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=-8.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED, 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 D009AC2F3A1 for ; Mon, 21 Jan 2019 12:38:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A27652085A for ; Mon, 21 Jan 2019 12:38:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728690AbfAUMim (ORCPT ); Mon, 21 Jan 2019 07:38:42 -0500 Received: from foss.arm.com ([217.140.101.70]:33274 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728163AbfAUMil (ORCPT ); Mon, 21 Jan 2019 07:38:41 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id F17EE80D; Mon, 21 Jan 2019 04:38:40 -0800 (PST) Received: from lakrids.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 782BD3F614; Mon, 21 Jan 2019 04:38:39 -0800 (PST) Date: Mon, 21 Jan 2019 12:38:37 +0000 From: Mark Rutland To: Elena Reshetova Cc: akpm@linux-foundation.org, aryabinin@virtuozzo.com, anders.roxell@linaro.org, dvyukov@google.com, linux-kernel@vger.kernel.org, keescook@chromium.org, peterz@infradead.org Subject: Re: [PATCH] kcov: convert kcov.refcount to refcount_t Message-ID: <20190121123836.GC47506@lakrids.cambridge.arm.com> References: <1547634429-772-1-git-send-email-elena.reshetova@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1547634429-772-1-git-send-email-elena.reshetova@intel.com> User-Agent: Mutt/1.11.1+11 (2f07cb52) (2018-12-01) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 16, 2019 at 12:27:09PM +0200, Elena Reshetova wrote: > atomic_t variables are currently used to implement reference > counters with the following properties: > - counter is initialized to 1 using atomic_set() > - a resource is freed upon counter reaching zero > - once counter reaches zero, its further > increments aren't allowed > - counter schema uses basic atomic operations > (set, inc, inc_not_zero, dec_and_test, etc.) > > Such atomic variables should be converted to a newly provided > refcount_t type and API that prevents accidental counter overflows > and underflows. This is important since overflows and underflows > can lead to use-after-free situation and be exploitable. > > The variable kcov.refcount is used as pure reference counter. > Convert it to refcount_t and fix up the operations. > > **Important note for maintainers: > > Some functions from refcount_t API defined in lib/refcount.c > have different memory ordering guarantees than their atomic > counterparts. > The full comparison can be seen in > https://lkml.org/lkml/2017/11/15/57 and it is hopefully soon > in state to be merged to the documentation tree. > Normally the differences should not matter since refcount_t provides > enough guarantees to satisfy the refcounting use cases, but in > some rare cases it might matter. > Please double check that you don't have some undocumented > memory guarantees for this variable usage. > > For the kcov.refcount it might make a difference > in following places: > - kcov_put(): decrement in refcount_dec_and_test() only > provides RELEASE ordering and control dependency on success > vs. fully ordered atomic counterpart > > Suggested-by: Kees Cook > Reviewed-by: David Windsor > Reviewed-by: Hans Liljestrand > Signed-off-by: Elena Reshetova Just to check, has this been tested with CONFIG_REFCOUNT_FULL and something poking kcov? Given lib/refcount.c is instrumented, the refcount_*() calls will recurse back into the kcov code. It looks like that's fine, given these are only manipulated in setup/teardown paths, but it would be nice to be sure. Thanks, Mark. > --- > kernel/kcov.c | 9 +++++---- > 1 file changed, 5 insertions(+), 4 deletions(-) > > diff --git a/kernel/kcov.c b/kernel/kcov.c > index c2277db..051e86e 100644 > --- a/kernel/kcov.c > +++ b/kernel/kcov.c > @@ -20,6 +20,7 @@ > #include > #include > #include > +#include > #include > > /* Number of 64-bit words written per one comparison: */ > @@ -44,7 +45,7 @@ struct kcov { > * - opened file descriptor > * - task with enabled coverage (we can't unwire it from another task) > */ > - atomic_t refcount; > + refcount_t refcount; > /* The lock protects mode, size, area and t. */ > spinlock_t lock; > enum kcov_mode mode; > @@ -228,12 +229,12 @@ EXPORT_SYMBOL(__sanitizer_cov_trace_switch); > > static void kcov_get(struct kcov *kcov) > { > - atomic_inc(&kcov->refcount); > + refcount_inc(&kcov->refcount); > } > > static void kcov_put(struct kcov *kcov) > { > - if (atomic_dec_and_test(&kcov->refcount)) { > + if (refcount_dec_and_test(&kcov->refcount)) { > vfree(kcov->area); > kfree(kcov); > } > @@ -312,7 +313,7 @@ static int kcov_open(struct inode *inode, struct file *filep) > if (!kcov) > return -ENOMEM; > kcov->mode = KCOV_MODE_DISABLED; > - atomic_set(&kcov->refcount, 1); > + refcount_set(&kcov->refcount, 1); > spin_lock_init(&kcov->lock); > filep->private_data = kcov; > return nonseekable_open(inode, filep); > -- > 2.7.4 >