A much more lax approach to privelege separation (#816)

* A much more lax approach to privelege separation

Compared to #814.

* Correct ErrFailed for privdrop package

* privdrop stubs for Windows

* Assign *syscall.SysProcAttr instead of *syscall.Credential

* chown(2) agent's binary when privelege dropping was requested

* privdrop: expose Chown for both Windows and non-Windows

* Clarify to which isolations does --user apply

internal/commands/worker/run.go

@@ -3,16 +3,27 @@ package worker
 import (
 	"errors"
 	"fmt"
+	"github.com/cirruslabs/cirrus-cli/pkg/privdrop"
 	"github.com/spf13/cobra"
+	"runtime"
 )
 
 var ErrRun = errors.New("run failed")
 
+var username string
+
 func NewRunCmd() *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "run",
 		Short: "Run persistent worker",
 		RunE: func(cmd *cobra.Command, args []string) error {
+			// Initialize privilege dropping on macOS, if requested
+			if username != "" {
+				if err := privdrop.Initialize(username); err != nil {
+					return err
+				}
+			}
+
 			worker, err := buildWorker(cmd.ErrOrStderr())
 			if err != nil {
 				return err
@@ -24,6 +35,22 @@ func NewRunCmd() *cobra.Command {
 		},
 	}
 
+	// We only need privilege dropping on macOS due to newly introduced
+	// "Local Network" permission, which cannot be disabled automatically,
+	// and according to the Apple's documentation[1], running Persistent
+	// Worker as a superuser is the only choice.
+	//
+	// Note that the documentation says that "macOS automatically allows
+	// local network access by:" and "Any daemon started by launchd". However,
+	// this is not true for daemons that have <key>UserName</key> set to non-root.
+	//
+	//nolint:lll // can't make the link shorter
+	// [1]: https://developer.apple.com/documentation/technotes/tn3179-understanding-local-network-privacy#macOS-considerations
+	if runtime.GOOS == "darwin" {
+		cmd.Flags().StringVar(&username, "user", "", "user name to drop privileges to"+
+			" when running external programs (only Tart, Parallels and unset isolation are currently supported)")
+	}
+
 	attachFlags(cmd)
 
 	return cmd

internal/executor/agent/agent.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"github.com/cirruslabs/cirrus-cli/pkg/privdrop"
 	"io"
 	"net/http"
 	"os"
@@ -77,10 +78,18 @@ func RetrieveBinary(
 	}
 
 	// Make the agent binary executable
-	if err := os.Chmod(tmpAgentFile.Name(), 0500); err != nil {
+	if err := tmpAgentFile.Chmod(0500); err != nil {
 		return "", err
 	}
 
+	// Make sure that the agent binary belongs to the privilege-dropped
+	// user and group, in case privilege dropping was requested
+	if chownTo := privdrop.ChownTo; chownTo != nil {
+		if err := tmpAgentFile.Chown(chownTo.UID, chownTo.GID); err != nil {
+			return "", err
+		}
+	}
+
 	if err := tmpAgentFile.Close(); err != nil {
 		return "", err
 	}

internal/executor/instance/persistentworker/isolation/none/none.go

@@ -8,6 +8,7 @@ import (
 	"github.com/cirruslabs/cirrus-cli/internal/executor/instance/persistentworker/pwdir"
 	"github.com/cirruslabs/cirrus-cli/internal/executor/instance/runconfig"
 	"github.com/cirruslabs/cirrus-cli/internal/logger"
+	"github.com/cirruslabs/cirrus-cli/pkg/privdrop"
 	"github.com/otiai10/copy"
 	"go.opentelemetry.io/otel/attribute"
 	"os"
@@ -80,6 +81,11 @@ func (pwi *PersistentWorkerInstance) Run(ctx context.Context, config *runconfig.
 		pwi.tempDir,
 	)
 
+	// Drop privileges for the spawned process, if requested
+	if sysProcAttr := privdrop.SysProcAttr; sysProcAttr != nil {
+		cmd.SysProcAttr = sysProcAttr
+	}
+
 	// Determine the working directory for the agent
 	if config.DirtyMode {
 		cmd.Dir = config.ProjectDir

internal/executor/instance/persistentworker/isolation/parallels/cli.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"github.com/cirruslabs/cirrus-cli/pkg/privdrop"
 	"os/exec"
 	"strings"
 )
@@ -17,6 +18,11 @@ var (
 func runParallelsCommand(ctx context.Context, commandName string, args ...string) (string, string, error) {
 	cmd := exec.CommandContext(ctx, commandName, args...)
 
+	// Drop privileges for the spawned process, if requested
+	if sysProcAttr := privdrop.SysProcAttr; sysProcAttr != nil {
+		cmd.SysProcAttr = sysProcAttr
+	}
+
 	var stdout, stderr bytes.Buffer
 	cmd.Stdout = &stdout
 	cmd.Stderr = &stderr

internal/executor/instance/persistentworker/isolation/tart/cmd.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"github.com/cirruslabs/cirrus-cli/pkg/privdrop"
 	"github.com/cirruslabs/echelon"
 	"io"
 	"os/exec"
@@ -60,6 +61,11 @@ func CmdWithLogger(
 
 	cmd := exec.CommandContext(ctx, tartCommandName, args...)
 
+	// Drop privileges for the spawned process, if requested
+	if sysProcAttr := privdrop.SysProcAttr; sysProcAttr != nil {
+		cmd.SysProcAttr = sysProcAttr
+	}
+
 	// Work around https://github.com/golang/go/issues/23019,
 	// most likely happens when running with --net-softnet
 	cmd.WaitDelay = time.Second

pkg/privdrop/common.go

@@ -0,0 +1,6 @@
+package privdrop
+
+type Chown struct {
+	UID int
+	GID int
+}

pkg/privdrop/privdrop.go

@@ -0,0 +1,51 @@
+//go:build !windows
+
+package privdrop
+
+import (
+	"errors"
+	"fmt"
+	userpkg "os/user"
+	"strconv"
+	"syscall"
+)
+
+var ErrFailed = errors.New("failed to initialize privilege dropping")
+
+var (
+	SysProcAttr *syscall.SysProcAttr
+	ChownTo     *Chown
+)
+
+func Initialize(username string) error {
+	user, err := userpkg.Lookup(username)
+	if err != nil {
+		return fmt.Errorf("%w: failed to lookup username %q: %v",
+			ErrFailed, username, err)
+	}
+
+	gid, err := strconv.Atoi(user.Gid)
+	if err != nil {
+		return fmt.Errorf("%w: failed to parse %s's group ID %q: %v",
+			ErrFailed, user.Name, user.Gid, err)
+	}
+
+	uid, err := strconv.Atoi(user.Uid)
+	if err != nil {
+		return fmt.Errorf("%w: failed to parse %s's user ID %q: %v",
+			ErrFailed, user.Name, user.Uid, err)
+	}
+
+	SysProcAttr = &syscall.SysProcAttr{
+		Credential: &syscall.Credential{
+			Uid: uint32(uid),
+			Gid: uint32(gid),
+		},
+	}
+	ChownTo = &Chown{
+		UID: uid,
+		GID: gid,
+	}
+
+	return nil
+}

pkg/privdrop/privdrop_unsupported.go

@@ -0,0 +1,17 @@
+//go:build windows
+
+package privdrop
+
+import (
+	"fmt"
+	"syscall"
+)
+
+var (
+	SysProcAttr *syscall.SysProcAttr
+	ChownTo     *Chown
+)
+
+func Initialize(username string) error {
+	return fmt.Errorf("privilege dropping is not supported on this platform")
+}