From 311535065e0144275c7f887d6dc4fe1ab9f8b17e Mon Sep 17 00:00:00 2001
From: Andrew Burke <31974658+atburke@users.noreply.github.com>
Date: Thu, 6 Jan 2022 17:26:31 -0800
Subject: [PATCH] Fall back to "/" when home directory doesn't exist for `tsh
 ssh` (#9413) (#9661)

This change drops ssh users into the root directory when their home directory doesn't exist. This is the same behavior as OpenSSH.
---
 lib/srv/exec_test.go | 15 ++++++++++++---
 lib/srv/reexec.go    | 26 +++++++++++++++++---------
 2 files changed, 29 insertions(+), 12 deletions(-)

diff --git a/lib/srv/exec_test.go b/lib/srv/exec_test.go
index d489196cb07b3..3949919069936 100644
--- a/lib/srv/exec_test.go
+++ b/lib/srv/exec_test.go
@@ -177,7 +177,7 @@ func (s *ExecSuite) TestOSCommandPrep(c *check.C) {
 	// Empty command (simple shell).
 	execCmd, err := s.ctx.ExecCommand()
 	c.Assert(err, check.IsNil)
-	cmd, err := buildCommand(execCmd, nil, nil, nil)
+	cmd, err := buildCommand(execCmd, s.usr, nil, nil, nil)
 	c.Assert(err, check.IsNil)
 	c.Assert(cmd, check.NotNil)
 	c.Assert(cmd.Path, check.Equals, "/bin/sh")
@@ -190,7 +190,7 @@ func (s *ExecSuite) TestOSCommandPrep(c *check.C) {
 	s.ctx.ExecRequest.SetCommand("ls -lh /etc")
 	execCmd, err = s.ctx.ExecCommand()
 	c.Assert(err, check.IsNil)
-	cmd, err = buildCommand(execCmd, nil, nil, nil)
+	cmd, err = buildCommand(execCmd, s.usr, nil, nil, nil)
 	c.Assert(err, check.IsNil)
 	c.Assert(cmd, check.NotNil)
 	c.Assert(cmd.Path, check.Equals, "/bin/sh")
@@ -203,11 +203,20 @@ func (s *ExecSuite) TestOSCommandPrep(c *check.C) {
 	s.ctx.ExecRequest.SetCommand("top")
 	execCmd, err = s.ctx.ExecCommand()
 	c.Assert(err, check.IsNil)
-	cmd, err = buildCommand(execCmd, nil, nil, nil)
+	cmd, err = buildCommand(execCmd, s.usr, nil, nil, nil)
 	c.Assert(err, check.IsNil)
 	c.Assert(cmd.Path, check.Equals, "/bin/sh")
 	c.Assert(cmd.Args, check.DeepEquals, []string{"/bin/sh", "-c", "top"})
 	c.Assert(cmd.SysProcAttr.Pdeathsig, check.Equals, syscall.SIGKILL)
+
+	// Missing home directory
+	s.usr.HomeDir = "/wrong/place"
+	root := string(os.PathSeparator)
+	expectedEnv[2] = fmt.Sprintf("HOME=%s", root)
+	cmd, err = buildCommand(execCmd, s.usr, nil, nil, nil)
+	c.Assert(err, check.IsNil)
+	c.Assert(cmd.Dir, check.Equals, root)
+	c.Assert(cmd.Env, check.DeepEquals, expectedEnv)
 }
 
 func (s *ExecSuite) TestLoginDefsParser(c *check.C) {
diff --git a/lib/srv/reexec.go b/lib/srv/reexec.go
index 2291e3dcc8192..b21fc86f61b17 100644
--- a/lib/srv/reexec.go
+++ b/lib/srv/reexec.go
@@ -215,8 +215,13 @@ func RunCommand() (io.Writer, int, error) {
 		pamEnvironment = pamContext.Environment()
 	}
 
+	localUser, err := user.Lookup(c.Login)
+	if err != nil {
+		return errorWriter, teleport.RemoteCommandFailure, trace.Wrap(err)
+	}
+
 	// Build the actual command that will launch the shell.
-	cmd, err := buildCommand(&c, tty, pty, pamEnvironment)
+	cmd, err := buildCommand(&c, localUser, tty, pty, pamEnvironment)
 	if err != nil {
 		return errorWriter, teleport.RemoteCommandFailure, trace.Wrap(err)
 	}
@@ -359,14 +364,10 @@ func RunAndExit(commandType string) {
 
 // buildCommand constructs a command that will execute the users shell. This
 // function is run by Teleport while it's re-executing.
-func buildCommand(c *ExecCommand, tty *os.File, pty *os.File, pamEnvironment []string) (*exec.Cmd, error) {
+func buildCommand(c *ExecCommand, localUser *user.User, tty *os.File, pty *os.File, pamEnvironment []string) (*exec.Cmd, error) {
 	var cmd exec.Cmd
 
-	// Lookup the UID and GID for the user.
-	localUser, err := user.Lookup(c.Login)
-	if err != nil {
-		return nil, trace.Wrap(err)
-	}
+	// Get UID and GID.
 	uid, err := strconv.Atoi(localUser.Uid)
 	if err != nil {
 		return nil, trace.Wrap(err)
@@ -423,11 +424,18 @@ func buildCommand(c *ExecCommand, tty *os.File, pty *os.File, pamEnvironment []s
 		cmd.Args = []string{shellPath, "-c", c.Command}
 	}
 
+	// Ensure that the user has a home directory.
+	// TODO: Generalize this to support Windows.
+	homeDir := string(os.PathSeparator)
+	if utils.IsDir(localUser.HomeDir) {
+		homeDir = localUser.HomeDir
+	}
+
 	// Create default environment for user.
 	cmd.Env = []string{
 		"LANG=en_US.UTF-8",
 		getDefaultEnvPath(localUser.Uid, defaultLoginDefsPath),
-		"HOME=" + localUser.HomeDir,
+		"HOME=" + homeDir,
 		"USER=" + c.Login,
 		"SHELL=" + shellPath,
 	}
@@ -450,7 +458,7 @@ func buildCommand(c *ExecCommand, tty *os.File, pty *os.File, pamEnvironment []s
 	cmd.Env = append(cmd.Env, pamEnvironment...)
 
 	// Set the home directory for the user.
-	cmd.Dir = localUser.HomeDir
+	cmd.Dir = homeDir
 
 	// If a terminal was requested, connect std{in,out,err} to the TTY and set
 	// the controlling TTY. Otherwise, connect std{in,out,err} to