serve ftp: use constant time comparison for password check GHSA-mfvx-7rcj-9m5g

The builtin authentication compared the configured username and password
with ==, whose run time depends on how much of the value matches, giving a
timing side-channel that could in principle help guess the password.

serve sftp, serve s3 and the auth proxy already use subtle.ConstantTimeCompare
so bring serve ftp in line with them. An empty configured password
still accepts any password.

Addresses GHSA-mfvx-7rcj-9m5g finding 4.

cmd/serve/ftp/ftp.go

@@ -5,6 +5,7 @@ package ftp
 
 import (
 	"context"
+	"crypto/subtle"
 	"errors"
 	"fmt"
 	"io"
@@ -333,7 +334,13 @@ func (d *driver) CheckPasswd(sctx *ftp.Context, user, pass string) (ok bool, err
 		d.userPass[user] = oPass
 		d.userPassMu.Unlock()
 	} else {
-		ok = d.opt.User == user && (d.opt.Pass == "" || d.opt.Pass == pass)
+		userOK := subtle.ConstantTimeCompare([]byte(d.opt.User), []byte(user))
+		// No password configured means any password is accepted
+		passOK := 1
+		if d.opt.Pass != "" {
+			passOK = subtle.ConstantTimeCompare([]byte(d.opt.Pass), []byte(pass))
+		}
+		ok = (userOK & passOK) == 1
 		if !ok {
 			fs.Infof(nil, "login failed: bad credentials")
 			return false, nil

cmd/serve/ftp/ftp_test.go

@@ -70,6 +70,36 @@ func TestFTP(t *testing.T) {
 	servetest.Run(t, "ftp", start)
 }
 
+// TestCheckPasswd checks the builtin authentication accepts only the
+// configured credentials, with an empty configured password accepting any
+// password.
+func TestCheckPasswd(t *testing.T) {
+	for _, test := range []struct {
+		name    string
+		optUser string
+		optPass string
+		user    string
+		pass    string
+		want    bool
+	}{
+		{name: "good", optUser: "user", optPass: "pass", user: "user", pass: "pass", want: true},
+		{name: "bad-pass", optUser: "user", optPass: "pass", user: "user", pass: "PASS", want: false},
+		{name: "bad-user", optUser: "user", optPass: "pass", user: "USER", pass: "pass", want: false},
+		{name: "wrong-length-pass", optUser: "user", optPass: "pass", user: "user", pass: "pass2", want: false},
+		{name: "empty-configured-pass", optUser: "user", optPass: "", user: "user", pass: "anything", want: true},
+		{name: "empty-configured-pass-bad-user", optUser: "user", optPass: "", user: "USER", pass: "anything", want: false},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			d := &driver{}
+			d.opt.User = test.optUser
+			d.opt.Pass = test.optPass
+			ok, err := d.CheckPasswd(nil, test.user, test.pass)
+			assert.NoError(t, err)
+			assert.Equal(t, test.want, ok)
+		})
+	}
+}
+
 func TestRc(t *testing.T) {
 	if israce.Enabled {
 		t.Skip("Skipping under race detector as underlying library is racy")