crypto/tls: let HTTP/1.1 clients connect to servers with NextProtos "h2"go1.17beta1

Fixes #46310

Change-Id: Idd5e30f05c439f736ae6f3904cbb9cc2ba772315
Reviewed-on: https://go-review.googlesource.com/c/go/+/325432
Trust: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>

1 files changed, 34 insertions, 8 deletions

diff --git a/src/crypto/tls/handshake_server.go b/src/crypto/tls/handshake_server.go
index b231981e09..43f30e2fef 100644
--- a/src/crypto/tls/handshake_server.go
+++ b/src/crypto/tls/handshake_server.go
@@ -217,15 +217,13 @@ func (hs *serverHandshakeState) processClientHello() error {
 		c.serverName = hs.clientHello.serverName
 	}
 
-	if len(c.config.NextProtos) > 0 && len(hs.clientHello.alpnProtocols) > 0 {
-		selectedProto := mutualProtocol(hs.clientHello.alpnProtocols, c.config.NextProtos)
-		if selectedProto == "" {
-			c.sendAlert(alertNoApplicationProtocol)
-			return fmt.Errorf("tls: client requested unsupported application protocols (%s)", hs.clientHello.alpnProtocols)
-		}
-		hs.hello.alpnProtocol = selectedProto
-		c.clientProtocol = selectedProto
+	selectedProto, err := negotiateALPN(c.config.NextProtos, hs.clientHello.alpnProtocols)
+	if err != nil {
+		c.sendAlert(alertNoApplicationProtocol)
+		return err
 	}
+	hs.hello.alpnProtocol = selectedProto
+	c.clientProtocol = selectedProto
 
 	hs.cert, err = c.config.getCertificate(clientHelloInfo(hs.ctx, c, hs.clientHello))
 	if err != nil {
@@ -277,6 +275,34 @@ func (hs *serverHandshakeState) processClientHello() error {
 	return nil
 }
 
+// negotiateALPN picks a shared ALPN protocol that both sides support in server
+// preference order. If ALPN is not configured or the peer doesn't support it,
+// it returns "" and no error.
+func negotiateALPN(serverProtos, clientProtos []string) (string, error) {
+	if len(serverProtos) == 0 || len(clientProtos) == 0 {
+		return "", nil
+	}
+	var http11fallback bool
+	for _, s := range serverProtos {
+		for _, c := range clientProtos {
+			if s == c {
+				return s, nil
+			}
+			if s == "h2" && c == "http/1.1" {
+				http11fallback = true
+			}
+		}
+	}
+	// As a special case, let http/1.1 clients connect to h2 servers as if they
+	// didn't support ALPN. We used not to enforce protocol overlap, so over
+	// time a number of HTTP servers were configured with only "h2", but
+	// expected to accept connections from "http/1.1" clients. See Issue 46310.
+	if http11fallback {
+		return "", nil
+	}
+	return "", fmt.Errorf("tls: client requested unsupported application protocols (%s)", clientProtos)
+}
+
 // supportsECDHE returns whether ECDHE key exchanges can be used with this
 // pre-TLS 1.3 client.
 func supportsECDHE(c *Config, supportedCurves []CurveID, supportedPoints []uint8) bool {