From mboxrd@z Thu Jan 1 00:00:00 1970 Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751173AbeABQa7 (ORCPT + 1 other); Tue, 2 Jan 2018 11:30:59 -0500 Received: from mx2.suse.de ([195.135.220.15]:34158 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750970AbeABQa6 (ORCPT ); Tue, 2 Jan 2018 11:30:58 -0500 From: Nicolai Stange To: "David S. Miller" , Alexey Kuznetsov , Hideaki YOSHIFUJI Cc: Mohamed Ghannam , Michal Kubecek , Miroslav Benes , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Nicolai Stange Subject: [PATCH] net: ipv4: emulate READ_ONCE() on ->hdrincl bit-field in raw_sendmsg() Date: Tue, 2 Jan 2018 17:30:20 +0100 Message-Id: <20180102163020.32473-1-nstange@suse.de> X-Mailer: git-send-email 2.13.6 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: Commit 8f659a03a0ba ("net: ipv4: fix for a race condition in raw_sendmsg") fixed the issue of possibly inconsistent ->hdrincl handling due to concurrent updates by reading this bit-field member into a local variable and using the thus stabilized value in subsequent tests. However, aforementioned commit also adds the (correct) comment that /* hdrincl should be READ_ONCE(inet->hdrincl) * but READ_ONCE() doesn't work with bit fields */ because as it stands, the compiler is free to shortcut or even eliminate the local variable at its will. Note that I have not seen anything like this happening in reality and thus, the concern is a theoretical one. However, in order to be on the safe side, emulate a READ_ONCE() on the bit-field by introducing an intermediate local variable and doing a READ_ONCE() from it: int __hdrincl = inet->hdrincl; int hdrincl = READ_ONCE(__hdrincl); This breaks the chain in the sense that the compiler is not allowed to replace subsequent reads from hdrincl with reloads from inet->hdrincl. Fixes: 8f659a03a0ba ("net: ipv4: fix for a race condition in raw_sendmsg") Signed-off-by: Nicolai Stange --- Compile-tested only (with inspection of compiler output on x86_64). Applicable to linux-next-20180102. net/ipv4/raw.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) diff --git a/net/ipv4/raw.c b/net/ipv4/raw.c index 5b9bd5c33d9d..e84290c28c0c 100644 --- a/net/ipv4/raw.c +++ b/net/ipv4/raw.c @@ -513,16 +513,18 @@ static int raw_sendmsg(struct sock *sk, struct msghdr *msg, size_t len) int err; struct ip_options_data opt_copy; struct raw_frag_vec rfv; - int hdrincl; + int hdrincl, __hdrincl; err = -EMSGSIZE; if (len > 0xFFFF) goto out; /* hdrincl should be READ_ONCE(inet->hdrincl) - * but READ_ONCE() doesn't work with bit fields + * but READ_ONCE() doesn't work with bit fields. + * Emulate it by doing the READ_ONCE() from an intermediate int. */ - hdrincl = inet->hdrincl; + __hdrincl = inet->hdrincl; + hdrincl = READ_ONCE(__hdrincl); /* * Check the flags. */ -- 2.13.6