection.
loginArgs = append(loginArgs, "--pty")
@ -421,6 +415,52 @@ func tryLoginWithSU(logf logger.Logf, ia incubatorArgs) (bool, error) {
return true, unix.Exec(su, loginArgs, os.Environ())
}
// findSUCommand finds the su command and returns the path to it, if and only
// if.
//
// 1. We're running on Linux
// 2. The su command is on the path
// 3. The su command supports the -h, --login, and --command flags
//
// This also returns a boolean indicating whether or not su supports the --pty
// flag.
func findSUCommand(logf logger.Logf) (string, bool) {
// Currently, we only support using su on Linux. This potentially could
// work on BSDs as well, but requires testing.
if runtime.GOOS != "linux" {
return "", false
}
su, err := exec.LookPath("su")
if err != nil {
logf("can't find su command")
return "", false
}
// Get help text to inspect supported flags.
out, err := exec.Command(su, "-h").CombinedOutput()
if err != nil {
logf("%s doesn't support -h, don't use", su)
return "", false
}
supportsFlag := func(flag string) bool {
return bytes.Contains(out, []byte(flag))
}
// Make sure su supports the necessary flags.
if !supportsFlag("--login") {
logf("%s doesn't support --login, don't use", su)
return "", false
}
if !supportsFlag("--command") {
logf("%s doesn't support --command, don't use", su)
return "", false
}
return su, supportsFlag("--pty")
}
// dropPrivilegesAndRun is a last resort if we couldn't use login or su. It
// drops privileges for the current process, registers a new session with the
// OS, sets its UID, GID and groups to the specified values, and then launches

62
ssh/tailssh/tailssh_integration_test.go
View File

@ -1,9 +1,6 @@
// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
//go:build integrationtest
// +build integrationtest
package tailssh
import (
@ -23,6 +20,7 @@
"net/netip"
"os"
"os/exec"
"path"
"runtime"
"strings"
"testing"
@ -81,25 +79,20 @@ func TestMain(m *testing.M) {
m.Run()
}
var homeDir = "/home/testuser"
func init() {
if runtime.GOOS == "darwin" {
homeDir = "/Users/testuser"
}
}
func TestIntegrationSSH(t *testing.T) {
debugTest.Store(true)
t.Cleanup(func() {
debugTest.Store(false)
})
homeDir := "/home/testuser"
if runtime.GOOS == "darwin" {
homeDir = "/Users/testuser"
}
_, err := exec.LookPath("su")
suPresent := err == nil
// Some operating systems like Fedora seem to require login to be present
// in order for su to work.
_, err = exec.LookPath("login")
loginPresent := err == nil
tests := []struct {
cmd string
want []string
@ -112,15 +105,11 @@ func TestIntegrationSSH(t *testing.T) {
{
cmd: "pwd",
want: []string{homeDir},
skip: runtime.GOOS != "linux" || !suPresent || !loginPresent,
skip: !expectPAMToCreateHomeDir(),
},
}
for _, test := range tests {
if test.skip {
continue
}
// run every test both without and with a shell
for _, shell := range []bool{false, true} {
shellQualifier := "no_shell"
@ -129,6 +118,10 @@ func TestIntegrationSSH(t *testing.T) {
}
t.Run(fmt.Sprintf("%s_%s", test.cmd, shellQualifier), func(t *testing.T) {
if test.skip {
t.SkipNow()
}
s := testSession(t)
if shell {
@ -159,7 +152,12 @@ func TestIntegrationSFTP(t *testing.T) {
debugTest.Store(false)
})
filePath := "/tmp/sftptest.dat"
baseDir := homeDir
if !expectPAMToCreateHomeDir() {
baseDir = "/tmp"
}
filePath := path.Join(baseDir, "sftptest.dat")
wantText := "hello world"
cl := testClient(t)
@ -168,6 +166,11 @@ func TestIntegrationSFTP(t *testing.T) {
t.Fatalf("can't get sftp client: %s", err)
}
_, err = scl.Stat(baseDir)
if err != nil {
t.Fatalf("can't stat %s: %s", homeDir, err)
}
file, err := scl.Create(filePath)
if err != nil {
t.Fatalf("can't create file: %s", err)
@ -203,6 +206,21 @@ func TestIntegrationSFTP(t *testing.T) {
}
}
// expectPAMToCreateHomeDir tells us whether the test can expect pam_mkhomedir
// to actually make the home directory. This returns true if and only if both
// su and login commands are available on the path.
func expectPAMToCreateHomeDir() bool {
_, err := exec.LookPath("su")
suPresent := err == nil
// Some operating systems like Fedora seem to require login to be present
// in order for su to work.
_, err = exec.LookPath("login")
loginPresent := err == nil
return suPresent && loginPresent
}
type session struct {
*ssh.Session

17
ssh/tailssh/testcontainers/Dockerfile
View File

@ -15,8 +15,14 @@ RUN authconfig --enablemkhomedir --update || echo "might not be fedora"
COPY . .
# RUN echo "Test doDropPrivileges once"
# RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.run TestDoDropPrivileges
RUN echo "First run tests normally."
RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.run TestIntegration TestDoDropPrivileges
# RUN rm -Rf /home/testuser
# RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.run TestIntegrationSSH
RUN rm -Rf /home/testuser
RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.run TestIntegrationSFTP
RUN echo "Then run tests as non-root user testuser and make sure tests still pass."
RUN chown testuser:groupone /tmp/tailscalessh.log
@ -26,11 +32,14 @@ RUN echo "Then remove the login command and make sure tests still pass."
RUN chown root:root /tmp/tailscalessh.log
RUN rm `which login`
RUN rm -Rf /home/testuser
RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.run TestIntegration TestDoDropPrivileges
RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.run TestIntegrationSSH
RUN rm -Rf /home/testuser
RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.run TestIntegrationSFTP
RUN echo "Then remove the su command and make sure tests still pass."
RUN ls -l /tmp/sftptest.dat
RUN chown root:root /tmp/tailscalessh.log
RUN rm `which su`
RUN rm -Rf /home/testuser
RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.run TestIntegration TestDoDropPrivileges
RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.run TestIntegrationSSH
RUN rm -Rf /home/testuser
RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.run TestIntegrationSFTP