client: fix race condition in use of go-getter

go-getter creates a circular dependency between a Client and Getter,
which means each is inherently thread-unsafe if you try to re-use
on or the other.

This PR fixes Nomad to no longer make use of the default Getter objects
provided by the go-getter package. Nomad must create a new Client object
on every artifact download, as the Client object controls the Src and Dst
among other things. When Caling Client.Get, the Getter modifies its own
Client reference, creating the circular reference and race condition.

We can still achieve most of the desired connection caching behavior by
re-using a shared HTTP client with transport pooling enabled.

.changelog/12036.txt

+```release-note:security
+Fix race condition in use of go-getter that could cause a client agent to download the wrong artifact into the wrong destination. [CVE-2022-24686](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2022-24686)
+```

client/allocrunner/taskrunner/getter/getter.go

@@ -6,22 +6,20 @@ import (
 	"net/http"
 	"net/url"
 	"strings"
-	"sync"
 
+	"github.com/hashicorp/go-cleanhttp"
 	gg "github.com/hashicorp/go-getter"
 
 	"github.com/hashicorp/nomad/nomad/structs"
 )
 
-var (
-	// getters is the map of getters suitable for Nomad. It is initialized once
-	// and the lock is used to guard access to it.
-	getters map[string]gg.Getter
-	lock    sync.Mutex
-
-	// supported is the set of download schemes supported by Nomad
-	supported = []string{"http", "https", "s3", "hg", "git", "gcs"}
-)
+// httpClient is a shared HTTP client for use across all http/https Getter
+// instantiations. The HTTP client is designed to be thread-safe, and using a pooled
+// transport will help reduce excessive connections when clients are downloading lots
+// of artifacts.
+var httpClient = &http.Client{
+	Transport: cleanhttp.DefaultPooledTransport(),
+}
 
 const (
 	// gitSSHPrefix is the prefix for downloading via git using ssh
@@ -35,53 +33,36 @@ type EnvReplacer interface {
 	ClientPath(string, bool) (string, bool)
 }
 
-func makeGetters(headers http.Header) map[string]gg.Getter {
-	getters := make(map[string]gg.Getter, len(supported))
-	for _, getter := range supported {
-		switch {
-		case getter == "http" && len(headers) > 0:
-			fallthrough
-		case getter == "https" && len(headers) > 0:
-			getters[getter] = &gg.HttpGetter{
-				Netrc:  true,
-				Header: headers,
-			}
-		default:
-			if defaultGetter, ok := gg.Getters[getter]; ok {
-				getters[getter] = defaultGetter
-			}
-		}
-	}
-	return getters
-}
-
 // getClient returns a client that is suitable for Nomad downloading artifacts.
 func getClient(src string, headers http.Header, mode gg.ClientMode, dst string) *gg.Client {
-	client := &gg.Client{
-		Src:   src,
-		Dst:   dst,
-		Mode:  mode,
-		Umask: 060000000,
+	return &gg.Client{
+		Src:     src,
+		Dst:     dst,
+		Mode:    mode,
+		Umask:   060000000,
+		Getters: createGetters(headers),
 	}
+}
 
-	switch len(headers) {
-	case 0:
-		// When no headers are present use the memoized getters, creating them
-		// on demand if they do not exist yet.
-		lock.Lock()
-		if getters == nil {
-			getters = makeGetters(nil)
-		}
-		lock.Unlock()
-		client.Getters = getters
-	default:
-		// When there are headers present, we must create fresh gg.HttpGetter
-		// objects, because that is where gg stores the headers to use in its
-		// artifact HTTP GET requests.
-		client.Getters = makeGetters(headers)
+func createGetters(header http.Header) map[string]gg.Getter {
+	httpGetter := &gg.HttpGetter{
+		Netrc:  true,
+		Client: httpClient,
+		Header: header,
+	}
+	// Explicitly create fresh set of supported Getter for each Client, because
+	// go-getter is not thread-safe. Use a shared HTTP client for http/https Getter,
+	// with pooled transport which is thread-safe.
+	//
+	// If a getter type is not listed here, it is not supported (e.g. file).
+	return map[string]gg.Getter{
+		"git":   new(gg.GitGetter),
+		"gcs":   new(gg.GCSGetter),
+		"hg":    new(gg.HgGetter),
+		"s3":    new(gg.S3Getter),
+		"http":  httpGetter,
+		"https": httpGetter,
 	}
-
-	return client
 }
 
 // getGetterUrl returns the go-getter URL to download the artifact.