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=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 B8CADC432BE for ; Sat, 28 Aug 2021 19:45:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A3DC760551 for ; Sat, 28 Aug 2021 19:45:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234433AbhH1TqU (ORCPT ); Sat, 28 Aug 2021 15:46:20 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44100 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230253AbhH1TqT (ORCPT ); Sat, 28 Aug 2021 15:46:19 -0400 Received: from desiato.infradead.org (desiato.infradead.org [IPv6:2001:8b0:10b:1:d65d:64ff:fe57:4e05]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 389A5C061756; Sat, 28 Aug 2021 12:45:28 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=XaDJo/Xp/thzakG+AyFw4gfOeYqXSd+OvToP2IAZADw=; b=iaSYaDXqR2I0RfntlzqVugj0Nj rXYt5kYTwyGKSPywFnw4fyUzTY9LAxxbRjtQkmlvftPwsUj5uPz2F5YspCjIPUg0EU+uNCyGQ3m1z lWBYQwuSheYbXudKlCIc8TNjxaN9C6VQgIvcLqNM4nad3zEOcmW0XTeahuIU8N5XEWm3P7Ut9+abN Q8fQ7zTfRTZ3R7RoF/QygqFwsUOnFQLkR+lOFf7wq4T+6b6moR3EFv7YMAVF9pd5Ox3uQjzGFISAV ivzAchA/SXjNJ8WNIC5WtqGO0JdesSU+G3xE61VeRun7z2ArlsfJGMdbJCaLwzEcpgePCOVpe7U2B vRqaWBQQ==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=worktop.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1mK4Fb-00Dvei-M3; Sat, 28 Aug 2021 19:44:23 +0000 Received: by worktop.programming.kicks-ass.net (Postfix, from userid 1000) id A026C98679D; Sat, 28 Aug 2021 21:44:21 +0200 (CEST) Date: Sat, 28 Aug 2021 21:44:21 +0200 From: Peter Zijlstra To: Sean Christopherson Cc: Ingo Molnar , Arnaldo Carvalho de Melo , Will Deacon , Mark Rutland , Catalin Marinas , Marc Zyngier , Guo Ren , Nick Hu , Greentime Hu , Vincent Chen , Paul Walmsley , Palmer Dabbelt , Albert Ou , Thomas Gleixner , Borislav Petkov , x86@kernel.org, Paolo Bonzini , Boris Ostrovsky , Juergen Gross , Alexander Shishkin , Jiri Olsa , Namhyung Kim , James Morse , Alexandru Elisei , Suzuki K Poulose , "H. Peter Anvin" , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Stefano Stabellini , linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-csky@vger.kernel.org, linux-riscv@lists.infradead.org, kvm@vger.kernel.org, xen-devel@lists.xenproject.org, Artem Kashkanov , Like Xu , Zhu Lingshan Subject: Re: [PATCH v2 01/13] perf: Ensure perf_guest_cbs aren't reloaded between !NULL check and deref Message-ID: <20210828194421.GB4353@worktop.programming.kicks-ass.net> References: <20210828003558.713983-1-seanjc@google.com> <20210828003558.713983-2-seanjc@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210828003558.713983-2-seanjc@google.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Aug 27, 2021 at 05:35:46PM -0700, Sean Christopherson wrote: > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h > index 2d510ad750ed..6b0405e578c1 100644 > --- a/include/linux/perf_event.h > +++ b/include/linux/perf_event.h > @@ -1237,6 +1237,14 @@ extern void perf_event_bpf_event(struct bpf_prog *prog, > u16 flags); > > extern struct perf_guest_info_callbacks *perf_guest_cbs; > +static inline struct perf_guest_info_callbacks *perf_get_guest_cbs(void) > +{ > + /* Reg/unreg perf_guest_cbs waits for readers via synchronize_rcu(). */ > + lockdep_assert_preemption_disabled(); > + > + /* Prevent reloading between a !NULL check and dereferences. */ > + return READ_ONCE(perf_guest_cbs); > +} Nice.. > extern int perf_register_guest_info_callbacks(struct perf_guest_info_callbacks *callbacks); > extern int perf_unregister_guest_info_callbacks(struct perf_guest_info_callbacks *callbacks); > > diff --git a/kernel/events/core.c b/kernel/events/core.c > index 464917096e73..2126f6327321 100644 > --- a/kernel/events/core.c > +++ b/kernel/events/core.c > @@ -6491,14 +6491,19 @@ struct perf_guest_info_callbacks *perf_guest_cbs; > > int perf_register_guest_info_callbacks(struct perf_guest_info_callbacks *cbs) > { > - perf_guest_cbs = cbs; > + if (WARN_ON_ONCE(perf_guest_cbs)) > + return -EBUSY; > + > + WRITE_ONCE(perf_guest_cbs, cbs); > + synchronize_rcu(); You're waiting for all NULL users to go away? :-) IOW, we can do without this synchronize_rcu() call. > return 0; > } > EXPORT_SYMBOL_GPL(perf_register_guest_info_callbacks); > > int perf_unregister_guest_info_callbacks(struct perf_guest_info_callbacks *cbs) > { > - perf_guest_cbs = NULL; if (WARN_ON_ONCE(perf_guest_cbs != cbs)) return -EBUSY; ? > + WRITE_ONCE(perf_guest_cbs, NULL); > + synchronize_rcu(); > return 0; > } > EXPORT_SYMBOL_GPL(perf_unregister_guest_info_callbacks); Yes, this ought to work fine.