vendor: update buildkit to 539be170

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

vendor/golang.org/x/crypto/ssh/client.go

@@ -77,7 +77,7 @@ func NewClientConn(c net.Conn, addr string, config *ClientConfig) (Conn, <-chan
 	}
 
 	conn := &connection{
-		sshConn: sshConn{conn: c},
+		sshConn: sshConn{conn: c, user: fullConf.User},
 	}
 
 	if err := conn.clientHandshake(addr, &fullConf); err != nil {
@@ -115,12 +115,25 @@ func (c *connection) clientHandshake(dialAddress string, config *ClientConfig) e
 
 // verifyHostKeySignature verifies the host key obtained in the key
 // exchange.
-func verifyHostKeySignature(hostKey PublicKey, result *kexResult) error {
+func verifyHostKeySignature(hostKey PublicKey, algo string, result *kexResult) error {
 	sig, rest, ok := parseSignatureBody(result.Signature)
 	if len(rest) > 0 || !ok {
 		return errors.New("ssh: signature parse error")
 	}
 
+	// For keys, underlyingAlgo is exactly algo. For certificates,
+	// we have to look up the underlying key algorithm that SSH
+	// uses to evaluate signatures.
+	underlyingAlgo := algo
+	for sigAlgo, certAlgo := range certAlgoNames {
+		if certAlgo == algo {
+			underlyingAlgo = sigAlgo
+		}
+	}
+	if sig.Format != underlyingAlgo {
+		return fmt.Errorf("ssh: invalid signature algorithm %q, expected %q", sig.Format, underlyingAlgo)
+	}
+
 	return hostKey.Verify(result.H, sig)
 }