snapshot of https://github.com/go-gitea/gitea.git (main, 2026-09-15; blobs > 131072B omitted)

modules/ssh/ssh.go

@@ -0,0 +1,374 @@
+// Copyright 2017 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package ssh
+
+import (
+	"bytes"
+	"context"
+	"encoding/pem"
+	"errors"
+	"io"
+	"net"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"sync"
+	"syscall"
+
+	asymkey_model "gitea.dev/models/asymkey"
+	"gitea.dev/modules/generate"
+	"gitea.dev/modules/graceful"
+	"gitea.dev/modules/log"
+	"gitea.dev/modules/process"
+	"gitea.dev/modules/setting"
+	"gitea.dev/modules/util"
+
+	gossh "golang.org/x/crypto/ssh"
+)
+
+const giteaPermissionExtensionKeyID = "gitea-perm-ext-key-id"
+
+func getExitStatusFromError(err error) int {
+	if err == nil {
+		return 0
+	}
+
+	exitErr, ok := errors.AsType[*exec.ExitError](err)
+	if !ok {
+		return 1
+	}
+
+	waitStatus, ok := exitErr.Sys().(syscall.WaitStatus)
+	if !ok {
+		// This is a fallback and should at least let us return something useful
+		// when running on Windows, even if it isn't completely accurate.
+		if exitErr.Success() {
+			return 0
+		}
+
+		return 1
+	}
+
+	return waitStatus.ExitStatus()
+}
+
+func sessionHandler(session *sshSession) int {
+	// the conn permissions are the ones of the key which really authenticated, see publicKeyHandler
+	keyID := session.conn.Permissions.Extensions[giteaPermissionExtensionKeyID]
+
+	log.Trace("SSH: Payload: %v", session.rawCmd)
+
+	args := []string{"--config=" + setting.CustomConf, "serv", "key-" + keyID}
+	log.Trace("SSH: Arguments: %v", args)
+
+	ctx, cancel := context.WithCancel(session.ctx)
+	defer cancel()
+
+	gitProtocol := ""
+	for _, env := range session.env {
+		if strings.HasPrefix(env, "GIT_PROTOCOL=") {
+			_, gitProtocol, _ = strings.Cut(env, "=")
+			break
+		}
+	}
+
+	cmd := process.CommandContext(ctx, setting.AppPath, args...)
+	cmd.Env = append(
+		os.Environ(),
+		"SSH_ORIGINAL_COMMAND="+session.rawCmd,
+		"SKIP_MINWINSVC=1",
+		"GIT_PROTOCOL="+gitProtocol,
+	)
+
+	stdout, err := cmd.StdoutPipe()
+	if err != nil {
+		log.Error("SSH: StdoutPipe: %v", err)
+		return 1
+	}
+	defer stdout.Close()
+
+	stderr, err := cmd.StderrPipe()
+	if err != nil {
+		log.Error("SSH: StderrPipe: %v", err)
+		return 1
+	}
+	defer stderr.Close()
+
+	stdin, err := cmd.StdinPipe()
+	if err != nil {
+		log.Error("SSH: StdinPipe: %v", err)
+		return 1
+	}
+	defer stdin.Close()
+
+	wg := &sync.WaitGroup{}
+
+	if err = cmd.Start(); err != nil {
+		log.Error("SSH: Start: %v", err)
+		return 1
+	}
+
+	go func() {
+		defer stdin.Close()
+		if _, err := io.Copy(stdin, session); err != nil {
+			log.Error("Failed to write session to stdin. %s", err)
+		}
+	}()
+
+	wg.Go(func() {
+		defer stdout.Close()
+		if _, err := io.Copy(session, stdout); err != nil {
+			log.Error("Failed to write stdout to session. %s", err)
+		}
+	})
+
+	wg.Go(func() {
+		defer stderr.Close()
+		if _, err := io.Copy(session.Stderr(), stderr); err != nil {
+			log.Error("Failed to write stderr to session. %s", err)
+		}
+	})
+
+	// Ensure all the output has been written before we wait on the command
+	// to exit.
+	wg.Wait()
+
+	// Wait for the command to exit and log any errors we get
+	err = cmd.Wait()
+	if err != nil {
+		// Cannot use errors.Is here because ExitError doesn't implement Is
+		// Thus errors.Is will do equality test NOT type comparison
+		if _, ok := err.(*exec.ExitError); !ok {
+			log.Error("SSH: Wait: %v", err)
+		}
+	}
+
+	return getExitStatusFromError(err)
+}
+
+func keyPermissions(keyID int64) *gossh.Permissions {
+	return &gossh.Permissions{Extensions: map[string]string{
+		giteaPermissionExtensionKeyID: strconv.FormatInt(keyID, 10),
+	}}
+}
+
+// publicKeyHandler only offers the candidate keys, it does not verify them. x/crypto assigns the
+// returned Permissions to the ssh conn once it verified the signature for that key, so a user
+// offering keys A (with a private key) and B (without one) authenticates and is served as A.
+func publicKeyHandler(ctx context.Context, conn gossh.ConnMetadata, key gossh.PublicKey) (*gossh.Permissions, error) {
+	if log.IsDebug() { // <- FingerprintSHA256 is kinda expensive so only calculate it if necessary
+		log.Debug("Handle Public Key: Fingerprint: %s from %s", gossh.FingerprintSHA256(key), conn.RemoteAddr())
+	}
+
+	if conn.User() != setting.SSH.BuiltinServerUser {
+		log.Warn("Invalid SSH username %s - must use %s for all git operations via ssh", conn.User(), setting.SSH.BuiltinServerUser)
+		log.Warn("Failed authentication attempt from %s", conn.RemoteAddr())
+		return nil, util.ErrPermissionDenied
+	}
+
+	// check if we have a certificate
+	if cert, ok := key.(*gossh.Certificate); ok {
+		if log.IsDebug() { // <- FingerprintSHA256 is kinda expensive so only calculate it if necessary
+			log.Debug("Handle Certificate: %s Fingerprint: %s is a certificate", conn.RemoteAddr(), gossh.FingerprintSHA256(key))
+		}
+
+		if len(setting.SSH.TrustedUserCAKeys) == 0 {
+			log.Warn("Certificate Rejected: No trusted certificate authorities for this server")
+			log.Warn("Failed authentication attempt from %s", conn.RemoteAddr())
+			return nil, util.ErrPermissionDenied
+		}
+
+		if cert.CertType != gossh.UserCert {
+			log.Warn("Certificate Rejected: Not a user certificate")
+			log.Warn("Failed authentication attempt from %s", conn.RemoteAddr())
+			return nil, util.ErrPermissionDenied
+		}
+
+		// look for the exact principal
+	principalLoop:
+		for _, principal := range cert.ValidPrincipals {
+			pkey, err := asymkey_model.SearchPublicKeyByContentExact(ctx, principal)
+			if err != nil {
+				if asymkey_model.IsErrKeyNotExist(err) {
+					log.Debug("Principal Rejected: %s Unknown Principal: %s", conn.RemoteAddr(), principal)
+					continue principalLoop
+				}
+				log.Error("SearchPublicKeyByContentExact: %v", err)
+				return nil, util.ErrPermissionDenied
+			}
+
+			c := &gossh.CertChecker{
+				IsUserAuthority: func(auth gossh.PublicKey) bool {
+					marshaled := auth.Marshal()
+					for _, k := range setting.SSH.TrustedUserCAKeysParsed {
+						if bytes.Equal(marshaled, k.Marshal()) {
+							return true
+						}
+					}
+
+					return false
+				},
+			}
+
+			// check the CA of the cert
+			if !c.IsUserAuthority(cert.SignatureKey) {
+				if log.IsDebug() {
+					log.Debug("Principal Rejected: %s Untrusted Authority Signature Fingerprint %s for Principal: %s", conn.RemoteAddr(), gossh.FingerprintSHA256(cert.SignatureKey), principal)
+				}
+				continue principalLoop
+			}
+
+			// validate the cert for this principal
+			if err := c.CheckCert(principal, cert); err != nil {
+				// User is presenting an invalid certificate - STOP any further processing
+				log.Error("Invalid Certificate KeyID %s with Signature Fingerprint %s presented for Principal: %s from %s", cert.KeyId, gossh.FingerprintSHA256(cert.SignatureKey), principal, conn.RemoteAddr())
+				log.Warn("Failed authentication attempt from %s", conn.RemoteAddr())
+
+				return nil, util.ErrPermissionDenied
+			}
+
+			if log.IsDebug() { // <- FingerprintSHA256 is kinda expensive so only calculate it if necessary
+				log.Debug("Successfully authenticated: %s Certificate Fingerprint: %s Principal: %s", conn.RemoteAddr(), gossh.FingerprintSHA256(key), principal)
+			}
+			return keyPermissions(pkey.ID), nil
+		}
+
+		log.Warn("From %s Fingerprint: %s is a certificate, but no valid principals found", conn.RemoteAddr(), gossh.FingerprintSHA256(key))
+		log.Warn("Failed authentication attempt from %s", conn.RemoteAddr())
+		return nil, util.ErrPermissionDenied
+	}
+
+	if log.IsDebug() { // <- FingerprintSHA256 is kinda expensive so only calculate it if necessary
+		log.Debug("Handle Public Key: %s Fingerprint: %s is not a certificate", conn.RemoteAddr(), gossh.FingerprintSHA256(key))
+	}
+
+	pkey, err := asymkey_model.SearchPublicKeyByContent(ctx, strings.TrimSpace(string(gossh.MarshalAuthorizedKey(key))))
+	if err != nil {
+		if asymkey_model.IsErrKeyNotExist(err) {
+			log.Warn("Unknown public key: %s from %s", gossh.FingerprintSHA256(key), conn.RemoteAddr())
+			log.Warn("Failed authentication attempt from %s", conn.RemoteAddr())
+			return nil, util.ErrPermissionDenied
+		}
+		log.Error("SearchPublicKeyByContent: %v", err)
+		return nil, util.ErrPermissionDenied
+	}
+
+	if log.IsDebug() { // <- FingerprintSHA256 is kinda expensive so only calculate it if necessary
+		log.Debug("Successfully authenticated: %s Public Key Fingerprint: %s", conn.RemoteAddr(), gossh.FingerprintSHA256(key))
+	}
+	return keyPermissions(pkey.ID), nil
+}
+
+// sshConnectionFailed logs a failed connection
+// - this mainly exists to give a nice function name in logging
+func sshConnectionFailed(conn net.Conn, err error) {
+	// Log the underlying error with a specific message
+	log.Warn("Failed connection from %s with error: %v", conn.RemoteAddr(), err)
+	// Log with the standard failed authentication from message for simpler fail2ban configuration
+	log.Warn("Failed authentication attempt from %s", conn.RemoteAddr())
+}
+
+// Listen starts an SSH server listening on given port.
+func Listen(host string, port int, ciphers, keyExchanges, macs []string) {
+	hostKeyFiles := make([]string, 0, len(setting.SSH.ServerHostKeys))
+	for _, key := range setting.SSH.ServerHostKeys {
+		_, err := os.Stat(key)
+		if err != nil {
+			if !errors.Is(err, os.ErrNotExist) {
+				log.Fatal("Unable to check if %s exists. Error: %v", setting.SSH.ServerHostKeys, err)
+			}
+			continue
+		}
+		hostKeyFiles = append(hostKeyFiles, key)
+	}
+
+	if len(hostKeyFiles) == 0 {
+		hostKeyDir := filepath.Dir(setting.SSH.ServerHostKeys[0])
+		err := os.MkdirAll(hostKeyDir, os.ModePerm)
+		if err != nil {
+			log.Error("Failed to create dir %s: %v", hostKeyDir, err)
+		}
+		hostKeyFiles, err = InitDefaultHostKeys(hostKeyDir)
+		if err != nil {
+			log.Fatal("Failed to generate private key: %v", err)
+		}
+	}
+
+	var hostSigners []gossh.Signer
+	for _, keyFile := range hostKeyFiles {
+		pemBytes, err := os.ReadFile(keyFile)
+		if err == nil {
+			var signer gossh.Signer
+			if signer, err = gossh.ParsePrivateKey(pemBytes); err == nil {
+				log.Info("Adding SSH host key: %s", keyFile)
+				hostSigners = append(hostSigners, signer)
+				continue
+			}
+		}
+		log.Error("Failed to load SSH host key %s: %v", keyFile, err)
+	}
+
+	if len(hostSigners) == 0 {
+		log.Fatal("No usable SSH host key, tried: %v", hostKeyFiles)
+	}
+
+	srv := &sshServer{
+		addr:        net.JoinHostPort(host, strconv.Itoa(port)),
+		hostSigners: hostSigners,
+		config:      gossh.Config{Ciphers: ciphers, KeyExchanges: keyExchanges, MACs: macs},
+	}
+
+	go func() {
+		_, _, finished := process.GetManager().AddTypedContext(graceful.GetManager().HammerContext(), "Service: Built-in SSH server", process.SystemProcessType, true)
+		defer finished()
+		listen(srv)
+	}()
+}
+
+// GenKeyPair make a pair of public and private keys for SSH access.
+// Public key is encoded in the format for inclusion in an OpenSSH authorized_keys file.
+// Private Key generated is PEM encoded
+func GenKeyPair(keyPath string, keyType generate.SSHKeyType, bits int) error {
+	publicKey, privateKeyPEM, err := generate.NewSSHKey(keyType, bits)
+	if err != nil {
+		return err
+	}
+
+	public := gossh.MarshalAuthorizedKey(publicKey)
+	privateKeyBuf := &bytes.Buffer{}
+	err = pem.Encode(privateKeyBuf, privateKeyPEM)
+	if err != nil {
+		return err
+	}
+
+	err = os.WriteFile(keyPath, privateKeyBuf.Bytes(), 0o600)
+	if err != nil {
+		return err
+	}
+
+	return os.WriteFile(keyPath+".pub", public, 0o644)
+}
+
+// InitDefaultHostKeys mirrors how ssh-keygen -A operates
+// it runs checks if public and private keys are already defined and creates new ones if not present
+// key naming does not follow the OpenSSH convention due to existing settings being gitea.{KeyType} so generation follows gitea convention
+func InitDefaultHostKeys(path string) (keyFiles []string, _ error) {
+	var errs []error
+	keyTypes := []generate.SSHKeyType{generate.SSHKeyRSA, generate.SSHKeyECDSA, generate.SSHKeyED25519}
+	for _, keyType := range keyTypes {
+		keyPath := filepath.Join(path, "gitea."+string(keyType))
+		_, errStatPriv := os.Stat(keyPath)
+		if errStatPriv != nil {
+			err := GenKeyPair(keyPath, keyType, 0)
+			if err != nil {
+				errs = append(errs, err)
+				continue
+			}
+		}
+		keyFiles = append(keyFiles, keyPath)
+	}
+	return keyFiles, errors.Join(errs...)
+}