From patchwork Tue Jan 5 21:17:43 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Tom Parkin X-Patchwork-Id: 357367 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-13.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH, MAILING_LIST_MULTI, SPF_HELO_NONE, SPF_PASS, USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 72053C433E0 for ; Tue, 5 Jan 2021 21:18:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3CC3222D6E for ; Tue, 5 Jan 2021 21:18:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731415AbhAEVS2 (ORCPT ); Tue, 5 Jan 2021 16:18:28 -0500 Received: from mail.katalix.com ([3.9.82.81]:35392 "EHLO mail.katalix.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725879AbhAEVS2 (ORCPT ); Tue, 5 Jan 2021 16:18:28 -0500 Received: from localhost.localdomain (82-69-49-219.dsl.in-addr.zen.co.uk [82.69.49.219]) (Authenticated sender: tom) by mail.katalix.com (Postfix) with ESMTPSA id 8CBC97D533; Tue, 5 Jan 2021 21:17:46 +0000 (GMT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=katalix.com; s=mail; t=1609881466; bh=L5DgvzoaChhKRPQ/BcHR0cjVGKcZ5Fa2JdUS6eRWkLE=; h=From:To:Cc:Subject:Date:Message-Id:From; z=From:=20Tom=20Parkin=20|To:=20netdev@vger.ke rnel.org|Cc:=20gnault@redhat.com,=0D=0A=09jchapman@katalix.com,=0D =0A=09Tom=20Parkin=20|Subject:=20[PATCH]=20pp p:=20fix=20refcount=20underflow=20on=20channel=20unbridge|Date:=20 Tue,=20=205=20Jan=202021=2021:17:43=20+0000|Message-Id:=20<2021010 5211743.8404-1-tparkin@katalix.com>; b=o3GiYoYCQqBWnpDp+tHapvEqfsMyRnk8FIw9jI6PqaCHyDl5hEOuhbByuzaNx86wa LolsbdLcHRZWEayle+OULk9py+7OpK1n/QAJS5Tq+Vt/QGV4lpyey80rSwM9nPGPwp BfqlH/gdIyCtumGsFj1IymcOk+ZgxeMKBRIRgwjY/p3UL7d68FUdctuuU9cfPO/eBA AeOBI5jOkiB8yWnr8Dx29Ef8ffOd0EMrvnGLS+M//Ilv2FFPTRhtuNJhZuAdKNDEnc 7R21CvvsdNAUvBnIv1nwdlXUYLjOaqkrnH/JUWsNg53Snpmt9bQMV//h+cWqYGfZhl XXZw0H1rPb2zQ== From: Tom Parkin To: netdev@vger.kernel.org Cc: gnault@redhat.com, jchapman@katalix.com, Tom Parkin Subject: [PATCH] ppp: fix refcount underflow on channel unbridge Date: Tue, 5 Jan 2021 21:17:43 +0000 Message-Id: <20210105211743.8404-1-tparkin@katalix.com> X-Mailer: git-send-email 2.17.1 Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org When setting up a channel bridge, ppp_bridge_channels sets the pch->bridge field before taking the associated reference on the bridge file instance. This opens up a refcount underflow bug if ppp_bridge_channels called via. iotcl runs concurrently with ppp_unbridge_channels executing via. file release. The bug is triggered by ppp_bridge_channels taking the error path through the 'err_unset' label. In this scenario, pch->bridge is set, but the reference on the bridged channel will not be taken because the function errors out. If ppp_unbridge_channels observes pch->bridge before it is unset by the error path, it will erroneously drop the reference on the bridged channel and cause a refcount underflow. To avoid this, ensure that ppp_bridge_channels holds a reference on each channel in advance of setting the bridge pointers. --- v2: * rework ppp_bridge_channels code to avoid the race condition in preference to holding ppp_mutex while calling ppp_unbridge_channels --- drivers/net/ppp/ppp_generic.c | 20 ++++++++++++++++---- 1 file changed, 16 insertions(+), 4 deletions(-) diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c index 09c27f7773f9..d019fa40b494 100644 --- a/drivers/net/ppp/ppp_generic.c +++ b/drivers/net/ppp/ppp_generic.c @@ -617,12 +617,15 @@ static struct bpf_prog *compat_ppp_get_filter(struct sock_fprog32 __user *p) */ static int ppp_bridge_channels(struct channel *pch, struct channel *pchb) { + bool drop_ref = false; + write_lock_bh(&pch->upl); if (pch->ppp || rcu_dereference_protected(pch->bridge, lockdep_is_held(&pch->upl))) { write_unlock_bh(&pch->upl); return -EALREADY; } + refcount_inc(&pchb->file.refcnt); rcu_assign_pointer(pch->bridge, pchb); write_unlock_bh(&pch->upl); @@ -632,19 +635,28 @@ static int ppp_bridge_channels(struct channel *pch, struct channel *pchb) write_unlock_bh(&pchb->upl); goto err_unset; } + refcount_inc(&pch->file.refcnt); rcu_assign_pointer(pchb->bridge, pch); write_unlock_bh(&pchb->upl); - refcount_inc(&pch->file.refcnt); - refcount_inc(&pchb->file.refcnt); - return 0; err_unset: write_lock_bh(&pch->upl); - RCU_INIT_POINTER(pch->bridge, NULL); + /* Re-check pch->bridge with upl held since a racing unbridge might already + * have cleared it and dropped the reference on pch->bridge. + */ + if (rcu_dereference_protected(pch->bridge, lockdep_is_held(&pch->upl))) { + RCU_INIT_POINTER(pch->bridge, NULL); + drop_ref = true; + } write_unlock_bh(&pch->upl); synchronize_rcu(); + + if (drop_ref) + if (refcount_dec_and_test(&pchb->file.refcnt)) + ppp_destroy_channel(pchb); + return -EALREADY; }