pkg/plugins: use a dummy hostname for local connections

For local communications (npipe://, unix://), the hostname is not used,
but we need valid and meaningful hostname.

The current code used the socket path as hostname, which gets rejected by
go1.20.6 and go1.19.11 because of a security fix for [CVE-2023-29406 ][1],
which was implemented in  https://go.dev/issue/60374.

Prior versions go Go would clean the host header, and strip slashes in the
process, but go1.20.6 and go1.19.11 no longer do, and reject the host
header.

Before this patch, tests would fail on go1.20.6:

    === FAIL: pkg/authorization TestAuthZRequestPlugin (15.01s)
    time="2023-07-12T12:53:45Z" level=warning msg="Unable to connect to plugin: //tmp/authz2422457390/authz-test-plugin.sock/AuthZPlugin.AuthZReq: Post \"http://%2F%2Ftmp%2Fauthz2422457390%2Fauthz-test-plugin.sock/AuthZPlugin.AuthZReq\": http: invalid Host header, retrying in 1s"
    time="2023-07-12T12:53:46Z" level=warning msg="Unable to connect to plugin: //tmp/authz2422457390/authz-test-plugin.sock/AuthZPlugin.AuthZReq: Post \"http://%2F%2Ftmp%2Fauthz2422457390%2Fauthz-test-plugin.sock/AuthZPlugin.AuthZReq\": http: invalid Host header, retrying in 2s"
    time="2023-07-12T12:53:48Z" level=warning msg="Unable to connect to plugin: //tmp/authz2422457390/authz-test-plugin.sock/AuthZPlugin.AuthZReq: Post \"http://%2F%2Ftmp%2Fauthz2422457390%2Fauthz-test-plugin.sock/AuthZPlugin.AuthZReq\": http: invalid Host header, retrying in 4s"
    time="2023-07-12T12:53:52Z" level=warning msg="Unable to connect to plugin: //tmp/authz2422457390/authz-test-plugin.sock/AuthZPlugin.AuthZReq: Post \"http://%2F%2Ftmp%2Fauthz2422457390%2Fauthz-test-plugin.sock/AuthZPlugin.AuthZReq\": http: invalid Host header, retrying in 8s"
        authz_unix_test.go:82: Failed to authorize request Post "http://%2F%2Ftmp%2Fauthz2422457390%2Fauthz-test-plugin.sock/AuthZPlugin.AuthZReq": http: invalid Host header

[1]: https://github.com/advisories/GHSA-f8f7-69v5-w4vx

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit 6b7705d5b29e226a24902a8dcc488836faaee33c)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

pkg/plugins/client.go

@@ -18,6 +18,12 @@ import (
 
 const (
 	defaultTimeOut = 30
+
+	// dummyHost is a hostname used for local communication.
+	//
+	// For local communications (npipe://, unix://), the hostname is not used,
+	// but we need valid and meaningful hostname.
+	dummyHost = "plugin.moby.localhost"
 )
 
 func newTransport(addr string, tlsConfig *tlsconfig.Options) (transport.Transport, error) {
@@ -44,8 +50,12 @@ func newTransport(addr string, tlsConfig *tlsconfig.Options) (transport.Transpor
 		return nil, err
 	}
 	scheme := httpScheme(u)
-
-	return transport.NewHTTPTransport(tr, scheme, socket), nil
+	hostName := u.Host
+	if hostName == "" || u.Scheme == "unix" || u.Scheme == "npipe" {
+		// Override host header for non-tcp connections.
+		hostName = dummyHost
+	}
+	return transport.NewHTTPTransport(tr, scheme, hostName), nil
 }
 
 // NewClient creates a new plugin client (http).