feat(decl/proc-chain): add user and capabilities support

Add the capability to specify the user and the linux capabilities
a process in the process chain can be run with. Capabilities can
only be specified for the leaf process. Omitting capabilities is
equivalent to specify 'all=iep'. Each process in the chain runs
with real user/group ID equals to 0 (root). Specifying a user sets
the effective and the saved set-user/group-ID to the corresponding
user/group IDs. If a user specified in the chain doesn't exist, it
is created before running the test and deleted after test
execution. The securebit SECBBIT_NOROOT is enabled before creating
any child process: this is done in order to prevent the kernel from
ignoring the specified capabilities when the real user ID is zero
(see 'Capabilities and execution of programs by root' in
capabilities(7)). Users who wish to run the before and after script
or creating a 'process' test resource  must take into account to
provide at least CAP_SETPCAP in its permitted and effective set.

Signed-off-by: Leonardo Di Giovanna <[email protected]>

pkg/process/process.go

@@ -19,14 +19,18 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"io"
 	"os"
 	"os/exec"
+	"os/user"
 	"path/filepath"
 	"regexp"
+	"strconv"
 
 	"github.com/go-logr/logr"
 	"golang.org/x/sys/unix"
 
+	"github.com/falcosecurity/event-generator/pkg/capability"
 	"github.com/falcosecurity/event-generator/pkg/random"
 )
 
@@ -38,10 +42,18 @@ type Process struct {
 	// simExePath is the "simulated" executable path. This sets the executable path accessible through
 	// `readlink -f /proc/<pid/exepath` for the started process. If empty, it is randomly generated.
 	simExePath string
+	// username is the name of the user that must run the process. If empty, the current process user is used. If the
+	// specified user does not exist, it is created before running the test and deleted after test execution.
+	username string
+	// capabilities are the capabilities that must be set on the process executable file. The syntax follows the
+	// conventions specified by cap_from_text(3). Notice: an empty string defaults to 'all=iep'.
+	capabilities string
 	// cmd is the underlying command object.
 	cmd *exec.Cmd
 	// started is true if the process has been started; false otherwise.
 	started bool
+	// userCreated is true if the user has been created; false otherwise.
+	userCreated bool
 }
 
 // Description describes a process.
@@ -62,13 +74,21 @@ type Description struct {
 	Args string
 	// Env is the list of environment variables that must be provided to the process.
 	Env []string
+	// Username is the name of the user that must run the process. If empty, the current process user is used. If the
+	// specified user does not exist, it is created before running the test and deleted after test execution.
+	Username string
+	// Capabilities are the capabilities that must be set on the process executable file. The syntax follows the
+	// conventions specified by cap_from_text(3). Notice: an empty string defaults to 'all=iep'.
+	Capabilities string
 }
 
 var (
 	// defaultSimExePathPrefix defines the prefix used to generate a random simulated executable path.
 	defaultSimExePathPrefix = filepath.Join(os.TempDir(), "event-generator")
 )
 
+var allCaps = "all=iep"
+
 // New creates a new process.
 func New(ctx context.Context, procDesc *Description) *Process {
 	// If the user doesn't provide an executable path, we must generate a random path.
@@ -98,6 +118,14 @@ func New(ctx context.Context, procDesc *Description) *Process {
 		procArg0 = filepath.Base(exePath)
 	}
 
+	// If the user doesn't provide capabilities, we default them.
+	var capabilities string
+	if procDesc.Capabilities != "" {
+		capabilities = procDesc.Capabilities
+	} else {
+		capabilities = allCaps
+	}
+
 	// Evaluate process arguments.
 	procArgs := splitArgs(procDesc.Args)
 
@@ -108,11 +136,13 @@ func New(ctx context.Context, procDesc *Description) *Process {
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr
 	proc := &Process{
-		logger:     procDesc.Logger,
-		command:    procDesc.Command,
-		simExePath: simExePath,
-		cmd:        cmd,
-		started:    false,
+		logger:       procDesc.Logger,
+		command:      procDesc.Command,
+		simExePath:   simExePath,
+		username:     procDesc.Username,
+		capabilities: capabilities,
+		cmd:          cmd,
+		started:      false,
 	}
 	return proc
 }
@@ -151,19 +181,72 @@ func (p *Process) Start() (err error) {
 		return fmt.Errorf("error retrieving command path: %w", err)
 	}
 
-	// Create a hard link to the provided command path, named as specified by the user.
-	simExePath := p.simExePath
-	if err := os.Link(commandPath, simExePath); err != nil {
-		return fmt.Errorf("error creating process executable: %w", err)
-	}
-	defer func() {
+	// Determine if the process must be run by the current user/group or not. If a user different from the current one
+	// is requested to run it, the user is created at this stage, and deleted after the new process resources are
+	// released.
+	changeOwnership := false
+	userCreated := false
+	var procUID, procGID int
+	if username := p.username; username != "" {
+		userCreated, procUID, procGID, err = p.findOrCreateUser(username)
 		if err != nil {
-			if err := os.Remove(simExePath); err != nil {
-				p.logger.Error(err, "Error deleting process executable", "path", simExePath)
+			return fmt.Errorf("error finding or creating user %q: %w", username, err)
+		}
+
+		logger := p.logger.WithValues("user", username, "uid", procUID, "gid", procGID)
+		if userCreated {
+			defer p.deleteUserIfErr(username, &err)
+			logger.V(1).Info("Created user")
+		} else {
+			logger.V(1).Info("Found user")
+		}
+
+		// The current executable must be run with real IDs set to 0, so compare the obtained IDs with the effective
+		// ones instead of the real ones.
+		changeOwnership = procUID != os.Geteuid() || procGID != os.Getegid()
+	}
+
+	capabilities := p.capabilities
+	changeCapabilities := capabilities != allCaps
+
+	// Create the simulated executable file. It is either a hard link or a copy of the original command path, depending
+	// on the fact that it is required to change the file ownership/capabilities or not.
+	simExePath := p.simExePath
+	if changeOwnership || changeCapabilities {
+		// We are going to manipulate the file ownership and capabilities, so create a copy of the original instead of
+		// modifying the original one.
+		if err := copyFile(commandPath, simExePath); err != nil {
+			return fmt.Errorf("error copying process executable: %w", err)
+		}
+		defer p.removeFileIfErr(simExePath, &err)
+		p.logger.V(1).Info("Created process executable", "path", simExePath)
+
+		if changeOwnership {
+			if err := os.Chown(simExePath, procUID, procGID); err != nil {
+				return fmt.Errorf("error changing process executable ownership to %d:%d: %w", procUID, procGID, err)
 			}
 		}
-	}()
-	p.logger.V(1).Info("Created process executable", "path", simExePath)
+
+		if err := p.setFileCapabilities(simExePath, capabilities); err != nil {
+			return fmt.Errorf("error setting process executable file capabilities to %q: %w", capabilities, err)
+		}
+
+		if changeOwnership {
+			if err := os.Chmod(simExePath, 0o555|os.ModeSetuid|os.ModeSetgid); err != nil {
+				return fmt.Errorf("error changing process executable file mode: %w", err)
+			}
+		}
+	} else {
+		if err := os.Link(commandPath, simExePath); err != nil {
+			return fmt.Errorf("error creating process executable: %w", err)
+		}
+		defer p.removeFileIfErr(simExePath, &err)
+		p.logger.V(1).Info("Created process executable", "path", simExePath)
+
+		if err := p.setFileCapabilities(simExePath, capabilities); err != nil {
+			return fmt.Errorf("error setting process executable file capabilities to %q: %w", capabilities, err)
+		}
+	}
 
 	// If the user specified a custom process name, we will run the executable through a symbolic link, so create it.
 	exePath := p.cmd.Path
@@ -172,25 +255,160 @@ func (p *Process) Start() (err error) {
 			return fmt.Errorf("error creating symlink %q to process executable %q: %w", exePath, simExePath,
 				err)
 		}
-		defer func() {
-			if err != nil {
-				if err := os.Remove(exePath); err != nil {
-					p.logger.Error(err, "Error deleting symlink to process executable", "symlink", exePath)
-				}
-			}
-		}()
+		defer p.removeFileIfErr(exePath, &err)
+		p.logger.V(1).Info("Created symlink to process executable", "path", simExePath, "symlink", exePath)
 	}
-	p.logger.V(1).Info("Created symlink to process executable", "path", simExePath, "symlink", exePath)
 
-	// Run the process.
-	if err := p.cmd.Start(); err != nil {
+	// Run the executable but prevent the kernel from ignoring file capabilities when real/effective user ID is 0 (see
+	// 'Capabilities and execution of programs by root' in capabilities(7)).
+	// Notice: secure bits are inherited by child process, so whatever process is going to be spawned by our child will
+	// not inherit root privileges if our child doesn't clear the SECBIT_NOROOT first.
+	if err := capability.RunWithSecBitNoRootEnabled(p.cmd.Start); err != nil {
 		return err
 	}
 
 	p.started = true
+	p.userCreated = userCreated
 	return nil
 }
 
+// findOrCreateUser finds or creates the user with the provided username. It returns information about the user.
+func (p *Process) findOrCreateUser(username string) (created bool, uid, gid int, err error) {
+	if uid, gid, err = getUserIDs(username); err == nil {
+		return false, uid, gid, nil
+	}
+
+	var unknownUserErr user.UnknownUserError
+	if !errors.As(err, &unknownUserErr) {
+		return false, 0, 0, fmt.Errorf("error retrieving user id: %w", err)
+	}
+
+	// User does not exist, create it.
+	if err := addUser(username); err != nil {
+		_ = delUser(username) // addUser can leave an inconsistent state.
+		return false, 0, 0, fmt.Errorf("error creating user: %w", err)
+	}
+	defer p.deleteUserIfErr(username, &err)
+
+	// Retrieve the user ID of the created user.
+	if uid, gid, err = getUserIDs(username); err != nil {
+		return false, 0, 0, fmt.Errorf("error looking up for user after creation: %w", err)
+	}
+
+	return true, uid, gid, nil
+}
+
+// getUserIDs returns the user and group IDs of the user associated to the provided username.
+func getUserIDs(username string) (uid, gid int, err error) {
+	usr, err := user.Lookup(username)
+	if err != nil {
+		return 0, 0, fmt.Errorf("error looking up user %q: %w", username, err)
+	}
+
+	if uid, err = strconv.Atoi(usr.Uid); err != nil {
+		return 0, 0, fmt.Errorf("error parsing user uid %q: %w", usr.Uid, err)
+	}
+
+	if gid, err = strconv.Atoi(usr.Gid); err != nil {
+		return 0, 0, fmt.Errorf("error parsing user gid %q: %w", usr.Gid, err)
+	}
+
+	return uid, gid, nil
+}
+
+// addUser creates a user with the given username.
+func addUser(username string) error {
+	return capability.RunWithSecBitNoRootDisabled(func() error {
+		return exec.Command("sh", "-c", fmt.Sprintf("useradd %q", username)).Run() //nolint:gosec // Disable G204
+	})
+}
+
+// delUser deletes the user with the given username.
+func delUser(username string) error {
+	return capability.RunWithSecBitNoRootDisabled(func() error {
+		return exec.Command("sh", "-c", fmt.Sprintf("userdel %q", username)).Run() //nolint:gosec // Disable G204
+	})
+}
+
+// deleteUserIfErr deletes the user associated to the provided username if the provided error pointer points to an
+// error.
+func (p *Process) deleteUserIfErr(username string, err *error) { //nolint:gocritic // Disable ptrToRefParam
+	if *err != nil {
+		if err := delUser(username); err != nil {
+			p.logger.Error(err, "Error deleting user", "user", username)
+		}
+	}
+}
+
+// copyFile copies the file at src to dst.
+func copyFile(src, dst string) (err error) {
+	srcFile, err := os.Open(src) //nolint:gosec // Disable G304
+	if err != nil {
+		return fmt.Errorf("error opening source file: %w", err)
+	}
+	defer func() {
+		if e := srcFile.Close(); e != nil {
+			e := fmt.Errorf("error closing source file: %w", e)
+			if err != nil {
+				err = fmt.Errorf("%w; %w", err, e)
+			} else {
+				err = e
+			}
+		}
+	}()
+
+	dstFile, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE, 0o755) //nolint:gosec // Disable G304
+	if err != nil {
+		return fmt.Errorf("error creating destination file: %w", err)
+	}
+	defer func() {
+		if e := dstFile.Close(); e != nil {
+			e := fmt.Errorf("error closing destination file: %w", e)
+			if err != nil {
+				err = fmt.Errorf("%w; %w", err, e)
+			} else {
+				err = e
+			}
+		}
+	}()
+
+	if _, err := io.Copy(dstFile, srcFile); err != nil {
+		return fmt.Errorf("error copying data: %w", err)
+	}
+
+	return nil
+}
+
+// removeFileIfErr removes the file at the provided file path if the provided error pointer points to an error.
+func (p *Process) removeFileIfErr(filePath string, err *error) { //nolint:gocritic // Disable ptrToRefParam
+	if *err != nil {
+		if err := os.Remove(filePath); err != nil {
+			p.logger.Error(err, "Error deleting file", "path", filePath)
+		}
+	}
+}
+
+// setFileCapabilities sets the capability state of the file at the provided file path to the value obtained parsing
+// the provided capability string.
+func (p *Process) setFileCapabilities(filePath, capabilities string) error {
+	caps, err := capability.Parse(capabilities)
+	if err != nil {
+		return fmt.Errorf("error parsing capabilities: %w", err)
+	}
+
+	file, err := os.Open(filePath) //nolint:gosec // Disable G304
+	if err != nil {
+		return fmt.Errorf("error opening file: %w", err)
+	}
+	defer func() {
+		if err := file.Close(); err != nil {
+			p.logger.Error(err, "Error closing executable after setting capabilities")
+		}
+	}()
+
+	return caps.SetFd(file)
+}
+
 var errProcessNotStarted = fmt.Errorf("process not started")
 
 // Wait waits for process to exit and releases the related resources.
@@ -235,12 +453,22 @@ func (p *Process) Kill() error {
 
 // releaseResources releases the resources associated to the process, such as created executables.
 func (p *Process) releaseResources() {
-	simExePath := p.simExePath
-	exePath := p.cmd.Path
 	defer func() {
 		p.started = false
+		p.userCreated = false
 	}()
 
+	username := p.username
+	if p.userCreated {
+		if err := delUser(username); err != nil {
+			p.logger.Error(err, "Error deleting user", "user", username)
+		} else {
+			p.logger.V(1).Info("Deleted user", "user", username)
+		}
+	}
+
+	simExePath := p.simExePath
+	exePath := p.cmd.Path
 	if simExePath != exePath {
 		if err := os.Remove(exePath); err != nil {
 			p.logger.Error(err, "Error deleting symlink to process executable", "symlink", exePath)