cmd/dist,release/dist: sign release tarballs with an ECDSA key (#8759)

Pass an optional PEM-encoded ECDSA key to `cmd/dist` to sign all built
tarballs. The signature is stored next to the tarball with a `.sig`
extension.

Tested this with an `openssl`-generated key pair and verified the
resulting signature.

Updates #8760

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

cmd/dist/dist.go

@@ -6,6 +6,7 @@ package main
 
 import (
 	"context"
+	"crypto"
 	"errors"
 	"flag"
 	"log"
@@ -19,10 +20,10 @@ import (
 
 var synologyPackageCenter bool
 
-func getTargets() ([]dist.Target, error) {
+func getTargets(tgzSigner crypto.Signer) ([]dist.Target, error) {
 	var ret []dist.Target
 
-	ret = append(ret, unixpkgs.Targets()...)
+	ret = append(ret, unixpkgs.Targets(tgzSigner)...)
 	// Synology packages can be built either for sideloading, or for
 	// distribution by Synology in their package center. When
 	// distributed through the package center, apps can request

release/dist/cli/cli.go

@@ -6,6 +6,9 @@ package cli
 
 import (
 	"context"
+	"crypto"
+	"crypto/x509"
+	"encoding/pem"
 	"errors"
 	"flag"
 	"fmt"
@@ -23,7 +26,7 @@ import (
 // getTargets is a function that gets run in the Exec function of commands that
 // need to know the target list. Its execution is deferred in this way to allow
 // customization of command FlagSets with flags that influence the target list.
-func CLI(getTargets func() ([]dist.Target, error)) *ffcli.Command {
+func CLI(getTargets func(tgzSigner crypto.Signer) ([]dist.Target, error)) *ffcli.Command {
 	return &ffcli.Command{
 		Name:       "dist",
 		ShortUsage: "dist [flags] <command> [command flags]",
@@ -33,7 +36,7 @@ func CLI(getTargets func() ([]dist.Target, error)) *ffcli.Command {
 			{
 				Name: "list",
 				Exec: func(ctx context.Context, args []string) error {
-					targets, err := getTargets()
+					targets, err := getTargets(nil)
 					if err != nil {
 						return err
 					}
@@ -49,7 +52,11 @@ func CLI(getTargets func() ([]dist.Target, error)) *ffcli.Command {
 			{
 				Name: "build",
 				Exec: func(ctx context.Context, args []string) error {
-					targets, err := getTargets()
+					tgzSigner, err := parseSigningKey(buildArgs.tgzSigningKey)
+					if err != nil {
+						return err
+					}
+					targets, err := getTargets(tgzSigner)
 					if err != nil {
 						return err
 					}
@@ -61,6 +68,7 @@ func CLI(getTargets func() ([]dist.Target, error)) *ffcli.Command {
 					fs := flag.NewFlagSet("build", flag.ExitOnError)
 					fs.StringVar(&buildArgs.manifest, "manifest", "", "manifest file to write")
 					fs.BoolVar(&buildArgs.verbose, "verbose", false, "verbose logging")
+					fs.StringVar(&buildArgs.tgzSigningKey, "tgz-signing-key", "", "path to private signing key for release tarballs")
 					return fs
 				})(),
 				LongHelp: strings.TrimSpace(`
@@ -88,8 +96,9 @@ func runList(ctx context.Context, filters []string, targets []dist.Target) error
 }
 
 var buildArgs struct {
-	manifest string
-	verbose  bool
+	manifest      string
+	verbose       bool
+	tgzSigningKey string
 }
 
 func runBuild(ctx context.Context, filters []string, targets []dist.Target) error {
@@ -142,3 +151,21 @@ func runBuild(ctx context.Context, filters []string, targets []dist.Target) erro
 	fmt.Println("Done! Took", time.Since(st))
 	return nil
 }
+
+func parseSigningKey(path string) (crypto.Signer, error) {
+	if path == "" {
+		return nil, nil
+	}
+	raw, err := os.ReadFile(path)
+	if err != nil {
+		return nil, err
+	}
+	b, rest := pem.Decode(raw)
+	if b == nil {
+		return nil, fmt.Errorf("failed to decode PEM data in %q", path)
+	}
+	if len(rest) > 0 {
+		return nil, fmt.Errorf("trailing data in %q, please check that the key file was not corrupted", path)
+	}
+	return x509.ParseECPrivateKey(b.Bytes)
+}

release/dist/unixpkgs/pkgs.go

@@ -7,6 +7,9 @@ package unixpkgs
 import (
 	"archive/tar"
 	"compress/gzip"
+	"crypto"
+	"crypto/rand"
+	"crypto/sha512"
 	"errors"
 	"fmt"
 	"io"
@@ -22,6 +25,7 @@ import (
 type tgzTarget struct {
 	filenameArch string // arch to use in filename instead of deriving from goenv["GOARCH"]
 	goenv        map[string]string
+	signer       crypto.Signer
 }
 
 func (t *tgzTarget) arch() string {
@@ -65,7 +69,11 @@ func (t *tgzTarget) Build(b *dist.Build) ([]string, error) {
 		return nil, err
 	}
 	defer f.Close()
-	gw := gzip.NewWriter(f)
+	// Hash the final output we're writing to the file, after tar and gzip
+	// writers did their thing.
+	h := sha512.New()
+	hw := io.MultiWriter(f, h)
+	gw := gzip.NewWriter(hw)
 	defer gw.Close()
 	tw := tar.NewWriter(gw)
 	defer tw.Close()
@@ -146,7 +154,21 @@ func (t *tgzTarget) Build(b *dist.Build) ([]string, error) {
 		return nil, err
 	}
 
-	return []string{filename}, nil
+	files := []string{filename}
+
+	if t.signer != nil {
+		sig, err := t.signer.Sign(rand.Reader, h.Sum(nil), crypto.SHA512)
+		if err != nil {
+			return nil, err
+		}
+		sigFilename := out + ".sig"
+		if err := os.WriteFile(sigFilename, sig, 0644); err != nil {
+			return nil, err
+		}
+		files = append(files, filename+".sig")
+	}
+
+	return files, nil
 }
 
 type debTarget struct {

release/dist/unixpkgs/targets.go

@@ -4,6 +4,7 @@
 package unixpkgs
 
 import (
+	"crypto"
 	"fmt"
 	"sort"
 	"strings"
@@ -14,7 +15,7 @@ import (
 	_ "github.com/goreleaser/nfpm/rpm"
 )
 
-func Targets() []dist.Target {
+func Targets(signer crypto.Signer) []dist.Target {
 	var ret []dist.Target
 	for goosgoarch := range tarballs {
 		goos, goarch := splitGoosGoarch(goosgoarch)
@@ -23,6 +24,7 @@ func Targets() []dist.Target {
 				"GOOS":   goos,
 				"GOARCH": goarch,
 			},
+			signer: signer,
 		})
 	}
 	for goosgoarch := range debs {
@@ -53,6 +55,7 @@ func Targets() []dist.Target {
 			"GOARCH": "386",
 			"GO386":  "softfloat",
 		},
+		signer: signer,
 	})
 
 	sort.Slice(ret, func(i, j int) bool {