ipn/ipnlocal: blend existing host SSH keys + newly-generated types as needed

If the host only has RSA, use its RSA + generate ecdsa + ed25519, etc.

Perhaps fixes https://twitter.com/colek42c/status/1550554439299244032 and
something else that was reported.

Change-Id: I88dc475c8e3d95b6f25288ff7664b8e72655fd16
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/ssh.go

@@ -26,6 +26,7 @@ import (
 
 	"github.com/tailscale/golang-x-crypto/ssh"
 	"tailscale.com/envknob"
+	"tailscale.com/util/mak"
 )
 
 var useHostKeys = envknob.Bool("TS_USE_SYSTEM_SSH_HOST_KEYS")
@@ -36,34 +37,39 @@ var useHostKeys = envknob.Bool("TS_USE_SYSTEM_SSH_HOST_KEYS")
 var keyTypes = []string{"rsa", "ecdsa", "ed25519"}
 
 func (b *LocalBackend) GetSSH_HostKeys() (keys []ssh.Signer, err error) {
+	var existing map[string]ssh.Signer
 	if os.Geteuid() == 0 {
-		keys, err = b.getSystemSSH_HostKeys()
-		if err != nil || len(keys) > 0 {
-			return keys, err
-		}
-		// Otherwise, perhaps they don't have OpenSSH etc installed.
-		// Generate our own keys...
+		existing = b.getSystemSSH_HostKeys()
 	}
-	return b.getTailscaleSSH_HostKeys()
+	return b.getTailscaleSSH_HostKeys(existing)
 }
 
-func (b *LocalBackend) getTailscaleSSH_HostKeys() (keys []ssh.Signer, err error) {
-	root := b.TailscaleVarRoot()
-	if root == "" {
-		return nil, errors.New("no var root for ssh keys")
-	}
-	keyDir := filepath.Join(root, "ssh")
-	if err := os.MkdirAll(keyDir, 0700); err != nil {
-		return nil, err
-	}
+// getTailscaleSSH_HostKeys returns the three (rsa, ecdsa, ed25519) SSH host
+// keys, reusing the provided ones in existing if present in the map.
+func (b *LocalBackend) getTailscaleSSH_HostKeys(existing map[string]ssh.Signer) (keys []ssh.Signer, err error) {
+	var keyDir string // lazily initialized $TAILSCALE_VAR/ssh dir.
 	for _, typ := range keyTypes {
+		if s, ok := existing[typ]; ok {
+			keys = append(keys, s)
+			continue
+		}
+		if keyDir == "" {
+			root := b.TailscaleVarRoot()
+			if root == "" {
+				return nil, errors.New("no var root for ssh keys")
+			}
+			keyDir = filepath.Join(root, "ssh")
+			if err := os.MkdirAll(keyDir, 0700); err != nil {
+				return nil, err
+			}
+		}
 		hostKey, err := b.hostKeyFileOrCreate(keyDir, typ)
 		if err != nil {
-			return nil, err
+			return nil, fmt.Errorf("error creating SSH host key type %q in %q: %w", typ, keyDir, err)
 		}
 		signer, err := ssh.ParsePrivateKey(hostKey)
 		if err != nil {
-			return nil, err
+			return nil, fmt.Errorf("error parsing SSH host key type %q from %q: %w", typ, keyDir, err)
 		}
 		keys = append(keys, signer)
 	}
@@ -115,24 +121,21 @@ func (b *LocalBackend) hostKeyFileOrCreate(keyDir, typ string) ([]byte, error) {
 	return pemGen, err
 }
 
-func (b *LocalBackend) getSystemSSH_HostKeys() (ret []ssh.Signer, err error) {
-	// TODO(bradfitz): cache this?
+func (b *LocalBackend) getSystemSSH_HostKeys() (ret map[string]ssh.Signer) {
 	for _, typ := range keyTypes {
 		filename := "/etc/ssh/ssh_host_" + typ + "_key"
 		hostKey, err := ioutil.ReadFile(filename)
-		if os.IsNotExist(err) || len(bytes.TrimSpace(hostKey)) == 0 {
+		if err != nil || len(bytes.TrimSpace(hostKey)) == 0 {
 			continue
 		}
-		if err != nil {
-			return nil, err
-		}
 		signer, err := ssh.ParsePrivateKey(hostKey)
 		if err != nil {
-			return nil, fmt.Errorf("error reading private key %s: %w", filename, err)
+			b.logf("warning: error reading host key %s: %v (generating one instead)", filename, err)
+			continue
 		}
-		ret = append(ret, signer)
+		mak.Set(&ret, typ, signer)
 	}
-	return ret, nil
+	return ret
 }
 
 func (b *LocalBackend) getSSHHostKeyPublicStrings() (ret []string) {

ipn/ipnlocal/ssh_test.go

@@ -15,7 +15,7 @@ import (
 func TestSSHKeyGen(t *testing.T) {
 	dir := t.TempDir()
 	lb := &LocalBackend{varRoot: dir}
-	keys, err := lb.getTailscaleSSH_HostKeys()
+	keys, err := lb.getTailscaleSSH_HostKeys(nil)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -32,7 +32,7 @@ func TestSSHKeyGen(t *testing.T) {
 		t.Fatalf("keys = %v; want %v", got, want)
 	}
 
-	keys2, err := lb.getTailscaleSSH_HostKeys()
+	keys2, err := lb.getTailscaleSSH_HostKeys(nil)
 	if err != nil {
 		t.Fatal(err)
 	}