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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 831CEC47247 for ; Thu, 30 Apr 2020 22:07:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 66BD320836 for ; Thu, 30 Apr 2020 22:07:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728108AbgD3WHZ (ORCPT ); Thu, 30 Apr 2020 18:07:25 -0400 Received: from mga11.intel.com ([192.55.52.93]:23209 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727930AbgD3WGt (ORCPT ); Thu, 30 Apr 2020 18:06:49 -0400 IronPort-SDR: wFddU0H/qqyEsIFD2VY5QDn1+BVhIPD0+cLTNf1QHwEklus7DOtWRCJNnHk2Z/3pgAPPxq9Soa ZyiuRkl6DnKA== X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga006.jf.intel.com ([10.7.209.51]) by fmsmga102.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 30 Apr 2020 15:06:47 -0700 IronPort-SDR: EKa0MD1KFBL+6pqaxJP2pWCwCBDSir+lBLyLu+xgV2xbsbFCzrH2Wc2b2lTCwSQqAlfvQPv9WU oJNekRcfvZhw== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.73,337,1583222400"; d="scan'208";a="261926444" Received: from dnlloyd-mobl.amr.corp.intel.com (HELO [10.255.231.251]) ([10.255.231.251]) by orsmga006.jf.intel.com with ESMTP; 30 Apr 2020 15:06:46 -0700 Subject: Re: [PATCH v2 1/1] fs/splice: add missing callback for inaccessible pages To: Christian Borntraeger , Claudio Imbrenda , viro@zeniv.linux.org.uk Cc: david@redhat.com, akpm@linux-foundation.org, aarcange@redhat.com, linux-mm@kvack.org, frankja@linux.ibm.com, sfr@canb.auug.org.au, jhubbard@nvidia.com, linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org, jack@suse.cz, kirill@shutemov.name, peterz@infradead.org, sean.j.christopherson@intel.com, Ulrich.Weigand@de.ibm.com References: <20200430143825.3534128-1-imbrenda@linux.ibm.com> <1a3f5107-9847-73d4-5059-c6ef9d293551@de.ibm.com> From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: <1a3f5107-9847-73d4-5059-c6ef9d293551@de.ibm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org I was also wondering if Claudio was right about the debug patch having races. I went to go look how the s390 code avoids races when pages go from accessible->inaccessible. Because, if if all of the traps are in place to transform pages from inaccessible->accessible, the code *after* those traps is still vulnerable. What *keeps* pages accessible? The race avoidance is this, basically: down_read(&gmap->mm->mmap_sem); lock_page(page); ptep = get_locked_pte(gmap->mm, uaddr, &ptelock); ... > expected = expected_page_refs(page); > if (!page_ref_freeze(page, expected)) > return -EBUSY; > set_bit(PG_arch_1, &page->flags); > rc = uv_call(0, (u64)uvcb); > page_ref_unfreeze(page, expected); ... up_read(mmap_sem) / unlock_page() / unlock pte I'm assuming that after the uv_call(), the page is inaccessible and I/O devices will go boom if they touch the page. The page_ref_freeze() ensures that references come between the freeze/unfreeze are noticed, but it doesn't actually *stop* new ones for users that hold references already. For the page cache, especially, someone could do: page = find_get_page(); arch_make_page_accessible(); lock_page(); ... make_secure_pte(); unlock_page(); get_page(); // ^ OK because I have a ref // do DMA on inaccessible page Because the make_secure_pte() code isn't looking for a *specific* 'expected' value, it has no way of noticing that the extra ref snuck in there. I _think_ expected actually needs to be checked for having a specific (low) value so that if there's a *possibility* of a reference holder acquiring additional references, the page is known to be off-limits. mm/migrate.c has a few examples of this, but I'm not quite sure how bulletproof they are. Some of it appears to just be optimizations.