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gitea/modules/ssh/ssh.go
wxiaoguang d1c1e3cbcc
Fine tune ssh related comments and code (#32846)
Add more comments to explain the ssh problem, 
and rename `sshConn` to `sshSession`
2024-12-15 06:07:50 +00:00

442 lines
14 KiB
Go

// 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"
"reflect"
"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"
)
// The ssh auth overall works like this:
// NewServerConn:
// serverHandshake+serverAuthenticate:
// PublicKeyCallback:
// PublicKeyHandler (our code):
// reset(ctx.Permissions) and set ctx.Permissions.giteaKeyID = keyID
// pubKey.Verify
// return ctx.Permissions // only reaches here, the pub key is really authenticated
// set conn.Permissions from serverAuthenticate
// sessionHandler(conn)
//
// Then sessionHandler should only use the "verified keyID" from the original ssh conn, but not the ctx one.
// Otherwise, if a user provides 2 keys A (a correct one) and B (public key matches but no private key),
// then only A succeeds to authenticate, sessionHandler will see B's keyID
//
// After x/crypto >= 0.31.0 (fix CVE-2024-45337), the PublicKeyCallback will be called again for the verified key,
// it mitigates the misuse for most cases, it's still good for us to make sure we don't rely on that mitigation
// and do not misuse the PublicKeyCallback: we should only use the verified keyID from the verified ssh conn.
const giteaPermissionExtensionKeyID = "gitea-perm-ext-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()
}
// sessionPartial is the private struct from "gliderlabs/ssh/session.go"
// We need to read the original "conn" field from "ssh.Session interface" which contains the "*session pointer"
// https://github.com/gliderlabs/ssh/blob/d137aad99cd6f2d9495bfd98c755bec4e5dffb8c/session.go#L109-L113
// If upstream fixes the problem and/or changes the struct, we need to follow.
// If the struct mismatches, the builtin ssh server will fail during integration tests.
type sessionPartial struct {
sync.Mutex
gossh.Channel
conn *gossh.ServerConn
}
func ptr[T any](intf any) *T {
// https://pkg.go.dev/unsafe#Pointer
// (1) Conversion of a *T1 to Pointer to *T2.
// Provided that T2 is no larger than T1 and that the two share an equivalent memory layout,
// this conversion allows reinterpreting data of one type as data of another type.
v := reflect.ValueOf(intf)
p := v.UnsafePointer()
return (*T)(p)
}
func sessionHandler(session ssh.Session) {
// here can't use session.Permissions() because it only uses the value from ctx, which might not be the authenticated one.
// so we must use the original ssh conn, which always contains the correct (verified) keyID.
sshSession := ptr[sessionPartial](session)
keyID := sshSession.conn.Permissions.Extensions[giteaPermissionExtensionKeyID]
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 {
// The publicKeyHandler (PublicKeyCallback) only helps to provide the candidate keys to authenticate,
// It does NOT really verify here, so we could only record the related information here.
// After authentication (Verify), the "Permissions" will be assigned to the ssh conn,
// then we can use it in the "session handler"
// first, reset the ctx permissions (just like https://github.com/gliderlabs/ssh/pull/243 does)
// it shouldn't be reused across different ssh conn (sessions), each pub key should have its own "Permissions"
ctx.Permissions().Permissions = &gossh.Permissions{}
setPermExt := func(keyID int64) {
ctx.Permissions().Permissions.Extensions = map[string]string{
giteaPermissionExtensionKeyID: fmt.Sprint(keyID),
}
}
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)
}
setPermExt(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))
}
setPermExt(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
}