From patchwork Thu Jan 7 18:13:15 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Tom Parkin X-Patchwork-Id: 358682 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=-18.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER, INCLUDES_PATCH, MAILING_LIST_MULTI, SPF_HELO_NONE, SPF_PASS, URIBL_BLOCKED, 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 EA382C433E0 for ; Thu, 7 Jan 2021 18:14:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9D83C233FD for ; Thu, 7 Jan 2021 18:14:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727215AbhAGSOD (ORCPT ); Thu, 7 Jan 2021 13:14:03 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34994 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726386AbhAGSOD (ORCPT ); Thu, 7 Jan 2021 13:14:03 -0500 Received: from mail.katalix.com (mail.katalix.com [IPv6:2a05:d01c:827:b342:16d0:7237:f32a:8096]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id BBB12C0612F5 for ; Thu, 7 Jan 2021 10:13:22 -0800 (PST) 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 7FDE97D55C; Thu, 7 Jan 2021 18:13:21 +0000 (GMT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=katalix.com; s=mail; t=1610043201; bh=qHSpP77ivYRppJQ1QcmD9+jcu+Wq/FHiproLOhDTVMA=; 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=20net =20v3]=20ppp:=20fix=20refcount=20underflow=20on=20channel=20unbrid ge|Date:=20Thu,=20=207=20Jan=202021=2018:13:15=20+0000|Message-Id: =20<20210107181315.3128-1-tparkin@katalix.com>; b=bbsziuZ+PiY4PR68nkdv0DTuFGxVBpR82yE7d+DSWEi/f55+EHQLEtiC5lwPaaJfi mfqS6qtc0D1q5id3PmxD8n5Tk6vg3YYvkjWke+fFJOzRIFaAp3lE/sbLfdhFVs/hQz 41ZABAJr5hUMU2iQ+Qt4Oi8tXLxCjpTqOH8XtWH04vYxXgZdTFJo004e6tahYMpcq1 5WDkifAseOrCLar3UQdJRKmuPFUdHLpnCGpbqNQ71l9zjYJKl8oeVLQqrQSkS+lykS hnz9WyC97xw/s5uorqEksVmupLDPYeTScHQEFMXISpuNRSvUHyLpOHitGMwPoLYfjh X6q4frAUOtNVA== From: Tom Parkin To: netdev@vger.kernel.org Cc: gnault@redhat.com, jchapman@katalix.com, Tom Parkin Subject: [PATCH net v3] ppp: fix refcount underflow on channel unbridge Date: Thu, 7 Jan 2021 18:13:15 +0000 Message-Id: <20210107181315.3128-1-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. Signed-off-by: Tom Parkin Fixes: 4cf476ced45d ("ppp: add PPPIOCBRIDGECHAN and PPPIOCUNBRIDGECHAN ioctls") Acked-by: Guillaume Nault --- v3: * remove bool tracking variable in ppp_bridge_channels and re-read pch->bridge instead * add missing tags 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 | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-) diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c index 09c27f7773f9..d445ecb1d0c7 100644 --- a/drivers/net/ppp/ppp_generic.c +++ b/drivers/net/ppp/ppp_generic.c @@ -623,6 +623,7 @@ static int ppp_bridge_channels(struct channel *pch, struct channel *pchb) 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 +633,24 @@ 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); + /* Re-read pch->bridge with upl held in case it was modified concurrently */ + pchb = rcu_dereference_protected(pch->bridge, lockdep_is_held(&pch->upl)); RCU_INIT_POINTER(pch->bridge, NULL); write_unlock_bh(&pch->upl); synchronize_rcu(); + + if (pchb) + if (refcount_dec_and_test(&pchb->file.refcnt)) + ppp_destroy_channel(pchb); + return -EALREADY; }