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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, 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 5CF45C433E0 for ; Tue, 12 Jan 2021 18:37:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1E1502311F for ; Tue, 12 Jan 2021 18:37:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2392191AbhALSgj (ORCPT ); Tue, 12 Jan 2021 13:36:39 -0500 Received: from mail.skyhub.de ([5.9.137.197]:58318 "EHLO mail.skyhub.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2392178AbhALSgi (ORCPT ); Tue, 12 Jan 2021 13:36:38 -0500 Received: from zn.tnic (p200300ec2f0e8c00c6af2066566de43f.dip0.t-ipconnect.de [IPv6:2003:ec:2f0e:8c00:c6af:2066:566d:e43f]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.skyhub.de (SuperMail on ZX Spectrum 128k) with ESMTPSA id D26E01EC05D1; Tue, 12 Jan 2021 19:35:56 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=alien8.de; s=dkim; t=1610476557; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:in-reply-to:in-reply-to: references:references; bh=8ewCXROtelgxCjKkS72wMIEx2jAuhJ0XY23mMMysgdk=; b=D2EdN9d94gJJzsALxHhZpvIXQpjcwjD00kvBv/ok8NWwS4T1e5bCob+NLHGQKL2VKtnEkT XRYZRPeeduvxj28PnD68h5/ormAb1b7pjvugX8RE85PtgYCsODGIVG5VgwCTQk1fU24RZx EGY1JfpF2qpbdKtKsjeghqLKRl4tMUs= Date: Tue, 12 Jan 2021 19:35:50 +0100 From: Borislav Petkov To: Jarkko Sakkinen Cc: x86@kernel.org, linux-kernel@vger.kernel.org, linux-sgx@vger.kernel.org, Sean Christopherson , Haitao Huang , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Jethro Beekman , "Paul E. McKenney" Subject: Re: [PATCH v3] x86/sgx: Synchronize encl->srcu in sgx_encl_release(). Message-ID: <20210112183550.GK13086@zn.tnic> References: <20201216134920.21161-1-jarkko@kernel.org> <20210105145749.GF28649@zn.tnic> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-sgx@vger.kernel.org + paulmck. On Tue, Jan 12, 2021 at 02:08:10AM +0200, Jarkko Sakkinen wrote: > On Tue, Jan 05, 2021 at 03:57:49PM +0100, Borislav Petkov wrote: > > On Wed, Dec 16, 2020 at 03:49:20PM +0200, Jarkko Sakkinen wrote: > > > Add synchronize_srcu_expedited() to sgx_encl_release() to catch a grace > > > period initiated by sgx_mmu_notifier_release(). > > > > > > A trivial example of a failing sequence with tasks A and B: > > > > > > 1. A: -> sgx_release() > > > 2. B: -> sgx_mmu_notifier_release() > > > 3. B: -> list_del_rcu() > > > 3. A: -> sgx_encl_release() > > > 4. A: -> cleanup_srcu_struct() > > > > > > The loop in sgx_release() observes an empty list because B has removed its > > > entry in the middle, and calls cleanup_srcu_struct() before B has a chance > > > to calls synchronize_srcu(). > > > > Leading to what? NULL ptr? > > > > https://lkml.kernel.org/r/X9e2jOWz1hfXVpQ5@google.com > > > > already suggested that you should explain the bug better and add the > > splat but I'm still missing that explanation. > > OK, I'll try to explain it how I understand the issue. > > Consider this loop in the VFS release hook (sgx_release): > > /* > * Drain the remaining mm_list entries. At this point the list contains > * entries for processes, which have closed the enclave file but have > * not exited yet. The processes, which have exited, are gone from the > * list by sgx_mmu_notifier_release(). > */ > for ( ; ; ) { > spin_lock(&encl->mm_lock); > > if (list_empty(&encl->mm_list)) { > encl_mm = NULL; > } else { > encl_mm = list_first_entry(&encl->mm_list, > struct sgx_encl_mm, list); > list_del_rcu(&encl_mm->list); > } > > spin_unlock(&encl->mm_lock); > > /* The enclave is no longer mapped by any mm. */ > if (!encl_mm) > break; > > synchronize_srcu(&encl->srcu); > mmu_notifier_unregister(&encl_mm->mmu_notifier, encl_mm->mm); > kfree(encl_mm); > } > > > At this point all processes have closed the enclave file, but that doesn't > mean that they all have exited yet. > > Now, let's imagine that there is exactly one entry in the encl->mm_list. > and sgx_release() execution gets scheduled right after returning from > synchronize_srcu(). > > With some bad luck, some process comes and removes that last entry befoe > sgx_release() acquires mm_lock. The loop in sgx_release() just leaves > > /* The enclave is no longer mapped by any mm. */ > if (!encl_mm) > break; > > No synchronize_srcu(). > > After writing this, I think that the placement for synchronize_srcu() > in this patch is not best possible. It should be rather that the > above loop would also call synchronize_srcu() when leaving. > > I.e. the code change would result: > > for ( ; ; ) { > spin_lock(&encl->mm_lock); > > if (list_empty(&encl->mm_list)) { > encl_mm = NULL; > } else { > encl_mm = list_first_entry(&encl->mm_list, > struct sgx_encl_mm, list); > list_del_rcu(&encl_mm->list); > } > > spin_unlock(&encl->mm_lock); > > /* > * synchronize_srcu() is mandatory *even* when the list was > * empty, in order make sure that grace periods stays in > * sync even when another task took away the last entry > * (i.e. exiting process when it deletes its mm_list). > */ > synchronize_srcu(&encl->srcu); > > /* The enclave is no longer mapped by any mm. */ > if (!encl_mm) > break; > > mmu_notifier_unregister(&encl_mm->mmu_notifier, encl_mm->mm); > kfree(encl_mm); > } > > What do you think? Does this start to make more sense now? > I don't have logs for this but the bug can be also reasoned. It does. Now you need to write it up in a detailed form so that it is clear to readers months/years from now what exactly can happen. You can use a two-column format like CPU A CPU B Bla Blu This happens now here But this needs to happen there and so on. Also, from reading up a bit on this, Documentation/RCU/checklist.rst says "Use of the expedited primitives should be restricted to rare configuration-change operations that would not normally be undertaken while a real-time workload is running." so why are you using synchronize_srcu_expedited()? Grepping the tree reveals only a couple of call sites only... but I've almost no clue of RCU so lemme CC Paul. Thx. -- Regards/Gruss, Boris. https://people.kernel.org/tglx/notes-about-netiquette