x/crypto/ssh: fix host certificate principal evaluation to check for hostname only

SSH host certificates are expected to contain hostnames only,
not "host:port" format.

This change allows Go clients to connect to OpenSSH servers that
use host certificates.

Note, this change will break any clients that use ssh.NewClientConn()
with an `addr` that is not in `host:port` format (they will see a
"missing port in address" error).

Fixes bug 20273.

Change-Id: I5a306c6b7b419a737e1f0f9c5ca8c585e21a45a4
Reviewed-on: https://go-review.googlesource.com/43475
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 13 insertions, 11 deletions

diff --git a/ssh/certs.go b/ssh/certs.go
index 2fc8af1..b1f0220 100644
--- a/ssh/certs.go
+++ b/ssh/certs.go
@@ -298,8 +298,17 @@ func (c *CertChecker) CheckHostKey(addr string, remote net.Addr, key PublicKey)
 	if cert.CertType != HostCert {
 		return fmt.Errorf("ssh: certificate presented as a host key has type %d", cert.CertType)
 	}
+	if !c.IsHostAuthority(cert.SignatureKey, addr) {
+		return fmt.Errorf("ssh: no authorities for hostname: %v", addr)
+	}
+
+	hostname, _, err := net.SplitHostPort(addr)
+	if err != nil {
+		return err
+	}
 
-	return c.CheckCert(addr, cert)
+	// Pass hostname only as principal for host certificates (consistent with OpenSSH)
+	return c.CheckCert(hostname, cert)
 }
 
 // Authenticate checks a user certificate. Authenticate can be used as
@@ -316,6 +325,9 @@ func (c *CertChecker) Authenticate(conn ConnMetadata, pubKey PublicKey) (*Permis
 	if cert.CertType != UserCert {
 		return nil, fmt.Errorf("ssh: cert has type %d", cert.CertType)
 	}
+	if !c.IsUserAuthority(cert.SignatureKey) {
+		return nil, fmt.Errorf("ssh: certificate signed by unrecognized authority")
+	}
 
 	if err := c.CheckCert(conn.User(), cert); err != nil {
 		return nil, err
@@ -364,16 +376,6 @@ func (c *CertChecker) CheckCert(principal string, cert *Certificate) error {
 		}
 	}
 
-	// if this is a host cert, principal is the remote hostname as passed
-	// to CheckHostCert.
-	if cert.CertType == HostCert && !c.IsHostAuthority(cert.SignatureKey, principal) {
-		return fmt.Errorf("ssh: no authorities for hostname: %v", principal)
-	}
-
-	if cert.CertType == UserCert && !c.IsUserAuthority(cert.SignatureKey) {
-		return fmt.Errorf("ssh: certificate signed by unrecognized authority")
-	}
-
 	clock := c.Clock
 	if clock == nil {
 		clock = time.Now