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=-3.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=no 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 A907DC4360C for ; Wed, 2 Oct 2019 19:09:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8D2AD222C4 for ; Wed, 2 Oct 2019 19:09:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729592AbfJBTJN (ORCPT ); Wed, 2 Oct 2019 15:09:13 -0400 Received: from shadbolt.e.decadent.org.uk ([88.96.1.126]:36118 "EHLO shadbolt.e.decadent.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729380AbfJBTIW (ORCPT ); Wed, 2 Oct 2019 15:08:22 -0400 Received: from [192.168.4.242] (helo=deadeye) by shadbolt.decadent.org.uk with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.89) (envelope-from ) id 1iFjyo-00035r-Eq; Wed, 02 Oct 2019 20:08:06 +0100 Received: from ben by deadeye with local (Exim 4.92.1) (envelope-from ) id 1iFjyn-0003bm-MR; Wed, 02 Oct 2019 20:08:05 +0100 Content-Type: text/plain; charset="UTF-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit MIME-Version: 1.0 From: Ben Hutchings To: linux-kernel@vger.kernel.org, stable@vger.kernel.org CC: akpm@linux-foundation.org, Denis Kirjanov , "Linus Torvalds" , "Jiri Olsa" , "Namhyung Kim" , "Vince Weaver" , "Arnaldo Carvalho de Melo" , "Alexander Shishkin" , "Yabin Cui" , mark.rutland@arm.com, "Peter Zijlstra (Intel)" , "Ingo Molnar" , "Stephane Eranian" , "Arnaldo Carvalho de Melo" , "Thomas Gleixner" Date: Wed, 02 Oct 2019 20:06:51 +0100 Message-ID: X-Mailer: LinuxStableQueue (scripts by bwh) X-Patchwork-Hint: ignore Subject: [PATCH 3.16 24/87] perf/ring_buffer: Fix exposing a temporarily decreased data_head In-Reply-To: X-SA-Exim-Connect-IP: 192.168.4.242 X-SA-Exim-Mail-From: ben@decadent.org.uk X-SA-Exim-Scanned: No (on shadbolt.decadent.org.uk); SAEximRunCond expanded to false Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 3.16.75-rc1 review patch. If anyone has any objections, please let me know. ------------------ From: Yabin Cui commit 1b038c6e05ff70a1e66e3e571c2e6106bdb75f53 upstream. In perf_output_put_handle(), an IRQ/NMI can happen in below location and write records to the same ring buffer: ... local_dec_and_test(&rb->nest) ... <-- an IRQ/NMI can happen here rb->user_page->data_head = head; ... In this case, a value A is written to data_head in the IRQ, then a value B is written to data_head after the IRQ. And A > B. As a result, data_head is temporarily decreased from A to B. And a reader may see data_head < data_tail if it read the buffer frequently enough, which creates unexpected behaviors. This can be fixed by moving dec(&rb->nest) to after updating data_head, which prevents the IRQ/NMI above from updating data_head. [ Split up by peterz. ] Signed-off-by: Yabin Cui Signed-off-by: Peter Zijlstra (Intel) Cc: Alexander Shishkin Cc: Arnaldo Carvalho de Melo Cc: Arnaldo Carvalho de Melo Cc: Jiri Olsa Cc: Linus Torvalds Cc: Namhyung Kim Cc: Peter Zijlstra Cc: Stephane Eranian Cc: Thomas Gleixner Cc: Vince Weaver Cc: mark.rutland@arm.com Fixes: ef60777c9abd ("perf: Optimize the perf_output() path by removing IRQ-disables") Link: http://lkml.kernel.org/r/20190517115418.224478157@infradead.org Signed-off-by: Ingo Molnar Signed-off-by: Ben Hutchings --- kernel/events/ring_buffer.c | 24 ++++++++++++++++++++---- 1 file changed, 20 insertions(+), 4 deletions(-) --- a/kernel/events/ring_buffer.c +++ b/kernel/events/ring_buffer.c @@ -50,11 +50,18 @@ again: head = local_read(&rb->head); /* - * IRQ/NMI can happen here, which means we can miss a head update. + * IRQ/NMI can happen here and advance @rb->head, causing our + * load above to be stale. */ - if (!local_dec_and_test(&rb->nest)) + /* + * If this isn't the outermost nesting, we don't have to update + * @rb->user_page->data_head. + */ + if (local_read(&rb->nest) > 1) { + local_dec(&rb->nest); goto out; + } /* * Since the mmap() consumer (userspace) can run on a different CPU: @@ -86,9 +93,18 @@ again: rb->user_page->data_head = head; /* - * Now check if we missed an update -- rely on previous implied - * compiler barriers to force a re-read. + * We must publish the head before decrementing the nest count, + * otherwise an IRQ/NMI can publish a more recent head value and our + * write will (temporarily) publish a stale value. + */ + barrier(); + local_set(&rb->nest, 0); + + /* + * Ensure we decrement @rb->nest before we validate the @rb->head. + * Otherwise we cannot be sure we caught the 'last' nested update. */ + barrier(); if (unlikely(head != local_read(&rb->head))) { local_inc(&rb->nest); goto again;