Mitigate timing of disallowed users PAM logins.

When sshd decides to not allow a login (eg PermitRootLogin=no) and
it's using PAM, it sends a fake password to PAM so that the timing for
the failure is not noticeably different whether or not the password
is correct.  This behaviour can be detected by sending a very long
password string which is slower to hash than the fake password.

Mitigate by constructing an invalid password that is the same length
as the one from the client and thus takes the same time to hash.
Diff from djm@

auth-pam.c

@@ -232,7 +232,6 @@ static int sshpam_account_status = -1;
 static char **sshpam_env = NULL;
 static Authctxt *sshpam_authctxt = NULL;
 static const char *sshpam_password = NULL;
-static char badpw[] = "\b\n\r\177INCORRECT";
 
 /* Some PAM implementations don't implement this */
 #ifndef HAVE_PAM_GETENVLIST
@@ -795,12 +794,35 @@ sshpam_query(void *ctx, char **name, char **info,
 	return (-1);
 }
 
+/*
+ * Returns a junk password of identical length to that the user supplied.
+ * Used to mitigate timing attacks against crypt(3)/PAM stacks that
+ * vary processing time in proportion to password length.
+ */
+static char *
+fake_password(const char *wire_password)
+{
+	const char junk[] = "\b\n\r\177INCORRECT";
+	char *ret = NULL;
+	size_t i, l = wire_password != NULL ? strlen(wire_password) : 0;
+
+	if (l >= INT_MAX)
+		fatal("%s: password length too long: %zu", __func__, l);
+
+	ret = malloc(l + 1);
+	for (i = 0; i < l; i++)
+		ret[i] = junk[i % (sizeof(junk) - 1)];
+	ret[i] = '\0';
+	return ret;
+}
+
 /* XXX - see also comment in auth-chall.c:verify_response */
 static int
 sshpam_respond(void *ctx, u_int num, char **resp)
 {
 	Buffer buffer;
 	struct pam_ctxt *ctxt = ctx;
+	char *fake;
 
 	debug2("PAM: %s entering, %u responses", __func__, num);
 	switch (ctxt->pam_done) {
@@ -821,8 +843,11 @@ sshpam_respond(void *ctx, u_int num, char **resp)
 	    (sshpam_authctxt->pw->pw_uid != 0 ||
 	    options.permit_root_login == PERMIT_YES))
 		buffer_put_cstring(&buffer, *resp);
-	else
+	else {
-		buffer_put_cstring(&buffer, badpw);
+		fake = fake_password(*resp);
+		buffer_put_cstring(&buffer, fake);
+		free(fake);
+	}
 	if (ssh_msg_send(ctxt->pam_psock, PAM_AUTHTOK, &buffer) == -1) {
 		buffer_free(&buffer);
 		return (-1);
@@ -1166,6 +1191,7 @@ sshpam_auth_passwd(Authctxt *authctxt, const char *password)
 {
 	int flags = (options.permit_empty_passwd == 0 ?
 	    PAM_DISALLOW_NULL_AUTHTOK : 0);
+	char *fake = NULL;
 
 	if (!options.use_pam || sshpam_handle == NULL)
 		fatal("PAM: %s called when PAM disabled or failed to "
@@ -1181,7 +1207,7 @@ sshpam_auth_passwd(Authctxt *authctxt, const char *password)
 	 */
 	if (!authctxt->valid || (authctxt->pw->pw_uid == 0 &&
 	    options.permit_root_login != PERMIT_YES))
-		sshpam_password = badpw;
+		sshpam_password = fake = fake_password(password);
 
 	sshpam_err = pam_set_item(sshpam_handle, PAM_CONV,
 	    (const void *)&passwd_conv);
@@ -1191,6 +1217,7 @@ sshpam_auth_passwd(Authctxt *authctxt, const char *password)
 
 	sshpam_err = pam_authenticate(sshpam_handle, flags);
 	sshpam_password = NULL;
+	free(fake);
 	if (sshpam_err == PAM_SUCCESS && authctxt->valid) {
 		debug("PAM: password authentication accepted for %.100s",
 		    authctxt->user);