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ssh: don't advertise rsa-sha2 algorithms if we can't use them

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The server implementation looks at the HostKeys to advertise and
negotiate host key signature algorithms. A fundamental issue of the
Signer and AlgorithmSigner interfaces is that they don't expose the
supported signature algorithms, so really the server has to guess.

Currently, it would guess exclusively based on the PublicKey.Type,
regardless of whether the host key implemented AlgorithmSigner. This
means that a legacy Signer that only supports ssh-rsa still led the
server to negotiate rsa-sha2 algorithms. The server would then fail to
find a suitable host key to make the signature and crash.

This won't happen if only Signers from this package are used, but if a
custom Signer that doesn't support SignWithAlgorithm() but returns
"ssh-rsa" from PublicKey().Type() is used as a HostKey, the server is
vulnerable to DoS.

The only workable rules to determine what to advertise seems to be:

   1. a pure Signer will always Sign with the PublicKey.Type

   2. an AlgorithmSigner supports all algorithms associated with the
      PublicKey.Type

Rule number two means that we can't add new supported algorithms in the
future, which is not great, but it's too late to fix that.

rsaSigner was breaking rule number one, and although it would have been
fine where it's used, I didn't want to break our own interface contract.

It's unclear why we had separate test key entries for rsa-sha2
algorithms, since we can use the ssh-rsa key for those. The only test
that used them, TestCertTypes, seemed broken: the init was actually
failing at making the corresponding signers rsaSigners, and indeed the
test for the SHA-256 signer expected and checked a SHA-512 signature.

Pending CVE
For golang/go#49952

Change-Id: Ie658eefcadd87906e63fc7faae8249376aa96c79
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/392355
Trust: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
This commit is contained in:
Filippo Valsorda
2022-03-14 23:46:59 +00:00
parent fcc990c556
commit 1baeb1ce4c
12 changed files with 234 additions and 233 deletions
Download Patch File Download Diff File Expand all files Collapse all files
ssh/handshake.go
+59 -32
View File
@@ -455,21 +455,29 @@ func (t *handshakeTransport) sendKexInit() error {
}
io.ReadFull(rand.Reader, msg.Cookie[:])
if len(t.hostKeys) > 0 {
isServer := len(t.hostKeys) > 0
if isServer {
for _, k := range t.hostKeys {
algo := k.PublicKey().Type()
switch algo {
case KeyAlgoRSA:
msg.ServerHostKeyAlgos = append(msg.ServerHostKeyAlgos, []string{KeyAlgoRSASHA512, KeyAlgoRSASHA256, KeyAlgoRSA}...)
case CertAlgoRSAv01:
msg.ServerHostKeyAlgos = append(msg.ServerHostKeyAlgos, []string{CertAlgoRSASHA512v01, CertAlgoRSASHA256v01, CertAlgoRSAv01}...)
default:
msg.ServerHostKeyAlgos = append(msg.ServerHostKeyAlgos, algo)
// If k is an AlgorithmSigner, presume it supports all signature algorithms
// associated with the key format. (Ideally AlgorithmSigner would have a
// method to advertise supported algorithms, but it doesn't. This means that
// adding support for a new algorithm is a breaking change, as we will
// immediately negotiate it even if existing implementations don't support
// it. If that ever happens, we'll have to figure something out.)
// If k is not an AlgorithmSigner, we can only assume it only supports the
// algorithms that matches the key format. (This means that Sign can't pick
// a different default.)
keyFormat := k.PublicKey().Type()
if _, ok := k.(AlgorithmSigner); ok {
msg.ServerHostKeyAlgos = append(msg.ServerHostKeyAlgos, algorithmsForKeyFormat(keyFormat)...)
} else {
msg.ServerHostKeyAlgos = append(msg.ServerHostKeyAlgos, keyFormat)
}
}
} else {
msg.ServerHostKeyAlgos = t.hostKeyAlgorithms
}
packet := Marshal(msg)
// writePacket destroys the contents, so save a copy.
@@ -589,9 +597,9 @@ func (t *handshakeTransport) enterKeyExchange(otherInitPacket []byte) error {
var result *kexResult
if len(t.hostKeys) > 0 {
result, err = t.server(kex, t.algorithms, &magics)
result, err = t.server(kex, &magics)
} else {
result, err = t.client(kex, t.algorithms, &magics)
result, err = t.client(kex, &magics)
}
if err != nil {
@@ -618,33 +626,52 @@ func (t *handshakeTransport) enterKeyExchange(otherInitPacket []byte) error {
return nil
}
func (t *handshakeTransport) server(kex kexAlgorithm, algs *algorithms, magics *handshakeMagics) (*kexResult, error) {
var hostKey Signer
for _, k := range t.hostKeys {
kt := k.PublicKey().Type()
if kt == algs.hostKey {
hostKey = k
} else if signer, ok := k.(AlgorithmSigner); ok {
// Some signature algorithms don't show up as key types
// so we have to manually check for a compatible host key.
switch kt {
case KeyAlgoRSA:
if algs.hostKey == KeyAlgoRSASHA256 || algs.hostKey == KeyAlgoRSASHA512 {
hostKey = &rsaSigner{signer, algs.hostKey}
}
case CertAlgoRSAv01:
if algs.hostKey == CertAlgoRSASHA256v01 || algs.hostKey == CertAlgoRSASHA512v01 {
hostKey = &rsaSigner{signer, certToPrivAlgo(algs.hostKey)}
}
// algorithmSignerWrapper is an AlgorithmSigner that only supports the default
// key format algorithm.
//
// This is technically a violation of the AlgorithmSigner interface, but it
// should be unreachable given where we use this. Anyway, at least it returns an
// error instead of panicing or producing an incorrect signature.
type algorithmSignerWrapper struct {
Signer
}
func (a algorithmSignerWrapper) SignWithAlgorithm(rand io.Reader, data []byte, algorithm string) (*Signature, error) {
if algorithm != underlyingAlgo(a.PublicKey().Type()) {
return nil, errors.New("ssh: internal error: algorithmSignerWrapper invoked with non-default algorithm")
}
return a.Sign(rand, data)
}
func pickHostKey(hostKeys []Signer, algo string) AlgorithmSigner {
for _, k := range hostKeys {
if algo == k.PublicKey().Type() {
return algorithmSignerWrapper{k}
}
k, ok := k.(AlgorithmSigner)
if !ok {
continue
}
for _, a := range algorithmsForKeyFormat(k.PublicKey().Type()) {
if algo == a {
return k
}
}
}
return nil
}
r, err := kex.Server(t.conn, t.config.Rand, magics, hostKey)
func (t *handshakeTransport) server(kex kexAlgorithm, magics *handshakeMagics) (*kexResult, error) {
hostKey := pickHostKey(t.hostKeys, t.algorithms.hostKey)
if hostKey == nil {
return nil, errors.New("ssh: internal error: negotiated unsupported signature type")
}
r, err := kex.Server(t.conn, t.config.Rand, magics, hostKey, t.algorithms.hostKey)
return r, err
}
func (t *handshakeTransport) client(kex kexAlgorithm, algs *algorithms, magics *handshakeMagics) (*kexResult, error) {
func (t *handshakeTransport) client(kex kexAlgorithm, magics *handshakeMagics) (*kexResult, error) {
result, err := kex.Client(t.conn, t.config.Rand, magics)
if err != nil {
return nil, err
@@ -655,7 +682,7 @@ func (t *handshakeTransport) client(kex kexAlgorithm, algs *algorithms, magics *
return nil, err
}
if err := verifyHostKeySignature(hostKey, algs.hostKey, result); err != nil {
if err := verifyHostKeySignature(hostKey, t.algorithms.hostKey, result); err != nil {
return nil, err
}