Add some basic security policies with sensible defaults

This ommmit contains some security hardening measures for the Hugo build runtime.

There are some rarely used features in Hugo that would be good to have disabled by default. One example would be the "external helpers".

For `asciidoctor` and some others we use Go's `os/exec` package to start a new process.

These are a predefined set of binary names, all loaded from `PATH` and with a predefined set of arguments. Still, if you don't use `asciidoctor` in your project, you might as well have it turned off.

You can configure your own in the new `security` configuration section, but the defaults are configured to create a minimal amount of site breakage. And if that do happen, you will get clear instructions in the loa about what to do.

The default configuration is listed below. Note that almost all of these options are regular expression _whitelists_ (a string or a slice); the value `none` will block all.

```toml
[security]
  enableInlineShortcodes = false
  [security.exec]
    allow = ['^dart-sass-embedded$', '^go$', '^npx$', '^postcss$']
    osEnv = ['(?i)^(PATH|PATHEXT|APPDATA|TMP|TEMP|TERM)$']

  [security.funcs]
    getenv = ['^HUGO_']

  [security.http]
    methods = ['(?i)GET|POST']
    urls = ['.*']
```

2 files changed, 20 insertions, 13 deletions

diff --git a/modules/client.go b/modules/client.go
index fcb5957c3..1924cd5b4 100644
--- a/modules/client.go
+++ b/modules/client.go
@@ -28,6 +28,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/gohugoio/hugo/common/collections"
 	"github.com/gohugoio/hugo/common/hexec"
 
 	hglob "github.com/gohugoio/hugo/hugofs/glob"
@@ -79,7 +80,7 @@ func NewClient(cfg ClientConfig) *Client {
 		goModFilename = n
 	}
 
-	env := os.Environ()
+	var env []string
 	mcfg := cfg.ModuleConfig
 
 	config.SetEnvVars(&env,
@@ -87,12 +88,9 @@ func NewClient(cfg ClientConfig) *Client {
 		"GO111MODULE", "on",
 		"GOPROXY", mcfg.Proxy,
 		"GOPRIVATE", mcfg.Private,
-		"GONOPROXY", mcfg.NoProxy)
-
-	if cfg.CacheDir != "" {
-		// Module cache stored below $GOPATH/pkg
-		config.SetEnvVars(&env, "GOPATH", cfg.CacheDir)
-	}
+		"GONOPROXY", mcfg.NoProxy,
+		"GOPATH", cfg.CacheDir,
+	)
 
 	logger := cfg.Logger
 	if logger == nil {
@@ -609,16 +607,19 @@ func (c *Client) runGo(
 	}
 
 	stderr := new(bytes.Buffer)
-	cmd, err := hexec.SafeCommandContext(ctx, "go", args...)
+
+	argsv := collections.StringSliceToInterfaceSlice(args)
+	argsv = append(argsv, hexec.WithEnviron(c.environ))
+	argsv = append(argsv, hexec.WithStderr(io.MultiWriter(stderr, os.Stderr)))
+	argsv = append(argsv, hexec.WithStdout(stdout))
+	argsv = append(argsv, hexec.WithDir(c.ccfg.WorkingDir))
+	argsv = append(argsv, hexec.WithContext(ctx))
+
+	cmd, err := c.ccfg.Exec.New("go", argsv...)
 	if err != nil {
 		return err
 	}
 
-	cmd.Env = c.environ
-	cmd.Dir = c.ccfg.WorkingDir
-	cmd.Stdout = stdout
-	cmd.Stderr = io.MultiWriter(stderr, os.Stderr)
-
 	if err := cmd.Run(); err != nil {
 		if ee, ok := err.(*exec.Error); ok && ee.Err == exec.ErrNotFound {
 			c.goBinaryStatus = goBinaryStatusNotFound
@@ -727,6 +728,8 @@ type ClientConfig struct {
 	// Eg. "production"
 	Environment string
 
+	Exec *hexec.Exec
+
 	CacheDir     string // Module cache
 	ModuleConfig Config
 }
diff --git a/modules/client_test.go b/modules/client_test.go
index f801af07d..75e3c2b08 100644
--- a/modules/client_test.go
+++ b/modules/client_test.go
@@ -21,6 +21,8 @@ import (
 	"sync/atomic"
 	"testing"
 
+	"github.com/gohugoio/hugo/common/hexec"
+	"github.com/gohugoio/hugo/config/security"
 	"github.com/gohugoio/hugo/hugofs/glob"
 
 	"github.com/gohugoio/hugo/htesting"
@@ -53,7 +55,9 @@ github.com/gohugoio/hugoTestModules1_darwin/[email protected] github.com/gohugoio/h
 		ccfg := ClientConfig{
 			Fs:         hugofs.Os,
 			WorkingDir: workingDir,
+			CacheDir:   filepath.Join(workingDir, "modcache"),
 			ThemesDir:  themesDir,
+			Exec:       hexec.New(security.DefaultConfig),
 		}
 
 		withConfig(&ccfg)