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=-6.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,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 7695CC433DB for ; Fri, 15 Jan 2021 09:59:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3377C235F9 for ; Fri, 15 Jan 2021 09:59:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726852AbhAOJ73 (ORCPT ); Fri, 15 Jan 2021 04:59:29 -0500 Received: from mail.kernel.org ([198.145.29.99]:52534 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726439AbhAOJ72 (ORCPT ); Fri, 15 Jan 2021 04:59:28 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 9290923888; Fri, 15 Jan 2021 09:58:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1610704717; bh=9sIfGYn+CB58kDNMQLw6lzQnvI29J9O0n7ueauxjWrQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=oEeboGETw7lhuzTEW7iyXZDwW3hKbrpRHa4qkMlTbXdlmG07uYA1Sk4sPNXKSMP/f UfSLu9aymgt8t8wS1UmPPqNyp526rogbzVHgccdAx/B/73JilQy04U1We4ji7Bo43E kYGPvq1JnL45CzegKCrUeSRQGeYOXXvR6w/W/ux9opCB3bpwm50wY7C1zTSU47xAky p1yPuMwGPgPqrCaBGxdWewyA6SXOqEAfGXTchx7ZMO871Wiuc6I/IItWVVINinH2+9 s2QtUmcqmCK7hQz5l1iSq1mDauB6DrRj+s4/mlCzTcbych047lCTWwlS5THluqfAsU DrCJJ3+pN8rXQ== Date: Fri, 15 Jan 2021 11:58:32 +0200 From: Jarkko Sakkinen To: Haitao Huang Cc: Borislav Petkov , x86@kernel.org, linux-kernel@vger.kernel.org, linux-sgx@vger.kernel.org, Sean Christopherson , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Jethro Beekman Subject: Re: [PATCH v3] x86/sgx: Synchronize encl->srcu in sgx_encl_release(). Message-ID: References: <20201216134920.21161-1-jarkko@kernel.org> <20210105145749.GF28649@zn.tnic> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-sgx@vger.kernel.org On Wed, Jan 13, 2021 at 10:42:12PM -0600, Haitao Huang wrote: > On Mon, 11 Jan 2021 18:08:10 -0600, 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. > > > > /Jarkko > > I did this experiment just now and find it runs much much slower than both > original code and code with synchronize_srcu_expedited fix in this patch. > Haitao Yeah, but using expedited is not really in the scope of a bug fix. It's a change that needs to be considered separately. And it's more complicated than that. It also needs data to back it up. How we can test and compare? /Jarkko