From mboxrd@z Thu Jan 1 00:00:00 1970 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=-5.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,USER_AGENT_MUTT 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 057DCC10F13 for ; Tue, 16 Apr 2019 22:57:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C621120880 for ; Tue, 16 Apr 2019 22:57:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728965AbfDPW5a (ORCPT ); Tue, 16 Apr 2019 18:57:30 -0400 Received: from outgoing-auth-1.mit.edu ([18.9.28.11]:42281 "EHLO outgoing.mit.edu" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727760AbfDPW5a (ORCPT ); Tue, 16 Apr 2019 18:57:30 -0400 Received: from callcc.thunk.org (guestnat-104-133-0-109.corp.google.com [104.133.0.109] (may be forged)) (authenticated bits=0) (User authenticated as tytso@ATHENA.MIT.EDU) by outgoing.mit.edu (8.14.7/8.12.4) with ESMTP id x3GMvPsN006796 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Tue, 16 Apr 2019 18:57:26 -0400 Received: by callcc.thunk.org (Postfix, from userid 15806) id 55147420497; Tue, 16 Apr 2019 18:57:25 -0400 (EDT) Date: Tue, 16 Apr 2019 18:57:25 -0400 From: "Theodore Ts'o" To: Eric Biggers Cc: linux-fscrypt@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH v2] fscrypt: use READ_ONCE() to access ->i_crypt_info Message-ID: <20190416225725.GA14623@mit.edu> References: <20190411213215.163929-1-ebiggers@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190411213215.163929-1-ebiggers@kernel.org> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Thu, Apr 11, 2019 at 02:32:15PM -0700, Eric Biggers wrote: > From: Eric Biggers > > ->i_crypt_info starts out NULL and may later be locklessly set to a > non-NULL value by the cmpxchg() in fscrypt_get_encryption_info(). > > But ->i_crypt_info is used directly, which technically is incorrect. > It's a data race, and it doesn't include the data dependency barrier > needed to safely dereference the pointer on at least one architecture. > > Fix this by using READ_ONCE() instead. Note: we don't need to use > smp_load_acquire(), since dereferencing the pointer only requires a data > dependency barrier, which is already included in READ_ONCE(). We also > don't need READ_ONCE() in places where ->i_crypt_info is unconditionally > dereferenced, since it must have already been checked. > > Also downgrade the cmpxchg() to cmpxchg_release(), since RELEASE > semantics are sufficient on the write side. > > Signed-off-by: Eric Biggers Looks good, applied. - Ted