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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2DECAC77B7E for ; Fri, 28 Apr 2023 17:30:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1346430AbjD1Rai (ORCPT ); Fri, 28 Apr 2023 13:30:38 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58288 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1346387AbjD1Ra3 (ORCPT ); Fri, 28 Apr 2023 13:30:29 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 85FD42117 for ; Fri, 28 Apr 2023 10:29:41 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1682702980; 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:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=EIIwH7T02AjnTsmSMt7UxStS64AfVcp0/yWRRkLLOC0=; b=aCY2SKw31EMEilRsPZAXUqCUJizuAIExHYRqDwdp73WupGOKFqT058O60RoVBUo7CyvZyi 1fHs0SMSpMdYZNnA10YoAXjmn8XaD3dCANgw4tBNdaTD3s0YKkC/kL8KsMM48ZqjIbOqyo /etQjhIzCNdzKGVKI0mACkbIpTsY/Dk= Received: from mail-wm1-f70.google.com (mail-wm1-f70.google.com [209.85.128.70]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-325-c0DeQoamMVuptVhXBYU0Tw-1; Fri, 28 Apr 2023 13:29:39 -0400 X-MC-Unique: c0DeQoamMVuptVhXBYU0Tw-1 Received: by mail-wm1-f70.google.com with SMTP id 5b1f17b1804b1-3f1793d6363so36683895e9.1 for ; Fri, 28 Apr 2023 10:29:39 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1682702978; x=1685294978; h=content-transfer-encoding:in-reply-to:organization:from:references :cc:to:content-language:subject:user-agent:mime-version:date :message-id:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=EIIwH7T02AjnTsmSMt7UxStS64AfVcp0/yWRRkLLOC0=; b=UEsEVExlDZlJIKSx+6yBVvkpxYstJkaUtdlpnzBhePvRf9RZX/nSNzrcQFSaSKj58B M1RCN6PsGl3MIGI9r/h62v8WYcszAVzz4p/d0tm32GkifK5VZFAaOeq39j5MdZqjMlcX rdEXYyq1WO45Yq/97W86iamj88moaBlHSbpY8/zpnmXP5s9OCjGnSa7PGcJ7E+i0VASM hprI2Bk7AkxnEuI2zJ7sy4/CnUwdW6/ZUz20/73Mh0Ybxzk4yS5eSd2XWSpF6k2BZNPt wk7dYy6WgpYwXzzs82cJHPssnuPoxYxp3INDgKldCOsSwIhAzhZw70xacUIRY4SVemgl U1fg== X-Gm-Message-State: AC+VfDw6H2fsIiicRGoWrs5k8aW9DxgRuK/jg8c0oFc+d0mU7v/yqMZ6 z1AZgwi61e0idOvKrEXVcmpCGtTznmvcMM7EdSr75HXeV+rnFKMVPiJsLEEr9o2TWvFyao/CMZb N3Yz2dhhpKnUun3wgxvlvFAjoARYa1g== X-Received: by 2002:a7b:c015:0:b0:3f1:662a:93d0 with SMTP id c21-20020a7bc015000000b003f1662a93d0mr4869041wmb.15.1682702978230; Fri, 28 Apr 2023 10:29:38 -0700 (PDT) X-Google-Smtp-Source: ACHHUZ4h9gBJBIx2ZQJ+gm3oOoE/Rgj9KykEapquC53KsgU6XwysYEqKUCcp5L2ysNH2brGQpGxgmQ== X-Received: by 2002:a7b:c015:0:b0:3f1:662a:93d0 with SMTP id c21-20020a7bc015000000b003f1662a93d0mr4869002wmb.15.1682702977813; Fri, 28 Apr 2023 10:29:37 -0700 (PDT) Received: from ?IPV6:2003:cb:c726:9300:1711:356:6550:7502? (p200300cbc72693001711035665507502.dip0.t-ipconnect.de. [2003:cb:c726:9300:1711:356:6550:7502]) by smtp.gmail.com with ESMTPSA id q16-20020a7bce90000000b003f09d7b6e20sm25059003wmj.2.2023.04.28.10.29.35 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 28 Apr 2023 10:29:37 -0700 (PDT) Message-ID: <7e096879-4578-36df-4809-3b04f4c20587@redhat.com> Date: Fri, 28 Apr 2023 19:29:35 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.10.0 Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings by default Content-Language: en-US To: Lorenzo Stoakes Cc: Peter Xu , "Kirill A . Shutemov" , Jason Gunthorpe , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Jens Axboe , Matthew Wilcox , Dennis Dalessandro , Leon Romanovsky , Christian Benvenuti , Nelson Escobar , Bernard Metzler , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Ian Rogers , Adrian Hunter , Bjorn Topel , Magnus Karlsson , Maciej Fijalkowski , Jonathan Lemon , "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Christian Brauner , Richard Cochran , Alexei Starovoitov , Daniel Borkmann , Jesper Dangaard Brouer , John Fastabend , linux-fsdevel@vger.kernel.org, linux-perf-users@vger.kernel.org, netdev@vger.kernel.org, bpf@vger.kernel.org, Oleg Nesterov , John Hubbard , Jan Kara , Pavel Begunkov , Mika Penttila , David Howells , Christoph Hellwig References: <077c4b21-8806-455f-be98-d7052a584259@lucifer.local> <62ec50da-5f73-559c-c4b3-bde4eb215e08@redhat.com> <6ddc7ac4-4091-632a-7b2c-df2005438ec4@redhat.com> <20230428160925.5medjfxkyvmzfyhq@box.shutemov.name> <39cc0f26-8fc2-79dd-2e84-62238d27fd98@redhat.com> <20230428162207.o3ejmcz7rzezpt6n@box.shutemov.name> <173337c0-14f4-3246-15ff-7fbf03861c94@redhat.com> <40fc128f-1978-42db-b9c1-77ac3c2cebfe@lucifer.local> <3d7fcfab-e445-1dc7-f000-9fbe7bea04c0@redhat.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-perf-users@vger.kernel.org On 28.04.23 19:13, Lorenzo Stoakes wrote: > On Fri, Apr 28, 2023 at 07:05:38PM +0200, David Hildenbrand wrote: >> On 28.04.23 19:01, Lorenzo Stoakes wrote: >>> On Fri, Apr 28, 2023 at 06:51:46PM +0200, David Hildenbrand wrote: >>>> On 28.04.23 18:39, Peter Xu wrote: >>>>> On Fri, Apr 28, 2023 at 07:22:07PM +0300, Kirill A . Shutemov wrote: >>>>>> On Fri, Apr 28, 2023 at 06:13:03PM +0200, David Hildenbrand wrote: >>>>>>> On 28.04.23 18:09, Kirill A . Shutemov wrote: >>>>>>>> On Fri, Apr 28, 2023 at 05:43:52PM +0200, David Hildenbrand wrote: >>>>>>>>> On 28.04.23 17:34, David Hildenbrand wrote: >>>>>>>>>> On 28.04.23 17:33, Lorenzo Stoakes wrote: >>>>>>>>>>> On Fri, Apr 28, 2023 at 05:23:29PM +0200, David Hildenbrand wrote: >>>>>>>>>>>>>> >>>>>>>>>>>>>> Security is the primary case where we have historically closed uAPI >>>>>>>>>>>>>> items. >>>>>>>>>>>>> >>>>>>>>>>>>> As this patch >>>>>>>>>>>>> >>>>>>>>>>>>> 1) Does not tackle GUP-fast >>>>>>>>>>>>> 2) Does not take care of !FOLL_LONGTERM >>>>>>>>>>>>> >>>>>>>>>>>>> I am not convinced by the security argument in regard to this patch. >>>>>>>>>>>>> >>>>>>>>>>>>> >>>>>>>>>>>>> If we want to sells this as a security thing, we have to block it >>>>>>>>>>>>> *completely* and then CC stable. >>>>>>>>>>>> >>>>>>>>>>>> Regarding GUP-fast, to fix the issue there as well, I guess we could do >>>>>>>>>>>> something similar as I did in gup_must_unshare(): >>>>>>>>>>>> >>>>>>>>>>>> If we're in GUP-fast (no VMA), and want to pin a !anon page writable, >>>>>>>>>>>> fallback to ordinary GUP. IOW, if we don't know, better be safe. >>>>>>>>>>> >>>>>>>>>>> How do we determine it's non-anon in the first place? The check is on the >>>>>>>>>>> VMA. We could do it by following page tables down to folio and checking >>>>>>>>>>> folio->mapping for PAGE_MAPPING_ANON I suppose? >>>>>>>>>> >>>>>>>>>> PageAnon(page) can be called from GUP-fast after grabbing a reference. >>>>>>>>>> See gup_must_unshare(). >>>>>>>>> >>>>>>>>> IIRC, PageHuge() can also be called from GUP-fast and could special-case >>>>>>>>> hugetlb eventually, as it's table while we hold a (temporary) reference. >>>>>>>>> Shmem might be not so easy ... >>>>>>>> >>>>>>>> page->mapping->a_ops should be enough to whitelist whatever fs you want. >>>>>>>> >>>>>>> >>>>>>> The issue is how to stabilize that from GUP-fast, such that we can safely >>>>>>> dereference the mapping. Any idea? >>>>>>> >>>>>>> At least for anon page I know that page->mapping only gets cleared when >>>>>>> freeing the page, and we don't dereference the mapping but only check a >>>>>>> single flag stored alongside the mapping. Therefore, PageAnon() is fine in >>>>>>> GUP-fast context. >>>>>> >>>>>> What codepath you are worry about that clears ->mapping on pages with >>>>>> non-zero refcount? >>>>>> >>>>>> I can only think of truncate (and punch hole). READ_ONCE(page->mapping) >>>>>> and fail GUP_fast if it is NULL should be fine, no? >>>>>> >>>>>> I guess we should consider if the inode can be freed from under us and the >>>>>> mapping pointer becomes dangling. But I think we should be fine here too: >>>>>> VMA pins inode and VMA cannot go away from under GUP. >>>>> >>>>> Can vma still go away if during a fast-gup? >>>>> >>>> >>>> So, after we grabbed the page and made sure the the PTE didn't change (IOW, >>>> the PTE was stable while we processed it), the page can get unmapped (but >>>> not freed, because we hold a reference) and the VMA can theoretically go >>>> away (and as far as I understand, nothing stops the file from getting >>>> deleted, truncated etc). >>>> >>>> So we might be looking at folio->mapping and the VMA is no longer there. >>>> Maybe even the file is no longer there. >>>> >>> >>> This shouldn't be an issue though right? Because after a pup call unlocks the >>> mmap_lock we're in the same situation anyway. GUP doesn't generally guarantee >>> the mapping remains valid, only pinning the underlying folio. >> >> Yes. But the issue here is rather dereferencing something that has already >> been freed, eventually leading to undefined behavior. >> > > Is that an issue with interrupts disabled though? Will block page tables being > removed and as Kirill says (sorry I maybe misinterpreted you) we should be ok. Let's rule out page table freeing. If our VMA only spans a single page and falls into the same PMD as another VMA, an munmap() would not even free a single page table. However, if unmapping a page (flushing the TLB) would imply an IPI as Kirill said, we'd be fine. I recall that that's not the case for all architectures, but I might be just wrong. ... and now I'll stop reading mails until Tuesday :) -- Thanks, David / dhildenb