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=-6.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=ham 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 99B3DC10DCE for ; Wed, 18 Mar 2020 15:39:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6914320724 for ; Wed, 18 Mar 2020 15:39:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726796AbgCRPjI (ORCPT ); Wed, 18 Mar 2020 11:39:08 -0400 Received: from mga02.intel.com ([134.134.136.20]:34650 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726789AbgCRPjI (ORCPT ); Wed, 18 Mar 2020 11:39:08 -0400 IronPort-SDR: 2O+/lugaWeAa/Jnw7ikLYz/ma7KQ512glPZw3G0KGVlLoQdwfaysFqFekQpCh9SWtt22HnCg+y I4TDsxrmMlBg== X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga004.jf.intel.com ([10.7.209.38]) by orsmga101.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 18 Mar 2020 08:39:07 -0700 IronPort-SDR: rC98ifT26yv9xmM4iVasz6/oRt9/kUgsMmt2AXO6YqxfPXz/LUAo5n2hHQe0wphkMD5CyODk6v 1aibR8fuue9A== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.70,568,1574150400"; d="scan'208";a="391483775" Received: from mbeldzik-mobl.ger.corp.intel.com (HELO localhost) ([10.252.55.127]) by orsmga004.jf.intel.com with ESMTP; 18 Mar 2020 08:39:05 -0700 Date: Wed, 18 Mar 2020 17:39:03 +0200 From: Jarkko Sakkinen To: Sean Christopherson Cc: linux-sgx@vger.kernel.org Subject: Re: [PATCH] x86/sgx: Fix deadlock and race conditions between fork() and EPC reclaim Message-ID: <20200318153903.GA37333@linux.intel.com> References: <20200317051539.10447-1-sean.j.christopherson@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200317051539.10447-1-sean.j.christopherson@intel.com> Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo Sender: linux-sgx-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-sgx@vger.kernel.org On Mon, Mar 16, 2020 at 10:15:39PM -0700, Sean Christopherson wrote: > Drop the synchronize_srcu() from sgx_encl_mm_add() and replace it with a > mm_list versioning concept to avoid deadlock when adding a mm during > dup_mmap()/fork(), and to ensure copied PTEs are zapped. > > When dup_mmap() runs, it holds mmap_sem for write in both the old mm and > new mm. Invoking synchronize_srcu() while holding mmap_sem of a mm that > is already attached to the enclave will deadlock if the reclaimer is in > the process of walking mm_list, as the reclaimer will try to acquire > mmap_sem (of the old mm) while holding encl->srcu for read. > > INFO: task ksgxswapd:181 blocked for more than 120 seconds. > ksgxswapd D 0 181 2 0x80004000 > Call Trace: > __schedule+0x2db/0x700 > schedule+0x44/0xb0 > rwsem_down_read_slowpath+0x370/0x470 > down_read+0x95/0xa0 > sgx_reclaim_pages+0x1d2/0x7d0 > ksgxswapd+0x151/0x2e0 > kthread+0x120/0x140 > ret_from_fork+0x35/0x40 > > INFO: task fork_consistenc:18824 blocked for more than 120 seconds. > fork_consistenc D 0 18824 18786 0x00004320 > Call Trace: > __schedule+0x2db/0x700 > schedule+0x44/0xb0 > schedule_timeout+0x205/0x300 > wait_for_completion+0xb7/0x140 > __synchronize_srcu.part.22+0x81/0xb0 > synchronize_srcu_expedited+0x27/0x30 > synchronize_srcu+0x57/0xe0 > sgx_encl_mm_add+0x12b/0x160 > sgx_vma_open+0x22/0x40 > dup_mm+0x521/0x580 > copy_process+0x1a56/0x1b50 > _do_fork+0x85/0x3a0 > __x64_sys_clone+0x8e/0xb0 > do_syscall_64+0x57/0x1b0 > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > Furthermore, doing synchronize_srcu() in sgx_encl_mm_add() does not > prevent the new mm from having stale PTEs pointing at the EPC page to be > reclaimed. dup_mmap() calls vm_ops->open()/sgx_encl_mm_add() _after_ > PTEs are copied to the new mm, i.e. blocking fork() until reclaim zaps > the old mm is pointless as the stale PTEs have already been created in > the new mm. > > All other flows that walk mm_list can safely race with dup_mmap() or are > protected by a different mechanism. Add comments to all srcu readers > that don't check the list version to document why its ok for the flow to > ignore the version. > > Note, synchronize_srcu() is still needed when removing a mm from an > enclave, as the srcu readers must complete their walk before the mm can > be freed. Removing a mm is never done while holding mmap_sem. > > Signed-off-by: Sean Christopherson > --- > arch/x86/kernel/cpu/sgx/encl.c | 22 +++++++++++++++++++-- > arch/x86/kernel/cpu/sgx/encl.h | 1 + > arch/x86/kernel/cpu/sgx/reclaim.c | 33 +++++++++++++++++++++++++++++++ > 3 files changed, 54 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c > index d6a19bdd1921..b9a7c56f7c25 100644 > --- a/arch/x86/kernel/cpu/sgx/encl.c > +++ b/arch/x86/kernel/cpu/sgx/encl.c > @@ -196,6 +196,12 @@ int sgx_encl_mm_add(struct sgx_encl *encl, struct mm_struct *mm) > struct sgx_encl_mm *encl_mm; > int ret; > > + /* > + * This flow relies on mmap_sem to provide mutual exclusivity (for a > + * given mm) to prevent duplicate instances of an encl_mm on the list. > + */ > + lockdep_assert_held_write(&mm->mmap_sem); > + > if (atomic_read(&encl->flags) & SGX_ENCL_DEAD) > return -EINVAL; > > @@ -223,10 +229,22 @@ int sgx_encl_mm_add(struct sgx_encl *encl, struct mm_struct *mm) > > spin_lock(&encl->mm_lock); > list_add_rcu(&encl_mm->list, &encl->mm_list); > + /* > + * Ensure the mm is added to the list before updating the version. > + * Pairs with the smp_rmb() in sgx_reclaimer_block(). > + */ > + smp_wmb(); > + encl->mm_list_version++; > spin_unlock(&encl->mm_lock); > > - synchronize_srcu(&encl->srcu); > - > + /* > + * DO NOT call synchronize_srcu()! When this is called via dup_mmap(), > + * mmap_sem is held for write in both the old mm and new mm, and the > + * reclaimer may be holding srcu for read while waiting on down_read() > + * for the old mm's mmap_sem, i.e. synchronizing will deadlock. > + * Incrementing the list version ensures readers that must not race > + * with a mm being added will see the updated list. > + */ > return 0; > } > > diff --git a/arch/x86/kernel/cpu/sgx/encl.h b/arch/x86/kernel/cpu/sgx/encl.h > index 44b353aa8866..f0f72e591244 100644 > --- a/arch/x86/kernel/cpu/sgx/encl.h > +++ b/arch/x86/kernel/cpu/sgx/encl.h > @@ -74,6 +74,7 @@ struct sgx_encl { > struct mutex lock; > struct list_head mm_list; > spinlock_t mm_lock; > + unsigned long mm_list_version; > struct file *backing; > struct kref refcount; > struct srcu_struct srcu; > diff --git a/arch/x86/kernel/cpu/sgx/reclaim.c b/arch/x86/kernel/cpu/sgx/reclaim.c > index 39f0ddefbb79..3b4b849c5b2e 100644 > --- a/arch/x86/kernel/cpu/sgx/reclaim.c > +++ b/arch/x86/kernel/cpu/sgx/reclaim.c > @@ -155,6 +155,11 @@ static bool sgx_reclaimer_age(struct sgx_epc_page *epc_page) > bool ret = true; > int idx; > > + /* > + * Note, this can race with sgx_encl_mm_add(), but worst case scenario > + * a page will be reclaimed immediately after it's accessed in the new > + * process/mm. > + */ > idx = srcu_read_lock(&encl->srcu); > > list_for_each_entry_rcu(encl_mm, &encl->mm_list, list) { > @@ -184,10 +189,20 @@ static void sgx_reclaimer_block(struct sgx_epc_page *epc_page) > struct sgx_encl_page *page = epc_page->owner; > unsigned long addr = SGX_ENCL_PAGE_ADDR(page); > struct sgx_encl *encl = page->encl; > + unsigned long mm_list_version; > struct sgx_encl_mm *encl_mm; > struct vm_area_struct *vma; > int idx, ret; > > +retry: > + mm_list_version = encl->mm_list_version; > + /* > + * Ensure the list version is read before walking the list to prevent > + * beginning the walk with the old list using the new version. Pairs > + * with the smp_wmb() in sgx_encl_mm_add(). > + */ > + smp_rmb(); > + > idx = srcu_read_lock(&encl->srcu); > > list_for_each_entry_rcu(encl_mm, &encl->mm_list, list) { > @@ -207,6 +222,19 @@ static void sgx_reclaimer_block(struct sgx_epc_page *epc_page) > > srcu_read_unlock(&encl->srcu, idx); > > + /* > + * Redo the zapping if a mm was added to the list while zapping was in > + * progress. dup_mmap() copies the PTEs for VM_PFNMAP VMAs, i.e. the > + * new mm won't take a page fault and so won't see that the page is > + * tagged RECLAIMED. Note, vm_ops->open()/sgx_encl_mm_add() is called > + * _after_ PTEs are copied, and dup_mmap() holds the old mm's mmap_sem > + * for write, so the version check is only needed to protect against > + * dup_mmap() running after the list walk started but before the old > + * mm's PTEs were zapped. > + */ > + if (unlikely(encl->mm_list_version != mm_list_version)) > + goto retry; > + > mutex_lock(&encl->lock); > > if (!(atomic_read(&encl->flags) & SGX_ENCL_DEAD)) { > @@ -250,6 +278,11 @@ static const cpumask_t *sgx_encl_ewb_cpumask(struct sgx_encl *encl) > struct sgx_encl_mm *encl_mm; > int idx; > > + /* > + * Note, this can race with sgx_encl_mm_add(), but ETRACK has already > + * been executed, so CPUs running in the new mm will enter the enclave > + * in a different epoch. > + */ > cpumask_clear(cpumask); > > idx = srcu_read_lock(&encl->srcu); > -- > 2.24.1 > Please recheck the remarks that I made about inline comments in the source code. /Jarkko.