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 DB091C433DB for ; Wed, 13 Jan 2021 17:19:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9043A206EC for ; Wed, 13 Jan 2021 17:19:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727804AbhAMRTL (ORCPT ); Wed, 13 Jan 2021 12:19:11 -0500 Received: from mail.kernel.org ([198.145.29.99]:53156 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727484AbhAMRTK (ORCPT ); Wed, 13 Jan 2021 12:19:10 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 61A4E23437; Wed, 13 Jan 2021 17:18:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1610558309; bh=T2ryWm/RgQ4JTQ8kAJI8O40dQjLImKZXnrRkXQR5Mjs=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=PjpqngxqgFOkSDRqwYX+/b6/LxSRXocCno1ihzE+5pggwUuOjgHmzduKfrEPUr9hM P2eQyFlQ7MFZrGXNM4Pt4sDKhJ7fbXJKvJWc5MgQVKWkIvdl5dZYzQitj98Pwfh636 2eRk3GIzlbfOSNitwdbagLnkZ9DTld4DZYtiFUwNGL66+7eFbqIe3QZfrTeuuaVjq8 jNgkLlGKiSDgtTvcGIg6pgeput/PmK52Y/2cBeoE7lncHotFouBs9zZsAOwutryZT5 uATMrbCqWorfEhdXufeWmaeeVodBpe01bwti3wnPLPl6hztjCNKU+7e4g/0G8vT57m hQJ9VvlXxi5Zg== Date: Wed, 13 Jan 2021 19:18:23 +0200 From: Jarkko Sakkinen To: Borislav Petkov 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: References: <20201216134920.21161-1-jarkko@kernel.org> <20210105145749.GF28649@zn.tnic> <20210112183550.GK13086@zn.tnic> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210112183550.GK13086@zn.tnic> Precedence: bulk List-ID: X-Mailing-List: linux-sgx@vger.kernel.org On Tue, Jan 12, 2021 at 07:35:50PM +0100, Borislav Petkov wrote: > + 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. It spun out of this discussion: https://lore.kernel.org/linux-sgx/20201215213517.GA34761@kernel.org/raw My reasoning was that this is not a common case. The main loop that uses synchronize_srcu(). > > -- > Regards/Gruss, > Boris. /Jarkko