ssh/tailssh: only chdir incubator process to user's homedir when necessary and possible

Instead of changing the working directory before launching the incubator process,
this now just changes the working directory after dropping privileges, at which
point we're more likely to be able to enter the user's home directory since we're
running as the user.

For paths that use the 'login' or 'su -l' commands, those already take care of changing
the working directory to the user's home directory.

Fixes #13120

Signed-off-by: Percy Wegmann <percy@tailscale.com>

ssh/tailssh/incubator.go

@@ -115,6 +115,7 @@ func (ss *sshSession) newIncubatorCommand(logf logger.Logf) (cmd *exec.Cmd, err
 		"--gid=" + lu.Gid,
 		"--groups=" + groups,
 		"--local-user=" + lu.Username,
+		"--home-dir=" + lu.HomeDir,
 		"--remote-user=" + remoteUser,
 		"--remote-ip=" + ci.src.Addr().String(),
 		"--has-tty=false", // updated in-place by startWithPTY
@@ -179,6 +180,7 @@ type incubatorArgs struct {
 	gid                int
 	gids               []int
 	localUser          string
+	homeDir            string
 	remoteUser         string
 	remoteIP           string
 	ttyName            string
@@ -201,6 +203,7 @@ func parseIncubatorArgs(args []string) (incubatorArgs, error) {
 	flags.IntVar(&ia.gid, "gid", 0, "the gid of local-user")
 	flags.StringVar(&groups, "groups", "", "comma-separated list of gids of local-user")
 	flags.StringVar(&ia.localUser, "local-user", "", "the user to run as")
+	flags.StringVar(&ia.homeDir, "home-dir", "/", "the user's home directory")
 	flags.StringVar(&ia.remoteUser, "remote-user", "", "the remote user/tags")
 	flags.StringVar(&ia.remoteIP, "remote-ip", "", "the remote Tailscale IP")
 	flags.StringVar(&ia.ttyName, "tty-name", "", "the tty name (pts/3)")
@@ -565,7 +568,7 @@ const (
 // dropPrivileges calls doDropPrivileges with uid, gid, and gids from the given
 // incubatorArgs.
 func dropPrivileges(dlogf logger.Logf, ia incubatorArgs) error {
-	return doDropPrivileges(dlogf, ia.uid, ia.gid, ia.gids)
+	return doDropPrivileges(dlogf, ia.uid, ia.gid, ia.gids, ia.homeDir)
 }
 
 // doDropPrivileges contains all the logic for dropping privileges to a different
@@ -578,7 +581,7 @@ func dropPrivileges(dlogf logger.Logf, ia incubatorArgs) error {
 // be done by running:
 //
 //	go test -c ./ssh/tailssh/ && sudo ./tailssh.test -test.v -test.run TestDoDropPrivileges
-func doDropPrivileges(dlogf logger.Logf, wantUid, wantGid int, supplementaryGroups []int) error {
+func doDropPrivileges(dlogf logger.Logf, wantUid, wantGid int, supplementaryGroups []int, homeDir string) error {
 	dlogf("dropping privileges")
 	fatalf := func(format string, args ...any) {
 		dlogf("[unexpected] error dropping privileges: "+format, args...)
@@ -664,6 +667,13 @@ func doDropPrivileges(dlogf logger.Logf, wantUid, wantGid int, supplementaryGrou
 		// TODO(andrew-d): assert that our supplementary groups are correct
 	}
 
+	// Prefer to run in user's homedir if possible. We ignore a failure to Chdir,
+	// which just leaves us at "/" where we launched in the first place.
+	dlogf("attempting to chdir to user's home directory %q", homeDir)
+	if err := os.Chdir(homeDir); err != nil {
+		dlogf("failed to chdir to user's home directory %q, continuing in current directory", homeDir)
+	}
+
 	return nil
 }
 
@@ -680,16 +690,7 @@ func (ss *sshSession) launchProcess() error {
 	}
 
 	cmd := ss.cmd
-	homeDir := ss.conn.localUser.HomeDir
-	if _, err := os.Stat(homeDir); err == nil {
-		cmd.Dir = homeDir
-	} else if os.IsNotExist(err) {
-		// If the home directory doesn't exist, we can't chdir to it.
-		// Instead, we'll chdir to the root directory.
-		cmd.Dir = "/"
-	} else {
-		return err
-	}
+	cmd.Dir = "/"
 	cmd.Env = envForUser(ss.conn.localUser)
 	for _, kv := range ss.Environ() {
 		if acceptEnvPair(kv) {

ssh/tailssh/privs_test.go

@@ -49,7 +49,7 @@ func TestDoDropPrivileges(t *testing.T) {
 		f := os.NewFile(3, "out.json")
 
 		// We're in our subprocess; actually drop privileges now.
-		doDropPrivileges(t.Logf, input.UID, input.GID, input.AdditionalGroups)
+		doDropPrivileges(t.Logf, input.UID, input.GID, input.AdditionalGroups, "/")
 
 		additional, _ := syscall.Getgroups()
 

ssh/tailssh/tailssh_integration_test.go

@@ -122,13 +122,13 @@ func TestIntegrationSSH(t *testing.T) {
 		{
 			cmd:             "pwd",
 			want:            []string{homeDir},
-			skip:            !fallbackToSUAvailable(),
+			skip:            os.Getenv("SKIP_FILE_OPS") == "1" || !fallbackToSUAvailable(),
 			forceV1Behavior: false,
 		},
 		{
 			cmd:             "echo 'hello'",
 			want:            []string{"hello"},
-			skip:            !fallbackToSUAvailable(),
+			skip:            os.Getenv("SKIP_FILE_OPS") == "1" || !fallbackToSUAvailable(),
 			forceV1Behavior: false,
 		},
 	}

ssh/tailssh/testcontainers/Dockerfile

@@ -25,7 +25,7 @@ COPY tailssh.test .
 
 RUN chmod 755 tailscaled
 
-# RUN echo "First run tests normally."
+RUN echo "First run tests normally."
 RUN eval `ssh-agent -s` && TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.v -test.run TestSSHAgentForwarding
 RUN if echo "$BASE" | grep "ubuntu:"; then rm -Rf /home/testuser; fi
 RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.v -test.run TestIntegrationSFTP
@@ -35,9 +35,14 @@ RUN if echo "$BASE" | grep "ubuntu:"; then rm -Rf /home/testuser; fi
 RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.v -test.run TestIntegrationSSH
 
 RUN echo "Then run tests as non-root user testuser and make sure tests still pass."
+RUN touch /tmp/tailscalessh.log
 RUN chown testuser:groupone /tmp/tailscalessh.log
 RUN TAILSCALED_PATH=`pwd`tailscaled eval `su -m testuser -c ssh-agent -s` && su -m testuser -c "./tailssh.test -test.v -test.run TestSSHAgentForwarding"
 RUN TAILSCALED_PATH=`pwd`tailscaled su -m testuser -c "./tailssh.test -test.v -test.run TestIntegration TestDoDropPrivileges"
+RUN echo "Also, deny everyone access to the user's home directory and make sure non file-related tests still pass."
+RUN mkdir -p /home/testuser && chown testuser:groupone /home/testuser && chmod 0000 /home/testuser
+RUN TAILSCALED_PATH=`pwd`tailscaled SKIP_FILE_OPS=1 su -m testuser -c "./tailssh.test -test.v -test.run TestIntegrationSSH"
+RUN chmod 0755 /home/testuser
 RUN chown root:root /tmp/tailscalessh.log
 
 RUN if echo "$BASE" | grep "ubuntu:"; then \