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sshsigner.go
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sshsigner.go
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package sshsigner
import (
"context"
"crypto"
"crypto/rand"
"encoding/binary"
"errors"
"fmt"
"io"
pseudorand "math/rand"
"net/textproto"
"os"
"strings"
"sync"
"time"
"github.com/aws/aws-sdk-go/aws/arn"
"github.com/golang/protobuf/ptypes/empty"
"github.com/pardot/oidc"
sshsigner "github.com/pardot/sshsigner/proto/sshsigner/v1alpha1"
"github.com/sirupsen/logrus"
"golang.org/x/crypto/ssh"
"google.golang.org/grpc"
"google.golang.org/grpc/codes"
"google.golang.org/grpc/metadata"
"google.golang.org/grpc/status"
)
const (
validAfterDurationSkew = -1 * time.Minute
defaultMaxUserSignedValidDuration = 15 * time.Minute
defaultHostSignedValidDuration = 24 * time.Hour
nonceValidity = 24 * time.Hour
)
const (
ACRMultiFactor string = "http://schemas.openid.net/pape/policies/2007/06/multi-factor"
ACRMultiFactorPhysical string = "http://schemas.openid.net/pape/policies/2007/06/multi-factor-physical"
AMROTP string = "otp"
)
var _ sshsigner.SignerServer = (*SSHSigner)(nil)
// NonceRecorder is used to avoid replays
type NonceRecorder interface {
// RecordNonce should note that a given nonce was used, and indicate if it
// has already been used.
RecordNonce(ctx context.Context, nonce string, expires time.Time) (used bool, err error)
}
// TokenVerifier is used to verify a given JWT, returning claim information
type TokenVerifier interface {
VerifyRaw(ctx context.Context, audience string, rawToken string, opts ...oidc.VerifyOpt) (*oidc.Claims, error)
}
// SignerSource is used to retrieve a signer for signing operations, and
// information about public keys considered valid for this source. This can be
// used to wrap multiple signers / other keysets for use by this service
type SignerSource interface {
// Signer should return a usable crypto.Signer, that will be used for a
// signing operations.
Signer(context.Context) (crypto.Signer, error)
// PublicKeys returns a list of all public keys that should be considered
// valid for this source.
PublicKeys(context.Context) ([]crypto.PublicKey, error)
}
type staticSignerSource struct {
signer crypto.Signer
addlKeys []crypto.PublicKey
}
func (s *staticSignerSource) Signer(context.Context) (crypto.Signer, error) {
return s.signer, nil
}
func (s *staticSignerSource) PublicKeys(context.Context) ([]crypto.PublicKey, error) {
return append([]crypto.PublicKey{s.signer.Public()}, s.addlKeys...), nil
}
// NewStaticSignerSource returns a simple SignerSource that is bound to a single
// key
func NewStaticSignerSource(s crypto.Signer, addlKeys ...crypto.PublicKey) SignerSource {
return &staticSignerSource{
signer: s,
addlKeys: addlKeys,
}
}
type SignerOpt func(s *SSHSigner)
type SSHSigner struct {
Log logrus.FieldLogger
hostSignerSource SignerSource
userSignerSource SignerSource
remoteCacheFor time.Duration
remoteCacheSplay time.Duration
rand *pseudorand.Rand
userCertValidForMax time.Duration
hostCertValidFor time.Duration
NonceRec NonceRecorder
Verifier TokenVerifier
Audience string
ValidAWSAccounts []string
clock func() time.Time
}
// WithHostCertValidityPeriod sets the duration that host certs are valid for,
// after signing time. The default is 24 hours.
func WithHostCertValidityPeriod(p time.Duration) SignerOpt {
return func(s *SSHSigner) {
s.hostCertValidFor = p
}
}
// WithMaxUserCertValidityPeriod sets the maximum duration that user certs are
// valid for, after signing time. The default is 15 minutes. If the submitted
// claims expire in a time less than this, that time will be used instead.
func WithMaxUserCertValidityPeriod(p time.Duration) SignerOpt {
return func(s *SSHSigner) {
s.userCertValidForMax = p
}
}
// WithSignersCache will cache results for public key lookups for a fixed time.
// This can be used to reduce load for the public key endpoints
func WithSignersCache(cacheFor time.Duration) SignerOpt {
return func(s *SSHSigner) {
s.userSignerSource = &cacheKeySource{
SignerSource: s.userSignerSource,
CacheFor: cacheFor,
}
s.hostSignerSource = &cacheKeySource{
SignerSource: s.hostSignerSource,
CacheFor: cacheFor,
}
}
}
// WithCacheControl will return a Cache-Control header on requests to the
// user/host signing keys endpoint. This can be used for server control of how
// often the client fetches keys. The header is marked private, so intermediate
// proxies will ignore this. Max-Age will be set to the maxAge value, plus up to
// splay time.
func WithCacheControl(maxAge, splay time.Duration) SignerOpt {
return func(s *SSHSigner) {
s.remoteCacheFor = maxAge
s.remoteCacheSplay = splay
s.rand = pseudorand.New(pseudorand.NewSource(time.Now().UnixNano() ^ int64(os.Getpid())))
}
}
func New(l logrus.FieldLogger, userSigner SignerSource, hostSigner SignerSource, nonceRec NonceRecorder, v TokenVerifier, aud string, validAWSAccounts []string, opts ...SignerOpt) (*SSHSigner, error) {
ss := &SSHSigner{
Log: l.WithField("component", "sshsigner"),
Verifier: v,
Audience: aud,
ValidAWSAccounts: validAWSAccounts,
NonceRec: nonceRec,
userSignerSource: userSigner,
hostSignerSource: hostSigner,
userCertValidForMax: defaultMaxUserSignedValidDuration,
hostCertValidFor: defaultHostSignedValidDuration,
}
for _, o := range opts {
o(ss)
}
return ss, nil
}
func (s *SSHSigner) SignUserKey(ctx context.Context, req *sshsigner.SignUserKeyRequest) (*sshsigner.SignUserKeyResponse, error) {
logger := s.Log.WithFields(logrus.Fields{
"fn": "SignUserKey",
"ip": remoteAddrFromContext(ctx),
"publickey": req.PublicKey,
})
claims, err := s.authRequest(ctx)
if err != nil {
logger.WithError(err).WithField("at", "auth-request-error").Error()
return nil, status.Error(codes.Unauthenticated, err.Error())
}
if awsI, ok := claims.Extra["awsInstance"].(bool); ok && awsI {
return nil, status.Error(codes.PermissionDenied, "only users may use this RPC")
}
gc := groupClaims{}
if err := claims.Unmarshal(&gc); err != nil {
logger.WithError(err).WithField("at", "unmarshal-claims").Error()
return nil, status.Error(codes.Unauthenticated, err.Error())
}
if len(gc.Groups) == 0 {
return nil, status.Error(codes.PermissionDenied, "user must be member of at least one group")
}
email, _ := claims.Extra["email"].(string)
name, _ := claims.Extra["name"].(string)
logger = logger.WithFields(logrus.Fields{
"subject": claims.Subject,
"username": name,
"email": email,
"nonce": claims.Nonce,
})
nonce := claims.Nonce
if len(nonce) == 0 {
return nil, status.Error(codes.PermissionDenied, "empty nonce")
}
used, err := s.NonceRec.RecordNonce(ctx, nonce, time.Now().Add(nonceValidity))
if err != nil {
logger.WithError(err).WithField("at", "record-nonce-error").Error()
return nil, status.Error(codes.Internal, "failed to record nonce")
} else if used {
logger.WithError(err).WithField("at", "nonce-replay").Warn()
return nil, status.Error(codes.PermissionDenied, "invalid nonce")
}
principals := []string{}
// Groups are embedded as principals in the form group:%s
for _, group := range gc.Groups {
principals = append(principals, fmt.Sprintf("group:%s", group))
}
// ACR and AMR are embedded as principals in the form acr:%s and amr:%s.
// This allows some servers to require MFA (e.g., in production) while
// others do not (e.g., in non-production) without having separate
// signers.
if claims.ACR != "" {
principals = append(principals, fmt.Sprintf("acr:%s", claims.ACR))
for _, amr := range claims.AMR {
principals = append(principals, fmt.Sprintf("amr:%s", amr))
}
}
logger = logger.WithField("principals", principals)
now := s.now()
validBefore := now.Add(s.userCertValidForMax)
if claims.Expiry.Time().Before(validBefore) {
validBefore = claims.Expiry.Time()
}
sreq := sshCertReq{
CertType: ssh.UserCert,
Key: []byte(req.PublicKey),
ID: claims.Subject,
Principals: principals,
ValidAfter: now.Add(validAfterDurationSkew),
ValidBefore: validBefore,
Permissions: ssh.Permissions{
Extensions: map[string]string{
"permit-X11-forwarding": "",
"permit-agent-forwarding": "",
"permit-port-forwarding": "",
"permit-pty": "",
"permit-user-rc": "",
},
},
}
signer, err := s.userSignerSource.Signer(ctx)
if err != nil {
return nil, fmt.Errorf("getting signer: %v", err)
}
signed, err := signRequest(signer, sreq)
if err != nil {
logger.WithError(err).WithField("at", "sign-request-failed").Error()
return nil, status.Errorf(codes.Internal, "failed to sign user key")
}
logger.WithField("at", "success").Info()
return &sshsigner.SignUserKeyResponse{SignedCertificate: signed}, nil
}
func (s *SSHSigner) SignHostKey(ctx context.Context, req *sshsigner.SignHostKeyRequest) (*sshsigner.SignHostKeyResponse, error) {
logger := s.Log.WithFields(logrus.Fields{
"fn": "SignHostKey",
"ip": remoteAddrFromContext(ctx),
"publickey": req.PublicKey,
"hostnames": req.Hostnames,
})
claims, err := s.authRequest(ctx)
if err != nil {
logger.WithError(err).WithField("at", "auth-request-error").Error()
return nil, status.Error(codes.Unauthenticated, err.Error())
}
nonce := claims.Nonce
if len(nonce) == 0 {
return nil, status.Error(codes.PermissionDenied, "empty nonce")
}
used, err := s.NonceRec.RecordNonce(ctx, nonce, time.Now().Add(nonceValidity))
if err != nil {
logger.WithError(err).WithField("at", "record-nonce-error").Error()
return nil, status.Error(codes.Internal, "failed to record nonce")
} else if used {
logger.WithError(err).WithField("at", "nonce-replay").Warn()
return nil, status.Error(codes.PermissionDenied, "invalid nonce")
}
if awsI, ok := claims.Extra["awsInstance"].(bool); !ok || !awsI {
return nil, status.Error(codes.PermissionDenied, "only AWS roles may use this RPC")
}
arnc, _ := claims.Extra["arn"].(string)
md, _ := claims.Extra["md"].(map[string]interface{})
logger = logger.WithFields(logrus.Fields{
"subject": claims.Subject,
"arn": arnc,
"metadata": md,
"nonce": claims.Nonce,
})
if len(req.Hostnames) == 0 {
return nil, status.Error(codes.InvalidArgument, "at least one hostname must be specified")
}
parn, err := arn.Parse(arnc)
if err != nil {
return nil, status.Error(codes.InvalidArgument, "invalid ARN")
}
if parn.Partition != "aws" {
logger.WithField("at", "not-valid-partition").Error()
return nil, status.Error(codes.PermissionDenied, "invalid ARN")
} else if parn.Service != "sts" {
logger.WithField("at", "not-valid-service").Error()
return nil, status.Error(codes.PermissionDenied, "invalid ARN")
} else if !strings.HasPrefix(parn.Resource, "assumed-role/aws:ec2-instance/") {
logger.WithField("at", "not-valid-resource").Error()
return nil, status.Error(codes.PermissionDenied, "invalid ARN")
}
var validAWSAccount bool
for _, awsAccount := range s.ValidAWSAccounts {
if awsAccount == parn.AccountID {
validAWSAccount = true
break
}
}
if !validAWSAccount {
logger.WithField("at", "not-valid-account").WithField("account", parn.AccountID).Error()
return nil, status.Error(codes.PermissionDenied, "invalid ARN")
}
principals := req.Hostnames
logger = logger.WithField("principals", principals)
now := s.now()
sreq := sshCertReq{
CertType: ssh.HostCert,
Key: []byte(req.PublicKey),
ID: parn.String(),
Principals: principals,
ValidAfter: now.Add(validAfterDurationSkew),
ValidBefore: now.Add(s.hostCertValidFor),
}
signer, err := s.hostSignerSource.Signer(ctx)
if err != nil {
return nil, fmt.Errorf("getting signer: %v", err)
}
signed, err := signRequest(signer, sreq)
if err != nil {
logger.WithError(err).WithField("at", "sign-request-failed").Error()
return nil, status.Errorf(codes.Internal, "failed to sign host key")
}
logger.WithField("at", "success").Info()
return &sshsigner.SignHostKeyResponse{SignedCertificate: signed}, nil
}
func (s *SSHSigner) UserSigners(ctx context.Context, _ *empty.Empty) (*sshsigner.UserSignersResponse, error) {
logger := s.Log.WithFields(logrus.Fields{
"fn": "UserSigners",
"ip": remoteAddrFromContext(ctx),
})
keys, err := validAuthKeys(ctx, s.userSignerSource)
if err != nil {
logger.WithError(err).WithField("at", "get-ssh-keyset-error").Error()
return nil, status.Errorf(codes.Internal, "failed to retrieve keyset")
}
resp := &sshsigner.UserSignersResponse{}
for _, k := range keys {
resp.VerificationKeys = append(resp.VerificationKeys, &sshsigner.VerificationKey{
Key: ssh.MarshalAuthorizedKey(k),
})
}
if err := grpc.SendHeader(ctx, s.cacheControlMD()); err != nil {
logger.WithError(err).WithField("at", "send-cache-control-metadata").Error()
return nil, status.Errorf(codes.Internal, "failed to retrieve keyset")
}
return resp, nil
}
func (s *SSHSigner) HostSigners(ctx context.Context, _ *empty.Empty) (*sshsigner.HostSignersResponse, error) {
logger := s.Log.WithFields(logrus.Fields{
"fn": "HostSigners",
"ip": remoteAddrFromContext(ctx),
})
keys, err := validAuthKeys(ctx, s.hostSignerSource)
if err != nil {
logger.WithError(err).WithField("at", "get-ssh-keyset-error").Error()
return nil, status.Errorf(codes.Internal, "failed to retrieve keyset")
}
resp := &sshsigner.HostSignersResponse{}
for _, k := range keys {
resp.VerificationKeys = append(resp.VerificationKeys, &sshsigner.VerificationKey{
Key: ssh.MarshalAuthorizedKey(k),
})
}
if err := grpc.SendHeader(ctx, s.cacheControlMD()); err != nil {
logger.WithError(err).WithField("at", "send-cache-control-metadata").Error()
return nil, status.Errorf(codes.Internal, "failed to retrieve keyset")
}
return resp, nil
}
func (s *SSHSigner) now() time.Time {
if s.clock == nil {
return time.Now()
}
return s.clock()
}
func (s *SSHSigner) authRequest(ctx context.Context) (*oidc.Claims, error) {
md, ok := metadata.FromIncomingContext(ctx)
if !ok {
return nil, fmt.Errorf("metadata missing from incoming context")
}
authhdr := md.Get("authorization")
if len(authhdr) != 1 {
return nil, fmt.Errorf("want 1 authorization header, got %d", len(authhdr))
}
authsp := strings.SplitN(authhdr[0], " ", 2)
if len(authsp) != 2 {
return nil, fmt.Errorf("authorization header format incorrect")
}
if !strings.EqualFold(authsp[0], "bearer") {
return nil, fmt.Errorf("provided authorization token is not a bearer token")
}
claims, err := s.Verifier.VerifyRaw(ctx, s.Audience, authsp[1])
if err != nil {
return nil, fmt.Errorf("failed to verify token: %w", err)
}
return claims, nil
}
func (s *SSHSigner) cacheControlMD() metadata.MD {
md := metadata.New(map[string]string{})
if s.remoteCacheFor == 0 {
return md
}
splayedInterval := time.Duration(int64(s.remoteCacheFor) + s.rand.Int63n(int64(s.remoteCacheSplay)))
md.Set(textproto.CanonicalMIMEHeaderKey("Cache-Control"), fmt.Sprintf("private, max-age=%d", int64(splayedInterval.Seconds())))
return md
}
type sshCertReq struct {
// CertType of the certificate. Either ssh.UserCert or ssh.HostCert
CertType uint32
// Key to sign
Key []byte
// ID of the key. This is logged by OpenSSH. For user certificates, this is the
// token subject, which encodes their Aloha User ID. For host certificates,
// this is the IAM role that was assumed.
ID string
// Principals for the certificate. For user certificates, we use groups as
// principals, allowing hosts to make authz decisions like "only ops may login
// as root". For host certificates, this must be a list of hostnames, which the
// client will validate.
Principals []string
Permissions ssh.Permissions
ValidAfter time.Time
ValidBefore time.Time
}
// technically this shouldn't work, because we're not changing the flagged cert
// type to `rsa-sha2-256-cert-v01`. But, there is an exception for this exact
// path for "legacy reasons" so we can get away with it
// https://github.com/openssh/openssh-portable/blob/11d427162778c18fa42917893a75d178679a2389/ssh-rsa.c#L274-L287
type sshAlgorithmSigner struct {
algorithm string
signer ssh.AlgorithmSigner
}
func (s *sshAlgorithmSigner) PublicKey() ssh.PublicKey {
return s.signer.PublicKey()
}
func (s *sshAlgorithmSigner) Sign(rand io.Reader, data []byte) (*ssh.Signature, error) {
return s.signer.SignWithAlgorithm(rand, data, s.algorithm)
}
var signRequest = func(signer crypto.Signer, req sshCertReq) (string, error) {
if len(req.Principals) == 0 {
return "", fmt.Errorf("empty list of principals")
}
if len(req.ID) == 0 {
return "", fmt.Errorf("empty ID")
}
if len(req.Key) == 0 {
return "", fmt.Errorf("empty key")
}
buf := make([]byte, 8)
if _, err := rand.Read(buf); err != nil {
return "", fmt.Errorf("failed to read random bytes: %w", err)
}
serial := binary.LittleEndian.Uint64(buf)
pubKey, _, _, _, err := ssh.ParseAuthorizedKey([]byte(req.Key))
if err != nil {
return "", fmt.Errorf("failed to parse user public key: %w", err)
}
certificate := ssh.Certificate{
Serial: serial,
Key: pubKey,
KeyId: req.ID,
ValidPrincipals: req.Principals,
ValidAfter: uint64(req.ValidAfter.Unix()),
ValidBefore: uint64(req.ValidBefore.Unix()),
CertType: req.CertType,
Permissions: ssh.Permissions{
CriticalOptions: req.Permissions.CriticalOptions,
Extensions: req.Permissions.Extensions,
},
}
ca, err := ssh.NewSignerFromSigner(signer)
if err != nil {
return "", fmt.Errorf("creating ssh signer: %v", err)
}
as, ok := ca.(ssh.AlgorithmSigner)
if !ok {
return "", fmt.Errorf("SSH signer is not an algorithm signer, cannot proceed")
}
sas := &sshAlgorithmSigner{
signer: as,
algorithm: ssh.SigAlgoRSASHA2256,
}
err = certificate.SignCert(rand.Reader, sas)
if err != nil {
return "", fmt.Errorf("failed to sign user public key: %s", err)
}
marshaledCertificate := ssh.MarshalAuthorizedKey(&certificate)
if len(marshaledCertificate) == 0 {
return "", errors.New("failed to marshal signed certificate, empty result")
}
return string(marshaledCertificate), nil
}
func remoteAddrFromContext(ctx context.Context) string {
md, ok := metadata.FromIncomingContext(ctx)
if !ok {
return ""
}
v := md.Get("x-forwarded-for")
if len(v) == 0 {
return ""
}
return v[0]
}
// validAuthKeys builds a list of valid SSH public keys from the signer, and the
// KeySource if non-nil
func validAuthKeys(ctx context.Context, ks SignerSource) ([]ssh.PublicKey, error) {
keys := map[string]ssh.PublicKey{}
if ks != nil {
aks, err := ks.PublicKeys(ctx)
if err != nil {
return nil, fmt.Errorf("fetching from KeySource: %v", err)
}
for _, k := range aks {
sk, err := ssh.NewPublicKey(k)
if err != nil {
return nil, err
}
keys[string(sk.Marshal())] = sk
}
}
ret := []ssh.PublicKey{}
for _, v := range keys {
ret = append(ret, v)
}
return ret, nil
}
type groupClaims struct {
Groups []string `json:"groups"`
}
// cacheKeySource wraps a signer source, caching public key lookups
type cacheKeySource struct {
SignerSource
CacheFor time.Duration
cached []crypto.PublicKey
nextFetch time.Time
cacheMu sync.RWMutex
}
func (c *cacheKeySource) PublicKeys(ctx context.Context) ([]crypto.PublicKey, error) {
c.cacheMu.RLock()
if time.Now().Before(c.nextFetch) && c.cached != nil {
c.cacheMu.RUnlock()
return c.cached, nil
}
// we have no cache, re-lock for writing
c.cacheMu.RUnlock()
c.cacheMu.Lock()
defer c.cacheMu.Unlock()
// check again, in case another instance refreshed
if time.Now().Before(c.nextFetch) && c.cached != nil {
return c.cached, nil
}
// fetch
curr, err := c.SignerSource.PublicKeys(ctx)
if err != nil {
return nil, err
}
c.cached = curr
c.nextFetch = time.Now().Add(c.CacheFor)
return curr, err
}