1 files changed, 387 insertions, 0 deletions
diff --git a/modules/ssh/ssh.go b/modules/ssh/ssh.go
new file mode 100644
index 00000000..f8e4f569
--- /dev/null
+++ b/modules/ssh/ssh.go
@@ -0,0 +1,387 @@
+// Copyright 2017 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package ssh
+
+import (
+	"bytes"
+	"context"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/pem"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"sync"
+	"syscall"
+
+	asymkey_model "code.gitea.io/gitea/models/asymkey"
+	"code.gitea.io/gitea/modules/graceful"
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/process"
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/util"
+
+	"github.com/gliderlabs/ssh"
+	gossh "golang.org/x/crypto/ssh"
+)
+
+type contextKey string
+
+const giteaKeyID = contextKey("gitea-key-id")
+
+func getExitStatusFromError(err error) int {
+	if err == nil {
+		return 0
+	}
+
+	exitErr, ok := err.(*exec.ExitError)
+	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 ssh.Session) {
+	keyID := fmt.Sprintf("%d", session.Context().Value(giteaKeyID).(int64))
+
+	command := session.RawCommand()
+
+	log.Trace("SSH: Payload: %v", command)
+
+	args := []string{"--config=" + setting.CustomConf, "serv", "key-" + keyID}
+	log.Trace("SSH: Arguments: %v", args)
+
+	ctx, cancel := context.WithCancel(session.Context())
+	defer cancel()
+
+	gitProtocol := ""
+	for _, env := range session.Environ() {
+		if strings.HasPrefix(env, "GIT_PROTOCOL=") {
+			_, gitProtocol, _ = strings.Cut(env, "=")
+			break
+		}
+	}
+
+	cmd := exec.CommandContext(ctx, setting.AppPath, args...)
+	cmd.Env = append(
+		os.Environ(),
+		"SSH_ORIGINAL_COMMAND="+command,
+		"SKIP_MINWINSVC=1",
+		"GIT_PROTOCOL="+gitProtocol,
+	)
+
+	stdout, err := cmd.StdoutPipe()
+	if err != nil {
+		log.Error("SSH: StdoutPipe: %v", err)
+		return
+	}
+	defer stdout.Close()
+
+	stderr, err := cmd.StderrPipe()
+	if err != nil {
+		log.Error("SSH: StderrPipe: %v", err)
+		return
+	}
+	defer stderr.Close()
+
+	stdin, err := cmd.StdinPipe()
+	if err != nil {
+		log.Error("SSH: StdinPipe: %v", err)
+		return
+	}
+	defer stdin.Close()
+
+	process.SetSysProcAttribute(cmd)
+
+	wg := &sync.WaitGroup{}
+	wg.Add(2)
+
+	if err = cmd.Start(); err != nil {
+		log.Error("SSH: Start: %v", err)
+		return
+	}
+
+	go func() {
+		defer stdin.Close()
+		if _, err := io.Copy(stdin, session); err != nil {
+			log.Error("Failed to write session to stdin. %s", err)
+		}
+	}()
+
+	go func() {
+		defer wg.Done()
+		defer stdout.Close()
+		if _, err := io.Copy(session, stdout); err != nil {
+			log.Error("Failed to write stdout to session. %s", err)
+		}
+	}()
+
+	go func() {
+		defer wg.Done()
+		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)
+		}
+	}
+
+	if err := session.Exit(getExitStatusFromError(err)); err != nil && !errors.Is(err, io.EOF) {
+		log.Error("Session failed to exit. %s", err)
+	}
+}
+
+func publicKeyHandler(ctx ssh.Context, key ssh.PublicKey) bool {
+	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), ctx.RemoteAddr())
+	}
+
+	if ctx.User() != setting.SSH.BuiltinServerUser {
+		log.Warn("Invalid SSH username %s - must use %s for all git operations via ssh", ctx.User(), setting.SSH.BuiltinServerUser)
+		log.Warn("Failed authentication attempt from %s", ctx.RemoteAddr())
+		return false
+	}
+
+	// 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", ctx.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", ctx.RemoteAddr())
+			return false
+		}
+
+		if cert.CertType != gossh.UserCert {
+			log.Warn("Certificate Rejected: Not a user certificate")
+			log.Warn("Failed authentication attempt from %s", ctx.RemoteAddr())
+			return false
+		}
+
+		// 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", ctx.RemoteAddr(), principal)
+					continue principalLoop
+				}
+				log.Error("SearchPublicKeyByContentExact: %v", err)
+				return false
+			}
+
+			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", ctx.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, ctx.RemoteAddr())
+				log.Warn("Failed authentication attempt from %s", ctx.RemoteAddr())
+
+				return false
+			}
+
+			if log.IsDebug() { // <- FingerprintSHA256 is kinda expensive so only calculate it if necessary
+				log.Debug("Successfully authenticated: %s Certificate Fingerprint: %s Principal: %s", ctx.RemoteAddr(), gossh.FingerprintSHA256(key), principal)
+			}
+			ctx.SetValue(giteaKeyID, pkey.ID)
+
+			return true
+		}
+
+		log.Warn("From %s Fingerprint: %s is a certificate, but no valid principals found", ctx.RemoteAddr(), gossh.FingerprintSHA256(key))
+		log.Warn("Failed authentication attempt from %s", ctx.RemoteAddr())
+		return false
+	}
+
+	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", ctx.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), ctx.RemoteAddr())
+			log.Warn("Failed authentication attempt from %s", ctx.RemoteAddr())
+			return false
+		}
+		log.Error("SearchPublicKeyByContent: %v", err)
+		return false
+	}
+
+	if log.IsDebug() { // <- FingerprintSHA256 is kinda expensive so only calculate it if necessary
+		log.Debug("Successfully authenticated: %s Public Key Fingerprint: %s", ctx.RemoteAddr(), gossh.FingerprintSHA256(key))
+	}
+	ctx.SetValue(giteaKeyID, pkey.ID)
+
+	return true
+}
+
+// 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 a SSH server listens on given port.
+func Listen(host string, port int, ciphers, keyExchanges, macs []string) {
+	srv := ssh.Server{
+		Addr:             net.JoinHostPort(host, strconv.Itoa(port)),
+		PublicKeyHandler: publicKeyHandler,
+		Handler:          sessionHandler,
+		ServerConfigCallback: func(ctx ssh.Context) *gossh.ServerConfig {
+			config := &gossh.ServerConfig{}
+			config.KeyExchanges = keyExchanges
+			config.MACs = macs
+			config.Ciphers = ciphers
+			return config
+		},
+		ConnectionFailedCallback: sshConnectionFailed,
+		// We need to explicitly disable the PtyCallback so text displays
+		// properly.
+		PtyCallback: func(ctx ssh.Context, pty ssh.Pty) bool {
+			return false
+		},
+	}
+
+	keys := make([]string, 0, len(setting.SSH.ServerHostKeys))
+	for _, key := range setting.SSH.ServerHostKeys {
+		isExist, err := util.IsExist(key)
+		if err != nil {
+			log.Fatal("Unable to check if %s exists. Error: %v", setting.SSH.ServerHostKeys, err)
+		}
+		if isExist {
+			keys = append(keys, key)
+		}
+	}
+
+	if len(keys) == 0 {
+		filePath := filepath.Dir(setting.SSH.ServerHostKeys[0])
+
+		if err := os.MkdirAll(filePath, os.ModePerm); err != nil {
+			log.Error("Failed to create dir %s: %v", filePath, err)
+		}
+
+		err := GenKeyPair(setting.SSH.ServerHostKeys[0])
+		if err != nil {
+			log.Fatal("Failed to generate private key: %v", err)
+		}
+		log.Trace("New private key is generated: %s", setting.SSH.ServerHostKeys[0])
+		keys = append(keys, setting.SSH.ServerHostKeys[0])
+	}
+
+	for _, key := range keys {
+		log.Info("Adding SSH host key: %s", key)
+		err := srv.SetOption(ssh.HostKeyFile(key))
+		if err != nil {
+			log.Error("Failed to set Host Key. %s", err)
+		}
+	}
+
+	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) error {
+	privateKey, err := rsa.GenerateKey(rand.Reader, 4096)
+	if err != nil {
+		return err
+	}
+
+	privateKeyPEM := &pem.Block{Type: "RSA PRIVATE KEY", Bytes: x509.MarshalPKCS1PrivateKey(privateKey)}
+	f, err := os.OpenFile(keyPath, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0o600)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if err = f.Close(); err != nil {
+			log.Error("Close: %v", err)
+		}
+	}()
+
+	if err := pem.Encode(f, privateKeyPEM); err != nil {
+		return err
+	}
+
+	// generate public key
+	pub, err := gossh.NewPublicKey(&privateKey.PublicKey)
+	if err != nil {
+		return err
+	}
+
+	public := gossh.MarshalAuthorizedKey(pub)
+	p, err := os.OpenFile(keyPath+".pub", os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0o600)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if err = p.Close(); err != nil {
+			log.Error("Close: %v", err)
+		}
+	}()
+	_, err = p.Write(public)
+	return err
+}